PHYSICS NEWS BULLETIN OF THE INDIAN PHYSICS ASSOCIATION January – March 2020 Vol. 50 No. 1 ISSN: 0253 – 7583 www.tifr.res.in/~ipa1970
B.V. Sreekantan (1925-2019) INDIAN PHYSICS ASSOCIATION was founded in 1970 with the following aims and objectives: a) to help the advancement, dissemination and application of the knowledge of physics b) to promote active interaction among all persons, bodies, institutions (private and/or state owned) and industries interested achieving the advancement, dissemination and application of the knowledge of physics c) to disseminate information in the field of physics by publication of bulletins, reports, newsletters, journals incorporating research and teaching ideas, reviews, new developments, announcements regarding meetings, seminars, etc., and also by arranging special programmes for students or establishing student cadres d) to arrange seminars, lectures, debates, panel discussions, conferences and film shows on current research topics and other topics of national and local interest pertaining to research and teaching in physics e) to undertake and execute all other acts as mentioned in the constitution of IPA
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PHYSICS NEWS Vol. 50 No. 1 January – March 2020
Contents Editorial 2 From the President’s Desk 3 Articles
The Scientific Journey of B.V. Sreekantan 4 P.C. Agrawal Prof. B.V. Sreekantan - A Humane Visionary 14 Palahalli R. Vishwanath How to ‘see’ the invisible at collider 16 Rohini Godbole Quest for the “Top” 22 Naba K. Mondal Resources for Quantum Technologies 24 Aditi Sen (De) Subsurface detection by SHAllow RADar on MARS 29 S. K. Mishra and Rajiv R. Bharti ANN based identification of tsunamigenic earthquakes using seismic wave 35 Ajit Kundu, Pratap Mane and Siddhartha Mukhopadhyay News and Events GIPWG Activity Report 42 Book Review: Dr. B.V. Sreekantan-A Pioneer Physicist 44 Meet the Physicists 46 Backscatter: Pandemics, Public Health, and Physics 47
The opinions expresses in the articles in this issue are those of the authors and do not necessarily reflect the opinion of the Physics News or IPA
PHYSICS NEWS is funded by a grant from the Board of Research in Nuclear Sciences (BRNS) of the department of Atomic Energy, Government of India. Image Credits : Front cover - CORSIKA simulation of an extensive air shower initiated by a 1 TeV proton (www- zeuthen.desy.de/~jknapp/fs/proton_12_0deg.xz.png); Back cover - The High Altitude GAmma Ray (HAGAR) telescope array at Hanle, Ladakh (HAGAR team, TIFR)
Physics News
PHYSICS NEWS (ISSN : 0253-7583) January-March 2020
Vol. 50 No. 1 Editorial [email protected] We are very happy to bring out the first issue of the golden jubilee volume (Volume 50) of the IPA Physics News. In this issue we pay tribute to Prof. B.V. Sreekantan, EDITORS one of the pioneers of experimental cosmic ray research in India from deep mines to Arnab Bhattacharya high altitudes. He initiated X-ray and gamma-ray astronomy studies, which have
culminated in many major projects like the ASTROSAT satellite and the HAGAR Vandana Nanal telescope. He was closely associated with the IPA and served as its president from Aradhana Shrivastava 1977-79. We have two articles on Prof. Sreekantan by his close associates describing his role as a scientist and science administrator.
The January to March period covers the UN’s International Day of Women and Girls th th in Science (11 Feb.) and Women’s Day (8 March). Additionally, this year the theme CONSULTING EDITORS for India’s National Science Day (28th Feb.) was “Women in Science”. This issues Dipan K. Ghosh contains articles by two outstanding women physicists: Aditi De, the first woman physicist in the past 60 years to get prestigious Shanti Swarup Bhatnagar award in S. Kailas physical sciences and Rohini Godbole – recipient of the IPA’s R.D. Birla award of 2018 and the first woman physicist to be honoured with Padma award. The Hyderabad charter – recommendations from the gender working group of IPA to move towards S. Kailas achieving gender neutrality, is also reported in this issue. To add a personal touch to the newsletter, we are starting a new page “Meet the Physicists” – bringing you profiles of physicists across the country, from different disciplines and age groups. In this issue, keeping up with theme of 11th February, we feature 4 women physicists across India. We also have two general articles - ANN based identification of tsunamigenic earthquakes using seismic waves and Subsurface detections by SHAllow RADar (SHARAD) on MARS. We are glad that the recently introduced “backscatter” page has been very well received and in this issue we take a look at a topic in the news that certainly is causing great concern, the connection between physics and pandemics. We hope you enjoy the different articles in this issue. This is your newsletter and we really welcome your valuable feedback and suggestions to make Physics News grow in stature and strength. Please note that Physics News is now available online on the IPA webpage. Happy reading! Editorial Board
PHYSICS NEWS is published quarterly and is the official bulletin of Indian Physics Association, IIT Bombay, Mumbai – 400 076 PHYSICS NEWS is mailed free to all members. Copies can be purchased at the rate of ₹.150 per copy. Correspondence regarding subscription and other matter should be addressed to General Secretary, IPA - [email protected]
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From the President’s Desk
This issue highlights the works of one of the Pioneering Physicists of the country, prof. B.V. Sreekantan – who passed away recently. Prof. Sreekantan was intimately connected with IPA and worked as a President of IPA during 1977-79. His contributions to Physics research in India and to the IPA, are numerous and he will be missed by the community
Many members have appreciated on-line publication of Physics News Oct-Dec. 2019 issue. This is expected to widen the reach of Physics News and consequently, IPA both nationally and internationally.
A special seminar “New horizons in Physics” is being planned in Aug. 2020 to commemorate 50 years of IPA, with a goal to acquaint young researchers with exciting developments in various areas of Physics. We hope this occasion will also provide opportunities for improved networking with young physicists.
A.K. Mohanty
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The Scientific Journey of B.V. Sreekantan
P.C. Agrawal Senior Professor (Retd.) Tata Institute of Fundamental Research E-mail: [email protected]
P.C. Agrawal joined TIFR in 1962 and earned PhD from Mumbai University in 1972 on study of X- ray binaries with balloon- borne experiments under the mentor-ship of Prof B.V. Sreekantan. During the last four decades he has contributed significantly to advance understanding of a variety of cosmic X-ray sources with the balloon, rocket and satellite borne experiments. He initiated the proposal for the first Indian Astronomy satellite ASTROSAT and served as its Principal Investigator during 2001- 2011. ASTROSAT, in operation since its launch in 2015, carries three X-ray instruments developed at TIFR, including a major instrument designed by Prof.Agrawal’s team.
Abstract This article recounts the major scientific achievements of Prof B.V. Sreekantan from 1948 to 1992. He made noteworthy contributions to the development of many research programs for the studies of high energy cosmic rays and their interactions. He played a significant role in the initiation of major internationally competitive experiments like the first ever detection of cosmic ray produced neutrinos and proton decay experiment. He initiated an experiment at Ooty to detect Cerenkov light produced by very high energy Gamma-ray showers in the atmosphere, that has later culminated in the setting up of MACE Instrument at Hanle by BARC to study sources of TeV gamma-rays. He was instrumental in starting the balloon and rocket- borne experiments in the then emerging area of X-ray astronomy that resulted in TIFR group developing capability for building complex instruments for satellites and becoming an important center for research in X-ray astronomy. This led to the development and launch of the multiwavelength astronomy satellite ASTROSAT, launched by ISRO in 2015, which carried 3 major X-ray instruments developed by TIFR.
Joining Tata Institute of Fundamental Research During his long and illustrious science career at TIFR, (TIFR) and early Project : Sreekantan made notable contributions to several areas of research using a variety of techniques and particle detectors. Sreekantan was born on June 30, 1925 and pursued his During his entire research career, the focus of his research was education in Mysore and Bengaluru leading to the award of studies of cosmic rays and their interactions. However, when M.Sc. Degree from Mysore University in 1947. He had new areas of research opened up like X-ray Astronomy and developed interest in research and applied for Research TeV Gamma-ray Astronomy, he immediately sensed the studentship to Institutions like Indian Institute of Science, opportunity for making front ranking contributions in these Bengaluru and Tata Institute of Fundamental Research virgin fields and took a plunge in them. He initiated culture of (TIFR), Mumbai, founded by Dr. H.J. Bhabha in 1945. He was development of instruments for use in his experiments and laid selected in Indian Institute of Science for research in a firm foundation for conceiving and executing internationally Communication Engineering. Later he received a call from competitive experiments in the area of cosmic rays and their TIFR to appear for an interview. At TIFR he was examined on interactions. It may be mentioned that for his very first August 6,1948 by Committees chaired by Dr. Bhabha for his experiment to measure the life time of muons, he developed understanding and knowledge of Physics and Mathematics. all the required electronics. The detectors deployed in his After the interviews he was summoned to Dr. Bhabha’s room cosmic ray studies, neutrino detection and proton decay and told that he has been selected. Dr. Bhabha asked him experiments included almost every type of known radiation whether he would like to do theoretical or experimental work detector at that time such as- GM counters, plastic ccintillation in Physics. Sreekantan said that since Dr. Bhabha had counters, Neon flash tubes, proportional counters, cloud interviewed him, he would abide by his advice. Dr. Bhabha chamber, gas Cerenkov counter and water Cerenkov detectors. said that since Sreekantan was familiar with electronics, he When he initiated research in the new field of X-ray would advise to work in experimental physics since very few astronomy at TIFR using balloon and rocket-borne persons had knowledge of electronics at that time in India. instruments, his team developed the Sodium Iodide based X- Moreover he could always come back to theoretical research ray detector with anticoincidence & signal processing but the other way it was not possible. Sreekantan chose electronics and proportional counter based instruments. He experimental physics for his research [1]. had a very unique distinction of performing the experiments
4 Physics News at depths of several kms (in the deepest mines) for studies of Experiments in deep Underground Mines in Kolar cosmic ray muons and neutrinos, to heights of up to a few Gold Field (KGF): hundred kms to reveal the mysteries of X-ray emission from cosmic sources. His research covered particles with a wide (a) Muon intensity measurements and first detection of range of energy from MeV to PeV and photons with energy of cosmic ray produced Atmospheric Neutrinos : ~ keV to tens of TeV. The wide canvas of his research Dr. Bhabha had learnt about the discovery of a new unstable activities spanning experiments with varying degrees of particle in cosmic rays and wanted to check it by measuring complexities in detectors and associated instrumentation the angular distribution of muons as well as their flux at demonstrated his versatility as an Experimental Physicist and different depths in a mine. He thought that if a new particle a deep grasp of the underlying physics. besides the penetrating muon was present in the cosmic rays, In January 1975, Sreekantan succeeded M.G.K. Menon as the anomalies would be visible in the flux and angular Director of TIFR [2]. Even after becoming the Director, distribution. He suggested that the experiment be conducted in Sreekantan continued his deep interest and involvement in the the mine of Kolar Gold Field (KGF) at different depths The research programs initiated by him. He used to attend to his KGF mine was owned by a british company John Taylor & administrative duties in the morning and usually in late Sons, which readily agreed to provide a place in the mine for afternoons/ evenings visit senior colleagues, heading the setting up the experiment and also provided the necessary various research activities of his erstwhile group, to discuss infrastructure e.g. electric power, access to lift in the mine, the progress of the experiments and the science results. He accommodation for the TIFR staff etc. S. Naranan and P.V. thus continued his involvement in research until he moved to Ramanmurthy, who joined TIFR a few years later, joined Benagaluru in 1992, where he settled down [3]. Sreekantan in this experiment. The plan was to measure the intensity of muons at different depths and different angles in the mine using an array of GM counter operating in the coincidence mode, in order to deduce the flux and angular distribution of muons. The flux in the vertical direction was measured by the vertical axis GM counter tray, while at various other angles it was measured by tilting the axis of GM Counter trays. The first experiment was set up at a depth of 10 mwe (meter water equivalent) and then shifted to a depth of 503 mwe and finally to a depth of 1008 mwe. This covered the wide energy range of muona, > 3 GeV at 10 mwe and > 370 GeV at 1008 mwe. After 18 months of efforts by Sreekantan and his colleagues, the experiment became operational on 6 August 1951 and started taking the data. Sreekantan noted that as they moved from the surface level to deeper level in the mine, the coincidence count rate dropped drastically. At 10 mwe the count rate was 9400 counts/hour which reduced to 25.6 count/hour at 503 mwe and further reduced to 2.33 counts / hour at 1008 mwe , the deepest level at which the muon Figure 1: Plot of muon Intensity versus Delay (s) from intensity was measured. The angular distribution showed a 2 experiment of Sreekantan for measuring muon life time [4] Cos θ dependence at shallower depths which changed to Cos4θ dependence at deeper levels. These experiments were His first project in TIFR was to device and set up an completed by 1953 and results from these formed the Ph.D. experiment to measure the lifetime of µ mesons (muons) thesis of Sreekantan (submitted to Mumbai University.) produced by cosmic rays. Earlier two experiments had yielded values of 2.4 ± 0.3 s and 1.5 ± 0.3 s. A more accurate From the muon Intensity vs Depth measurements in the KGF measurement was required to check if there were other types mine, it was inferred that measurements of the muon intensity of unstable particles in cosmic rays. The experiment required at greater depths requires a muon telescope of larger area. A fabrication of Geiger Muller (GM) counters, pulse amplifiers, new muon telescope of 3m2 area using a combination of coincidence circuits etc., with s precision. Sreekantan Scintillation and GM Counters was set up by Ramanamurthy, designed and successfully fabricated the electronic circuits Narasimham and Miyake, a Japanese physicist of Osaka City required for the experiment using valves and other University. The intensity of Muons was measured at five components salvaged from the surplus communication and depths between 270 m (810 mwe) to 2760 m (8400 mwe). The signalling equipment purchased by TIFR from the flea market muon intensity fell drastically with the increasing depth. in Mumbai. The GM counters were made in TIFR by a group Coincidence counts recorded at 800 m were 1029 in 100 hours set up for this purpose. After two years of efforts the muon life which declined to 127 counts in 944 hours and 18 counts in time experiment became operational, ran for a year and 3000 hours at greater depths. At the deepest level of 2760 m measured a value of 2.24 ± 0.15 s for muon lifetime (Fig. 1), no count was registered in 2880 hours implying a count rate -3 which is very close to the present accepted value of 2.197083 of ≤ 3.5 x 10 count per hour. A plot of Muon Intensity as a ± 0.000015 s [4]. function of the depth of mine is shown in Fig. 2. It can be
5 Physics News concluded that no cosmic ray produced muon could reach at PC layers shown in Fig. 3 [8, 9]. The experiment was to look 2.760 km depth implying almost zero background. for events fully confined in the detector volume, which may be identified as due to decay of Proton. This experiment operated for almost 8 years and registered large number of neutrino events. In the second phase of the proton decay experiment, a larger detector with area of 6 m x 6 m made up from 4000 PCs arranged in 60 layers and sandwiched by 340 tons of steel plates was employed. This second experiment ran for 5.5 years and recorded large number of events most of which were due to muons produced by interactions of neutrinos in the rocks. Several candidate events that could not be explained by known processes were also recorded. If these are attributed to the decay of protons, it implies a value of 1.4 x 1031 years for proton lifetime [8]. The TIFR proton decay experiment created lot of excitement in the scientific world as the results had far reaching consequences for particle physics theories and fundamental physics. It also led to several other experiments worldwide in deep mines using detectors of larger volume and mass with high granularity (like water cerenkov detectors). However, no unambiguous event due to proton decay was observed implying a lower limit of > 1033 years for the proton life time. After submitting his Ph.D. thesis, Sreekantan spent about a year at MIT working with B. Rossi and his group. On his way to USA he spent some time visiting a few European laboratories engaged in cosmic ray research. At MIT initially he worked on the analysis of cloud chamber data to understand Figure. 2: Plot of Muon Intensity versus Mine Depth for the production of K meson, its decay products and their KGF mine experiment [5,9] properties. He also worked for some time at the Brook Heaven National Laboratory (BNL) to analyse data obtained by the It occurred to Menon, Ramanamurthy, Sreekantan and Miyake cloud chamber experiment set up by the MIT group at the that this will be an ideal location to detect and study neutrinos accelerator. This resulted in the publication of 3 papers on produced by cosmic ray interactions in the atmosphere [6]. decay products of K meson in collaboration with H.S. Bridge They planned to detect Neutrinos at large zenith angles or and others at MIT. those coming from the opposite side of the earth by looking for events that show muons produced by interactions of neutrinos in the rocks at large zenith angles or almost horizontally. A Neutrino detector was designed and set up at 2.3 km depth in KGF jointly by TIFR, Durham University and Osaka City University groups. The experiment used vertically stacked scintillation counters operating in coincidence with Neon Flash Tubes as track detector and lead or iron absorbers sandwiched between them. The experiment became operational in April 1965 and ran for several years. In this run 18 events were identified as Neutrino events [7]. This was the first detection of neutrinos produced by cosmic rays in the atmosphere. (b) Proton Decay Experiments in the KGF: Some models of Grand Unification Theory (GUT) had Figure 3: Front view of Phase 1 Proton decay detector at 2.3 predicted decay of protons with a life time of 1031 – 1034 years. km depth in KGF. Each square section seen here is of a Menon, Miyake, Narasimhan and Sreekantan thought that the proportional counter giving a grid size of 10 cm x 10 cm. decay of protons should be observable in deep KGF mine due Picture from [9]. to very low background of the charged particles. Motivated by this, the TIFR group in collaboration with the Japanese Study of Extensive Air Showers : scientists from Miyake’s group set up the first proton decay The period of late forties and fifties was a period of great experiment in KGF at a depth of 2.3 km. In the first phase the excitement in physics as several new particles were detector had a sensitive area of 4 m x 6 m and consisted of discovered, which provided understanding of the basic 1600 proportional counters (PCs) arranged in 34 orthogonal structure of matter and its constituents. In 1947, Cecil Powell layers with 140 tons of steel plates sandwiched between the and his colleagues at Bristol University used the nuclear
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emulsion technique to discover charged mesons, earlier detected by scintillation counters at a depth of 270 m and predicted by the Japanese physicist Yukawa. The emulsion muons with energy > 640 GeV were measured by water plates were exposed at high mountain altitude to cosmic rays Cerenkov detectors at a depth of 600 m. which interacted with silver and other nuclei producing secondary particles that left tracks in the emulsion plates. By measuring the tracks, Powell identified mesons from their decay into muon and neutrino. The discovery of so-called V- particles was also announced from the cloud chamber experiment. The V-particles were later identified as K mesons which also decay into and . Bhabha thought that these new particles can be studied with cloud chamber at a mountain site and suggested Ootacmund (Ooty)(altitude ~ 2300 m or 7500 ft) for setting up a cosmic ray laboratory. Bhabha approached Governor of Tamilnadu who readily agreed to provide land and space in the Raj Bhavan complex at Ooty for the new cosmic ray laboratory. (a) Study of energy spectrum and composition of cosmic rays at E > 10 12 eV: Figure 4: A view of the part of Extensive Air Shower array to study properties of high energy cosmic rays of > 1014 eV. After returning from MIT, Sreekantan devoted his efforts to setting up the new laboratory for studying high energy cosmic The EAS arrays at Ooty and KGF operated for many years and rays- the energy spectrum, composition and origin. Cosmic collected data on the shower characteristics. The measured 12 rays of high energies ( > 10 eV ) undergo interactions in the EAS properties were compared with those inferred from atmosphere with nuclei of Nitrogen and Oxygen producing Monte Carlo simulations of showers with different energy ± copious flux of secondary particles like π and K mesons that spectrum parameters and composition. The results suggested ± ± decay into muons (µ ) , electrons (e ) and neutrinos (, ̅ ). that at energies greater than ~8 x 1013 eV, heavy nuclei in o Neutral Pi mesons (π ) are also produced that decay into 2- cosmic rays disappear due to photodisintegration and consist gamma rays, that in turn undergo pair production. The high mainly of galactic origin protons up to ~ 7 x 1015 eV. As energy electrons radiate energetic photons by bremsstrahlung. energy increases, protons of galactic origin leak out causing This process continues and rapidly develops into a massive steepening of the energy spectrum and at higher energy the cascade or shower of particles that consists of a soft cosmic ray protons are mainly of extragalactic origin [11]. component due to electrons, a hard component due to Muons Following closure of the KGF mines the EAS array at KGF and strongly interacting nucleons near the core of the shower. was closed. The EAS array at Ooty was expanded several Bhabha and Heitler had developed a theory of these showers, times and in its present version known as GRAPES 3, consists which are now known as Extensive Air Showers (EAS). Large of 400 scintillation counters, each 1 m2 area, and 560 m2 area 15 EAS produced by very high energy cosmic rays (E > 10 eV) muon detector. Due to its large area it is very sensitive to any produce cascades spread over hundreds of square meters changes in the solar activity thus making it a very good which reach maximum particle density at mountain altitudes monitor of space weather. A very large solar flare occurred on and then decline. Hence, studies of EAS is best done at 3 to 5 June 22, 2015 and GRAPES 3 detected a peak in the muon km high mountain sites. Sreekantan and his EAS research intensity correlated with the geomagnetic field variation team at Ooty (which included M.V.S. Rao, G.T. Murthy, caused by arrival of the coronal mass ejection plasma at the B.K. Chaterjee and K. Sivaprasad) set up an array of plastic earth. scintillation counters along with neutron monitor type hadron detectors and muon detectors. Electronics with fast timing (b) Study of high energy interactions using cosmic rays capability to measure arrival time of the shower front and Study of high energy interactions of nucleons near the core of arrival direction, was an important feature of the EAS array. the shower to understand dependence of multiplicity, energy A photograph of a part of the EAS array is shown in Fig 4. and angular distribution was also an important objective of The EAS array was aimed at measuring the characteristics of EAS such as shower size, core location, arrival direction and cosmic ray studies at Ooty as they were the only means for energy of GeV range Muons, and from these the energy studying interactions at E > tens of GeVs. The accelerators spectrum and composition of the cosmic rays responsible for producing protons of high energy e.g. 30 GeV proton beam at generating the showers were inferred [10]. BNL and at CERN, came into operation only around 1970 or later. Bhabha suggested design and installation of a big size In parallel, an EAS array of scintillation counters was cloud chamber at Ooty. With the close involvement of established in 1965 at KGF, at the surface level to complement the studies at Ooty. The EAS group at KGF included B.S. Miyake, Sreekantan and his colleagues developed a 2 m x Acharya S. Naranan, M.V.S. Rao, K. Sivaprasad, 1.5 m x 1 m cloud chamber, the biggest such chamber in the B.V. Sreekantan and P.R. Vishwanath. Along with the world (Fig. 5), and made it operational in 1965. The TIFR detection of shower and its size by the surface array, high Multi-plate cloud chamber consisted of 21 iron plates that energy muons in the shower with energy > 220 GeV were correspond to ~ 3 interaction mean free paths for nucleons.
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Sreekantan, Ramanamurthy and Subramaniam also designed Contribution of Sreekantan to the Birth and Growth and set up an Air Cerenkov Counter (ACC) above the cloud of TeV Gamma-ray Astronomy in India: chamber for distinguishing the particles, mainly pions and In 1935 Carl Jansky accidentally discovered intense radio protons entering the cloud camber and interacting with the emission from the center of our Galaxy marking the birth of iron nuclei. Likewise a Total Absorption Scintillation Radio Astronomy. After the end of the second world war, a Spectrometer (TASS) was designed and installed below the large number of bright radio sources were discovered and their cloud chamber to measure the total energy of the particle. The positions determined. One of these bright sources was Taurus A which was identified with a nebulosity known as M1 or Crab nebula. This object was earlier identified with the position of a “Guest Star” sighted by the Chinese astronomers in the year 1054 AD which is now known as remnant of a star that exploded. Such objects are known as supernovae remnants (SNRs). It was immediately realized that Tau A is coincident with the SNR Crab nebula. The Russian physicist I.S. Shlovasky had explained in 1953 that radio emission from Tau A is due to synchrotron radiation emitted by high energy electrons spiralling in the magnetic field of the Crab nebula. Detection of polarization of the optical and radio emission confirmed the synchrotron hypothesis. In 1967 Radio Pulsars were discovered and soon afterwards in 1968 radio pulsations with 33 ms period were detected from a source NP0 535+26 in the Crab nebula. This was followed by detection of 33 ms Figure 5: The 2 m x 1.5 m x 1 m size chamber, the biggest in pulsed X-ray emission. The pulsars are explained as rotating the world, operated in coincidence with the EAS array+ACC+ neutron stars in which electrons accelerated to very high TASS, to study electron and nucleon initiated showers seen in energy (1012 – 1015 eV) by very high electric fields generated the picture. in the polar regions of the pulsar, produce synchrotron TASS at Ooty had an area of 1.4 m2 and consisted of 2 radiation. It was argued that protons of very high energy must identical parts each having 25 iron plates and 25 scintillation also be generated by the same process in the Crab and other detectors. Top layers of TASS measured the energy of the soft pulsars. In this picture the protons will undergo p-p collisions component and the remaining part absorbed the hard resulting in the production of π± and π0 mesons. The neutral component to estimate total energy of the incident particle. By meson will immediately decay in to 2 gamma rays with energy 1965 the cloud chamber, the ACC and TASS were fully of GeVs to TeVs, depending on the energy of the parent operational. The EAS array and coincidence between ACC protons. The Crab and other pulsars were thus predicted to be and TASS provided the trigger for the expansion of the cloud sources of very high energy gamma rays. chamber to study the cores of the showers. A picture of ACC The GeV to TeV gamma-rays will undergo pair production placed above the CC and TASS below it is shown in Fig 6. in the earth’s atmosphere and as they travel further they will The time structure of the nuclear component in the cores of develop into air showers of particles, similar to those produced the showers was investigated by S.C. Tonwar and Sreekantan by the cosmic ray protons. The showers of high energy from their interactions in the cloud chamber. Based on the electrons as they travel deeper, will emit a cone of Cerenkov measurements of the arrival time distribution of low energy radiation in the atmosphere. The TeV energy gamma-rays hadrons in the core of the showers of energy 1014 – 1016 eV, from the Crab pulsar and other cosmic sources should be they came to the important conclusion that their results could detectable by studying their Cerenkov light. Prompted by this be explained only if the cross-section for the production of the a Russian group had unsuccessfully tried detection of very nucleon-antinucleon increases considerably with energy [12]. high energy gamma-rays from Crab nebula and other cosmic Another important result was derived by R.H. Vatcha and sources by Atmospheric Cerenkov Technique (ACT). After Sreekantan from measurements of charged to neutral (C/N) the discovery of the Crab pulsar Sreekantan and his group ratio of high energy hadron in the cores of the EAS of 1014 – consisting of B.K. Chaterjee, G.T. Murty, P.V. Ramanmurthy 1016 eV. They compared their values with those obtained from and S.C. Tonwar set up an Atmospheric Cerankov Detector the Monte Carlo simulations and concluded that their results consisting of 8 mirrors of 1.5 m and 10 mirrors of 0.6 m size require copious production of baryons in the high energy with a photomultiplier (PMT) at the focal plane of each, to interactions [13]. Both these important findings were detect the Cerenkov light in coincidence mode. The telescope confirmed later by the accelerator experiments with the 30 could work only on clear moonless nights. The ACT telescope GeV proton beams at CERN and BNL. These results at Ooty ran for several years from which two important results demonstrated the importance of baryon production in very emerged. Pulsed TeV gamma-rays were detected at ~ 5 from high energy ~ 1015 eV hadron-nucleus interactions which the Crab Pulsar in an episodic 15 minute burst [14]. Transient result in production of 20-25 % particles as baryons. After the 88 ms pulsed Gamma-rays at a significance of ~ 4 were also arrival of high energy accelerators producing well defined observed from the bright gamma-ray source Vela pulsar [15]. particle beams, focus of high energy physics shifted from Due to unfavourable weather conditions at Ooty, clear nights cosmic rays to the accelerator beams. were limited and hence in early nineties it was decided to
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move the TeV gamma-ray instrument to Panchmarhi at an gamma-ray sources. A major advantage in setting up the elevation of ~ 1000 m but with more clear nights. The new telescope at Hanle was realization of lower energy threshold. ACT Telescope was made up of a cluster of 256 mirrors The MACE is at present undergoing tests and trial runs. When arranged in 7 orientable units with much larger area than the it is fully operational, MACE will propel India at the forefront one used in the Ooty telescope. During its operation over more of TeV gamma-ray astronomy enabling Indian astronomers to than a decade, besides the Crab nebula and pulsar, it could make new discoveries in this exciting field. Sreekantan’s early detect TeV gamma-rays from two well known Active Galactic experiments in this field sowed the seeds of India reaching to Nuclei (AGNs) namely Mkr 421 and Mkr 501 in regular as this stage in this branch of high energy astronomy. well as flaring states. Sreekantan’s Contributions to X-ray Astronomy: In 1962 R. Giacconi working at American Science & Engineering (AS&E) and B. Rossi at MIT and their team, carried out a rocket flight experiment with a GM counter to detect fluorescent X-rays generated by the solar X-rays shining on the Lunar surface. They failed to detect any lunar X-rays but to their surprise they detected a bright source of X- rays in the southern sky in the Scorpius constellation. This serendipitous discovery of the first extra-solar source now known as Sco X-1, marked the birth of the new field of X-ray astronomy which is now a mature branch of astronomy. Recognizing the importance of this epoch making discovery Giiacconi was awarded 2002 Nobel Prize in Physics. Following the discovery of Sco X-1, a series of rocket experiments carried out in USA and elsewhere resulted in detection of about 50 sources by 1965. Sreekantan arrived at Figure 6 : Front and Side views of Air Cerenkov Counter MIT in 1965 to work with the group of Rossi and found great (above) and Total Absorption Scintillation Spectrometer excitement around and he also joined the X-ray group at MIT (below) the Ooty Cloud Chamber for studying high energy that included Hale Bradt and Gordon Garmire. From estimate interactions of nucleons. of distance of ScoX-1 (a few hundred parsecs), and its 35 intensity, its X-ray luminosity (Lx) was estimated as ~ 10 Following the work of the TIFR group the high energy ergs/sec. There was no known physical process at that time astronomy group at the Bhabha Atomic Research Center that could explain such high Lx and hence the nature of Sco X- (BARC) had also set up a larger area Cerenkov telescope at 1 and other X-ray sources remained a mystery. A rocket Mount Abu and after a run of two decades observed similar experiment to measure precisely the position of Sco X-1 and limitations in sensitivity. ascertain its size was carried by the AS&E and MIT in 1965 In the early years of 2000, the European groups realized that using a modulation collimator, designed by the Japanese order of magnitude improvement in the sensitivity is required physicist M. Oda, with a proportional counter. The data in ACT instruments to detect and study in detail fainter analysis showed the size of Sco X-1 to be < 20´´ ruling out the sources. This was achieved in two ways : (1) the Cerenkov possibility of it being a supernova remnant. The position of telescope mirror area collecting the photons was increased by the source was measured to 10´´ accuracy [16]. The optical an order of magnitude (2) Cerenkov Imaging , which is very studies by the Palomar and Tokyo astronomers revealed a blue th efficient in discriminating between the proton generated 13 magnitude stellar object within the error circle that had Cerenkov showers and gamma-ray produced showers, was characteristics of an old nova-like star [17]. From the optical deployed in the new telescopes. Two major instruments HESS identification it was firmly established that Sco X-1 was truly (High Energy Stereoscopic System) and MAGIC (Major an X-ray star whose X-ray luminosity was ~ 1000 times its Atmospheric Imaging Cerenkov) were built and came into optical luminosity, thus further deepening the mystery of its regular operation between 2005-10. The effective area of each nature. This puzzle was resolved after the launch of the was several hundred square meter. This quantum jump in the UHURU X-ray satellite in 1971 which discovered X-ray sensitivity of the ACT instruments revolutionized the TeV binary pulsars Her X-1 and Cen X-3. The high Lx was Gamma-ray astronomy. In the following decade, all classes of explained as arising due to the accretion of the matter from the gamma-ray sources in the GeV range were detected and binary companion into the deep gravitational potential well of studied. Now about 100 sources of TeV gamma-rays are a neutron star or a black hole producing very high temperature known. plasma radiating X-rays. Steekantan participated in the rocket flight experiments and contributed to the understanding of the Buoyed by the success of HESS and Magic, BARC decided nature of Sco X-1. George Clark at MIT observed the Crab to set up, with the involvement of TIFR, MACE (Major nebula in a balloon experiment and detected X-rays in 15-60 Atmospheric Cerenkov Experiment), a large collection area keV band, while K. G. McCracken- an Australian scientist, (21 m size, 356 m2 area) fully steerable Cerenkov telescope reported detection of hard X-rays (20-58 keV) from Cyg X-1, based on imaging Cerekov technique at Hanle in Ladakh in another balloon experiment. Sreeekantan immediately region (altitude 4270 m or ~14000 ft) for studies of the TeV recognized the potential of balloon experiments in discovering
9 Physics News new phenomenon in the hard X-rays, a region which was seventies and later, a series of X-ray satellites having more largely unexplored. sensitive instruments, were launched by NASA, ESA and Japan which could study large number of sources for long After his return to TIFR in 1967, he realized that TIFR intervals. This completely transformed X-ray astronomy designed and fabricated plastic balloons of up to 2 million which grew at a rapid pace to be recognized as a mature branch cubic feet volume that could attain ~ 38 km altitude with about of astronomy. This resulted in a rapid decline in the number 100 kg payload. The X-rays of > 20 keV could penetrate to of balloon and rocket experiments as they were no more this altitude facilitating observation of sources in the balloon viable. Unfortunately until 1993, when the first successful experiments. He also noted that the balloon flights are flight of Polar Satellite Launch Vehicle (PSLV) occurred, conducted from Hyderabad located at geomagnetic latitude of India could launch a satellite of 120-150 kg mass with a 8o N implying a vertical cut-off energy of 16 GeV for protons payload of only 10-20 kg. Hence, Indian astronomers missed resulting in a very low cosmic ray produced X-ray the exciting phase of explosive growth of X-ray astronomy. background. Sreekantan discussed his plan of initiating balloon borne X-ray astronomy with Prof G.S. Gokhale, Head High Altitude Studies (HAS) section, who whole heartedly supported it. He also recognized that members of the HAS Section engaged in cosmic ray studies, are well versed in the development and use of a variety of radiation detectors and associated electronics. He then came to us and described his plan for balloon-borne X-ray astronomy experiments. Excited by the possibility of research in a virgin area, I offered to work on the new program. R.K. Manchanda, P.K. Kunte and later V.S. Iyengar who moved from PRL to TIFR, also joined the new research team. Working at a frantic pace, a 3 mm thick Sodium Iodide (Tl) X-ray detector of ~ 100 cm2 area with a plastic scintillator anticoincidence and associated signal processing electronics, were developed in about a year. The detector and electronics were mounted on an orientable platform to point at a source and track it. The X-ray instrument ready for launch, is shown in Fig 7. Since TIFR made balloons using only black plastic film, they could be used only in the day time. Sreekantan obtained approval of the then Director Prof. Menon to import two balloons for launches in the night. The first successful balloon experiment was carried out in April 1968 followed by the second experiment in December 1968 to study hard X-ray emission from Sco X-1 using the X- ray instrument shown in Fig 7. The experiment of 22nd December 1968 was very interesting as in the 90 minutes of observation of Sco X-1 its intensity varied significantly and this finding was published in Nature [18]. Detection of a hard component above 40 keV from Sco X-1, probably of non- thermal origin, was another important result that has subsequently been confirmed from other studies. Another Figure 7: Photograph of the first X-ray Astronomy Instrument balloon experiment on 6th April 1971, targeted Cyg X-1, the flown in Balloon experiments for studies of Hard X-rays from brightest X-ray source above 20 keV. The Cyg X-1 was X-ray sources during 1968-75 generating a lot of excitement as its optical companion was Sreekantan also embarked with S. Naranan and V.S. Iyengar identified as a 5.6 day period binary. on rocket experiments to study low energy X-rays from Sco From estimate of the mass of the blue supergiant binary X-1, Crab nebula etc. using spinning Centaur rockets in 1973- companion and parameters of the binary, mass of the X-ray 75. Some interesting results emerged from these experiments. emitting star was inferred to be ≥ 7 solar mass, implying that After my return from Caltech to TIFR in 1975, I undertook the it is a black hole. The Cyg X-1 was thus the first credible development of thin window soft X-ray Proportional candidate for a black hole. The TIFR experiment detected Counters, similar to those in the HEAO A-2 experiment, with rapid variations in the intensity of Cyg X-1 over time periods the deep involvement of K.P. Singh-for whom this was the of many minutes as well as recorded occurrence of an X-ray thesis project. The aim was to carry out a survey of Diffuse flare in it. This fitted well in the model of Cyg X-1 being a Soft X-ray Background in 0.1- 0,18 keV, 0.1-0.28 keV and 0.5 black hole and attracted international attention. These results – 3.0 keV bands to map its spatial and spectral structure. A appeared in papers in Nature [19] and Nature Physical rocket experiment with the low energy X-ray detector Sciences. Encouraged by the early successes, Sreekantan and payload, was successfully conducted in 1979 using RH - 560 his group continued balloon experiments till mid-seventies rocket of ISRO. Results from this experiment appeared in a and reported results on Her X-1, Aql X-1 etc. [20]. By early paper [21] and constituted the Ph.D. thesis of K.P.Singh.
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During 1975-87, when Sreekantan was serving as the Director, 6. M.G.K. Menon, P.V. RamanaMurthy, he encouraged me to develop detectors for a satellite payload B.V. Sreekantan and S. Miyake, Physics letters 5, 272 so that if an opportunity becomes available, we are not caught (1963) unprepared. I prepared a proposal for a satellite experiment for 7. C.V. Achar, M.G.K. Menon, V.S. Narasimham et al., a 50 kg instrument for the timing and spectral studies of Physics Letters 18, 196 (1965) Binary X-ray Stars. Sreekantan forwarded this proposal to the 8. M.R. Krishnaswamy, M.G.K. Menon, N.K. Mondal et ISRO Chairman who assured that as and when the launch al., Pramana 19, 525 (1982) becomes possible, it will be seriously considered. The detectors and electronics were designed and tested. After the 9. V.S. Narasimham, Proc. Indian National Science successful launch of PSLV in 1993, ISRO offered to include Academy 70A, 11 (2004) the X-ray payload, to be jointly made by TIFR and ISRO 10. H. Tanaka, S. R. Dugad, S. K. Gupta, et al., Journal of Satellite Center, for inclusion as a co-passenger on the top Phys. G: Nucl. Part. Phys. 39, 026201 (2012) deck of the IRS P-3 satellite. The condition was that the X-ray 11. S. Kawakami, Y. Aikawa, S.K. Gupta et al., payload should be delivered in 18 months for the integration Proceedings of the 25th International Cosmic Ray with the satellite for launch in March 1996. Thanks to the Conference 1997 (Transvaal, South Africa: preparatory work on the design of the payload, it could be Potchefstroom University), Edited by M. S. Potgieter, made ready and launched on 21st March 1996. During its 5 C. Raubenheimer, and D. J. Van der Walt, Vol. 4, 5 year life this instrument known as Indian X-ray Astronomy (1997) Experiment (IXAE) was used to study about 20 binary sources 12. S.C. Tonwar and B.V. Sreekantan, Journal of Physics and produced several new results. The most notable result was A: General Physics, Volume 4, 868 (1971) detection of quasi-periodic bursts every ~ 50 sec in the black 13. R.H. Vatcha and B.V. Sreekantan, Journal of Physics hole binary GRS 1915+105 interpreted as arising from the A 6,1050 (1973) disappearance of hot matter behind the event horizon of the 14. P.N. Bhat, P.V. Ramanamurthy, B.V. Sreekantan, Black Hole [22]. Success of IXAE prompted me to propose a Multiwavelength Astronomy Satellite ASTROSAT with 5 and P.R. Vishwanath, Nature 319, 127 (1986) coaligned instruments for simultaneous observations in 15. P.N. Bhat, S.K. Gupta, P.V. RamanaMurthy, visible, near UV, far UV, soft and hard X-ray spectral regions. B.V. Sreekantan et al., Astronomy and Astrophysics, Thanks to the constant encouragement and support from 178, 242 (1987) Sreekantan, 3 major X-ray instruments were designed and 16. H. Gursky, R. Giacconi, P. Gorenstein et al., fabricated in TIFR. The ASTROSAT weighing 1500 kg, of Astrophys. J. 146, 310 (1966) which about 800 kg is the instrument mass, was placed in a 60 17. A. Sandage, P. Osmer, R. Giacconi et al., Astrophys. inclined orbit on September 2015 by a PSLV launch. Since its launch more than 4 years ago, ASTROSAT has been working Journal 146, 316 (1966) and has made observations of about 600 sources so far. 18. P.C. Agrawal, S. Biswas, V.S. Iyengar et al., Nature ASTROSAT marks the maturity of the sapling that Sreekantan 224, 51 (1969) planted in 1968 and which has come of age and yielding 19. P.C. Agrawal, G.S. Gokhale, V.S. Iyengar et al., fruitful science results [23]. Nature 232, 38 (1971) References 20. R.K. Manchanda, V.S. Iyengar, N. Durgaprasad, et al., Nature Physical Science 244, 59 (1973) 1. B.V. Sreekantan, Resonance: Journal of Science 21. K.P. Singh, P.C. Agrawal, R.K. Manchanda et al., Education 16, 599 (2011) Astronomy & Astrophys.117, 319 (1983) 2. P.C. Agrawal, Current Science 108, 1731 (2015) 3. P. Vishwanath, B.V.Sreekantan - A Scientific 22. B. Paul, P.C. Agrawal, A.R. Rao et al., Astrophys. J. Biography (Gandhi Centre of Science and Human Lett. 492, L63 (1997) Values, Bhartividya Bhavan, Bengaluru, 2019) 23. P.C. Agrawal, J. Astrophys. Astronomy, 38, 27A 4. B.V. Sreekantan, Proc. Indian Academy of Sciences (2017) Section A 36, 97 (1952) {Image credit: Fig.s 1 & 2 –Indian Academy of Sciences, 5. B.V. Sreekantan, S. Naranan and Fig.3- Indian National Science Academy, Fig.s 4-7 & B.V.S. P.V. Ramanamurthy, Proc. Indian Academy of image –TIFR archives.} Sciences 43, 113 (1956)
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Sir John Cockcroft's visit to Tata Institute of Fundamental Research (TIFR)
Foundation Stone Laying Ceremony of the Pelletron
Building (a joint BARC-TIFR accelerator facility) at TIFR
Fred Hoyle and B. V. Sreekantan
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Release of the book “Collected scientific work of Homi Bhabha”
Inaugural function of the joint MSc. Physics Programme of University of Poona and TIFR, organised at Department of Physics, University of Poona (1985)
S. Chandrasekhar’s visit to TIFR
Image credits: TIFR archives
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Prof. B.V. Sreekantan - A Humane Visionary
Palahalli R. Vishwanath Professor (Retd.) Tata Institute of Fundamental Research E-mail: [email protected] Palahalli R Vishwanath was a professor in Tata Institute of fundamental Research (TIFR), Mumbai till 2002 and later a visiting professor in the Indian Institute of Astrophysics (IIA), Bangalore. He worked in cosmic ray physics, high energy physics and gamma-ray astronomy. He was last involved in setting up a gamma ray observatory in Ladakh in the high Himalayas. During the last decade and a half, he has been a science communicator in both English and Kannada. He has written many popular articles and books in Kannada on Astronomy and Physics. He interacts with school students and community, and has taken part in many popular TV programs dealing with science and scientific temper.
Abstract Prof. B.V. Sreekantan served as Director of TIFR from 1975 to 1987. He initiated several new programs and facilities at TIFR, therby contributing to allround growth of TIFR. He is remembered by the TIFR faculty and staff for his caring nature, pioneering scientific contibutions and great scientific leadership. This article highlights his role as a science administrator.
As Homi Jehangir Bhabha started to lead various scientific 30 years old- still holding on to its youth. Sri J.R.D. Tata, the activities in India in the 1940s, he tried to bring about a new Chairman of the Governing Council at that time, took administrative culture for the enterprises under him. He was enormous interest in the affairs of the institute. He believed in very clear that the institute had to be led by scientists [1] giving independence and had told Sreekantan to bring only "...The type of administration required for the growth of S & major issues to the Council. Sreekantan also benefited from T is quite different from the type of administration required for his closeness to both Prof. Menon and Dr. Raja Ramanna who the operation of industrial enterprises, and for administration yielded considerable influence in the government. This was of justice, finance and so on. It is an even more subtle matter very helpful to him in getting many projects going, as he put and must necessarily be done, as in the technologically it later " Of course they were very meritorious proposals but advanced countries, by scientists and technologists... " the huge jump in magnitude of a small institution does not Prof. Sreekantan was one of the earliest members to have happen just like that" joined TIFR when it was shifted from Bangalore to Bombay The Humane Administrator (the present Mumbai). He took up experimental research after the suggestion of Bhabha and was asked to find the lifetime of During Sreekantan’s directorship, he initiated several muons, an important problem in fundamental research at that important and far-reaching measures for the welfare of staff time. His very first experiment was lauded for the ingenuity in that amply demonstrated his compassionate and caring nature setting up a state of the art experiment in a resource starved and which earned him the love and admiration of TIFR staff. country. After this, he headed a series of experiments at KGF These included: (a) making contributory health service which culminated in the detection of the first ever atmospheric scheme of BARC accessible for TIFR staff and their families, neutrinos which brought international recognition to the (b) a pension scheme for retired TIFR staff, (c) constructing Indian efforts. Apart from starting various activities in the staff quarters at the BARC owned land at Mankhurd, field of High Energy Physics and Extensive Air Showers, (d) subsidized housing loan to employees, and (e) career Sreekantan also had the vision to start X-ray and Gamma Ray advancement measures especially for lower salaried staff like astronomy programs in the country. Thus Prof. Sreekantan watchmen, drivers as well as other support and auxiliary staff. headed various experimental programs and had enormous Both Prof. S.M.Chitre, a distinguished astrophysicist and a expertise in leading big groups. This was an advantage when colleague of Sreekantan and Prof. N.K.Mondal, a well known he, as the director, had to look into the much larger interests high energy physicist and a student of Sreekantan, have of the whole institute. Prof. B.V.Sreekantan became the third remarked on how Sreekantan would go round the campus rd director of the TIFR on 3 Jan 1975 and managed the affairs (housing complex) and the institute in the morning and look of TIFR till 1987 [2]. into situations where there could be problems. Sreekantan Taking over the mantles of an institution headed in the past tried to look into various problems personally and met by a person like Homi Bhabha must not have been easy. In individuals when needed. Sreekantan remembers that he had 1975 when Sreekantan took over, the institute was just about good relationship with the faculty and the members of various
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administration units. Prof. M.S. Raghunathan, a well-known (vi) SAMEER : The Microwave department was taken out of mathematician remarked that Sreekantan had a high regard for TIFR and was set up as a separate centre. Mr. R.V.S. Sitharam, the School of Mathematics and he had a cordial relationship who headed it in TIFR, also developed SAMEER [Society for with all the successive deans of Mathematics during his time. Applied Microwave Electronics Engineering and Research], Sreekantan also had good relationship with the employee's the new institution. association. Another example of Sreekantan's qualities as a visionary is The Visionary that he was a mentor for other institutions also. Sreekantan was a member and later the chairman of the Governing Sreekantan was responsible for several major initiatives as the Council of the Indian Institute of Astrophysics (IIA) for director: almost two decades starting from late 1980s. A major (i) RADIO ASTRONOMY: Prof. Govind Swarup, well achievement of IIA during his chairmanship was the known as the father of radio astronomy in the country, establishment of the two meter Himalayan Chandra Telescope remarked "...Soon after becoming the Director of TIFR, (HCT) at Hanle in eastern Ladakh at an altitude of about 4300 Prof. Sreekantan approved the construction of the Ooty masl. As in TIFR, he was always accessible to IIA staff and Synthesis Radio Telescope for mapping the angular listened to their problems. His openness and sympathetic distributions of radio galaxies. OSRT completed in early1984 approach to human issues won him affection and respect of all led to many discoveries... In May 1983, during his visit to in IIA. Ooty, Professor Sreekantan told me over a dinner to conceive a ‘truly innovative radio telescope’....In January, 1984, At the end of this article, it would be worthwhile to mention I...proposed construction of the Giant Metrewave Radio tributes from several of his colleagues and admirers. Telescope (GMRT), as a synthesis array of 25 km extant... A Prof. M.G.K. Menon, the director of TIFR just before detailed proposal for the GMRT was written in April Sreekantan, had this to say: While keeping the culture and 1984..and it became operational by 2000.. The growth of the working environment originally created by Homi Bhabha, Radio Astronomy Group at TIFR owes to the valuable Sreekantan has continued to transform the scientific encouragement and guidance given by Dr. Homi Bhabha, program..and he has continued to be the person that his Prof. M.G.K. Menon and Prof. B.V. Sreekantan " friends have always admired - a perfect gentleman, (ii) MOLECULAR BIOLOGY : Prof. Obaid Siddiqi wanted characterized by humility, modesty and a sense of objectivity to start another biology department outside Mumbai. While and fair play " there was some opposition to this in the department in Prof. S.K. Chitre was all praise for the many initiatives taken Mumbai, JRD Tata was very enthusiastic and Sreekantan by Sreekantan when he was the director: "Sreekantan would supported it. Bangalore was considered but the government always lend his ears for any new programs and encourage offered space in some smaller towns which however were not them if found interesting. There were so many activities in the suitable or international level laboratories like the ones institute when Sreekantan left. He has done more than his conceived. Later it was found that the University of share for experimental science in the country." Agricultural Sciences (UAS) had lot of space and that is how As Prof.Raghunathan reminisced "..The Mathematics Faculty National Centre for Biological Sciences (NCBS) came up in was very happy with him and perhaps his tenure was the Bangalore. period when the school was most comfortable with the (iii) PELLETRON : Prof. S.S. Kapoor, a well-known Nuclear director.... There was none of the aloofness or condescension Physicist remarked "It was with his support and that one encountered with some of the other directors." encouragement that a world class nuclear physics facility, the Dr N. Krishnan, a young physicist at the time Sreekantan was medium energy heavy ion accelerator facility consisting of the the director, says “..Sreekantan's unique situation is that he is pelletron and superconducting LINAC accelerator was probably the best loved director the institute has known". established in TIFR as a TIFR –BARC collaborative project According to Dr.S.S. Kapoor, " Professor Sreekantan will be and it was due to his vision and foresight that this facility is remembered not only for his personal pioneering scientific now on the world map of the nuclear physics facilities. contributions but also for his great motivating scientific (iv) HBSCE: Prof. B.M.Udgaonkar, a well known theorist, leadership." was very passionate about science education from the References beginning and was instrumental in starting the Pune University-TIFR program which ran for several years. 1. Homi Jehangir Bhabha – C. Deshmukh, National Book Trust, Sreekantan was very supportive of this joint venture. Later, he New Delhi 110070 (2003) also backed the initiative by Prof. Udgaonkar and 2. Dr. B.V. Sreekantan, a pioneer physicist – Palahalli Prof. V.G. Kulkarni regarding the Homi Bhabha Centre for Vishwanath, Gandhi Centre of Science and Human Values, Science education (HBSCE) Bhartiya Vidya Bhavan, Bengaluru 560001 (2019)
(v) National Facility for High-Field NMR : It is a premier facility in India and was set up in 1983 with support from various government departments. It houses state of the art NMR spectrometers and other related facilities.
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How to ‘see’ the invisible at collider?
Rohini Godbole Centre for High Energy Physics, Indian Institute of Science, Bangalore – 560012, India E-mail: [email protected]
Rohini Godbole is a Professor at the Centre for High Energy Physics at the Indian Institute of Science. She got her PhD from Stony Brook University, USA. She has been working for the last four decades on theoretical Elementary Particle Physics, specialising in different aspects of particle phenomenology. She has authored more than 250 research papers and several graduate level text books. She is a fellow of the Indian Science Academies and the World Academy of Sciences. She was awarded Padma Shri by Govt. of India in 2019. She was awarded R.D. Birla award of IPA in 2018 She has co-edited a book 'Lilavati's Daughters: Women Scientists of India', and 'A Girls' Guide to a Life in Science’. She has worked for over two decades in many arenas to increase, both, the proportion of women’s participation in science and its efficacy.
Abstract First I describe, very briefly, the current state of affairs in particle physics. This is done with a view to put in context some of my work related to proposals to study decay of the Higgs boson into final states that leave no evidence in the detector, the so called 'invisible decay', at the Large Hadron Collider (LHC) as well as the next generation electron-positron colliders that are currently under planning. This discussion will be done in a model independent way. Such a decay, if observed, will be a sure signal of Beyond the Standard Model (BSM) physics. Hence I will also discuss the framework of a specific model for BSM, viz. Supersymmetry, wherein the strength of the invisible decay is related to the amount of Dark Matter (DM) that is observed in the universe and also to the expected cross-section in the experiments that have been constructed to detect the relic DM in the universe. These two studies then can be used together to evaluate the region of the parameter space of SUSY models that can be probed at the LHC in its current and future runs.
Introduction1 SM, the so called BSM, is very limited too. The plot in the Fig. 1 (bottom) shows a recent compilation in which we see With the discovery of the spin-zero Higgs-like boson H in that the couplings of the Higgs to various SM particles relative 2012 [1] at the CERN LHC, the entire particle content of the to the predication of SM, extracted from the measured rates Standard Model (SM), the quantum gauge field theoretical are consistent with 1, the expectation for the SM. description of the fundamental particles and interactions This discovery of the Higgs was a very important step as it among them, based on a gauge group SU(3)C x SU(2)L x closed an important chapter in the story of the SM, confirming U(1)Y, has been experimentally observed. All measurements of the properties of H, to date, are compatible with H being the the mass generation mechanism. However, this is certainly not SM Higgs boson [2]. This discovery was the last element the last chapter. The SM framework is indeed capable of needed to confirm to us the correctness of our currently addressing issues of cosmology and the early universe. accepted description of ElectroWeak (EW) interactions as a However, there still remain a lot of observations such 1) as the Dark Matter or 2) observation of the Matter-Antimatter gauge theory [3] based on the gauge group SU(2)L x U(1)Y where the EW symmetry is broken spontaneously using the asymmetry in the Universe or 3) the closeness of MH to MW, Higgs mechanism [4]. MZ etc. The SM provides only qualitative explanations such as in the case of Matter-Antimatter asymmetry or none at all The mt - MW plot shown in Fig. 1 shows that the observed mass such as in the case of DM. Much before the 'direct' observation of the Higgs boson verifies the correctness of the SM at the of the Higgs boson, the good overlap between the grey and loop level. Since the fits use predictions of the SM including green areas in the top panel of Fig. 1, for a finite mass of the radiative corrections, the large overlap of the green regions Higgs, had already convinced the particle physicists about the with the blue regions not only tells us that the SM works basic consistency of the SM. Hence the community had been extremely well at loop level, but also that the space allowed in thinking for the past few decades about physics (particles and this plot for physics (particles and/or interactions) outside the interactions) possibilities beyond the SM, most of them on
1 This article is based on the talk given at R.D. Birla Award - 2018 presentation.
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aesthetic grounds but which also had ideas/constructs built in which were capable of answering these big questions of our field. This is indicated in Fig. 2 taken from [6]. Supersymmetry [7] has been the favoured option for the BSM among all these big ideas, because this theory is a natural extension of the symmetry ideas which have formed the basis of the 20th century fundamental physics and has within itself enough theoretical structures to 'naturally' provide solutions to all of the big problems mentioned above. Unfortunately in the seven and a half year since the discovery of the Higgs boson at the LHC, the experiments have confirmed the predictions of the SM in different sectors with high accuracy but have not yet found any evidence for any of the big ideas listed in the Fig. 2. This has led to a time of great introspection in the field of particle physics and the community is pondering over the next steps that the field should take. Since the paths indicated by symmetry/aesthetic considerations have not yielded any fruit, the consensus is that the community should be guided more than ever by experiments. Understanding the DM is on the top of the list of observational puzzles that lack solutions in the framework of the SM. I want to describe some of my work in this context. In the next section I will mention a few facts about the DM and also point out one way suggested by us to look for the evidence for this DM candidate, 'invisible' in the detector, by a study of the Higgs physics. I will then summarise results of our explorations of how a particular BSM model (viz. SUSY) can be investigated in complementary ways at the LHC through the 'invisible' decays of the Higgs or otherwise, as well as in experiments set up for direct detection (DD) of the DM. I will then conclude.
Figure 1: The top panel in the figure shows regions of the MW and mt plane allowed by direct measurements along with the region allowed by fits to the EW precision data from the LEP, Figure 2: Big questions in particle physics and score card of obtained without using the experimental measurements on the big ideas to answer them [6]. M ; m , M (grey shaded region) and the one where W t H DM and Invisible decay of the Higgs Boson experimental information on MH is used (blue region). The light and dark region correspond the regions enclosed by 68% Existence of DM is now confirmed through a series of and 95% c.l. contours. This plot is taken from cosmological and astrophysical measurements [8]. One of the http://gfitter.desy.de/Standard_Model/ The bottom panel first evidence for DM in the Universe came from observed shows a recent compilation [5] of the Higgs couplings relative rotational speeds of galaxies away from the centre, higher than to their SM values and also upper limits on branching ratio of expected from the luminous mass known to be present in the the Higgs into invisible, BSM or undetermined modes, galaxy. These indicated existence of matter which would extracted from measurements of decays into various final provide additional gravitational force to sustain the observed states. rotational speeds. Fig. 3 shows one of the astronomical
17 Physics News evidences of the DM and also the summary of information have specific prediction for DM relic energy density. The available from the CMB measurements by PLANCK [11] as controlling factor is the size of annihilation cross-section and presented in [10]. DM plays an important role in structure the DM particle mass. It can be shown that for DM particles formation and hence CMB data is able to provide us with Electro- Weak strength cross-section and with ~O (GeV) information about it. masses, can give rise to relic density of the right order of magnitude. In fact it can be shown that 's, where the In the early stages of the evolution (and expansion) of the annihilation indicated in the top panel of Fig. 4 is provided universe, when all the particles masses are negligible with through an exchange of a Z boson, can only provide 0.5% of respect to the temperature, the number of different particle the total energy density of the universe. Hence the SM does species are in equilibrium with the photon bath. Fig. 4 shows not contain a good DM candidate. Thus an understanding of two possible 2→2 interaction diagrams that are allowed with the observed DM density thus necessarily requires physics the DM particle (the electrically neutral particle which is beyond the SM. stable on cosmological scale) and X a SM particle, which is essentially massless and in equilibrium with the photon bath. Invisibly decaying Higgs boson, and the LHC When the interaction →XX is in equilibrium, the DM Let me discuss a particular case where the mass and the particles are constantly being replenished. As the Universe expands, though, it becomes increasingly harder for a DM interaction of are such that the annihilation process where particle to find a partner to annihilate with and the forward the mediator in the bottom panel of Fig. 4 is a Higgs boson, reaction shuts off. At this point, the DM number density can play an important part in giving rise to the right relic remains frozen in time. density of the DM particles . This also implies that the if M < MH/2 then the Higgs boson can also decay into a pair of . First question one may ask then is whether the experimental data obtained on the Higgs boson production and decay can be used to constrain the occurrence of such 'invisible' decays for the observed Higgs. Existence of such an additional decay mode will alter the total expected width of the Higgs and hence decrease the branching ratios of the Higgs into the SM particles. The latter have been measured accurately as indicated in the bottom panel of Fig. 1. Indeed a simultaneous fit to all the observed signals and the couplings of the Higgs with all the SM particles, while allowing for an invisible decay mode, restricts the branching ratio in to the 'invisible' channel to about 30% as seen in the bottom panel of Fig. 1.
Figure 3: The Top panel shows rotation curve [9] of spiral galaxy Messier 33 (yellow and blue points with error bars), and a predicted one from distribution of the visible matter (gray line). The discrepancy between the two curves can be accounted for by adding a dark matter halo surrounding the galaxy. The bottom panel shows the determination of the energy budget of the Universe, taken from [10], obtained using the data on Cosmic Microwave Background (CMB) measurements by PLANCK [11]. Figure 4: The top panel indicates representative DM The relic density of a particular type of DM particle is thus annihilation and scattering processes. The bottom panel determined by its mass and the strength of its interaction with indicates annihilation of a pair of DM particles through the various SM particles. Thus a specific BSM model will then exchange of a mediator.
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However, this is only a roundabout way of looking for these both, the prediction and measurement, of the SM backgrounds 'invisible' decays of the Higgs. Since the particles do not at the LHC. Thus one can just analyse dilepton + missing leave a signal in the detector, it will be impossible to look for transverse momentum events and look for excess over the SM the signal of a Higgs boson, unless it is produced in association prediction. Absence or presence of an excess over the SM with something else, where the something else leaves tracks prediction in the spectrum of Fig. 6 can then be converted into in the detector. This was the philosophy of our study proposed information on the product of the HZ production cross-section in [12] wherein we looked at production of an invisibly times the branching ratio of the Higgs into this invisible decaying Higgs boson in association with a vector boson, as channel. We had estimated the reach of a LHC run with beam indicated in the top panel of Fig. 5. The Higgs boson decays energy of 7000 GeV, for this product and then on the invisible invisibly and leave no tracks, but the Z does decay into a l+l- branching ratio assuming the SM value of the V H production pair which do. Since the two colliding protons at the LHC are cross section, the assumption being well justified moving in directions opposite to each other, the component of theoretically. the initial state momentum transverse to the beam direction, the so called transverse momentum, is zero. Conservation of Around the same time feasibility of using production of a three momentum then tells us that the transverse momentum Higgs boson along with two forward-backward jets, via vector of the leptons produced in the decay of the Z will in fact seem boson fusion process indicated in the bottom panel of Fig. 5, to have a nonzero value as the Higgs decay products are not to look for the invisible Higgs was investigated [13]. In this seen in the detector. The event where a H is produced in case the characteristic kinematic properties of the two jets are association with Z, with H decaying invisibly and Z decaying useful but the SM backgrounds are a little larger than our case. into two leptons, will be characterised by a missing transverse Table 1: Estimated reach for the invisible branching ratio of momentum and energetic lepton pair coming from the Z the Higgs at the LHC with different energies and luminosities decay. One can then use this feature to separate the signal from [14] the background. We simulated the signal as well as various Process 8 TeV 14 TeV 14 TeV 20 /fb 30 /fb 100 /fb VBH 0.34 0.32 0.17 Z (→l+l-) h 0.58 0.32 0.18 Z(b푏̅)h(substructure) - - 0.5 Z(b푏̅)h(b-jet cluster) - - 0.55
In a later publication [14] we estimated the LHC reach for the full beam energy, using both the above methods and also improved cuts for the background. Here we already used knowledge about the observed signal. In the Table I estimated reach for the LHC with different energies and luminosities are summarised. The current results from ATLAS and CMS have
Figure 5: The Top panel indicates the processes leading to production of a Higgs boson in association with a vector boson V = W/Z. The Bottom panel indicates production of the
Higgs boson in the vector boson fusion channel. background processes and investigated different kinematic Figure 6: Comparison of the expected signal (dashed variables. Fig. 6 shows comparison of calculated signal and histogram), assuming 100% branching ratio for the H into the dominant background, after all the cuts which got rid of invisible decay products with the dominant ZZ background most of the backgrounds. Even then the signal is overwhelmed (solid histogram) where one Z decays invisibly into a ̅ pair by the background. However, high accuracy is possible for [12].
19 Physics News reported an upper limit on the invisible branching ratio of the our exploration [15] of theoretically predicted values of the Higgs of 25% and 36% respectively [15]. Unfortunately, even invisible branching ratio of the Higgs, the expected relic though the methods we suggested have been found useful and density as well as expected values of the DD cross-sections, in in fact used at the LHC for directly searching for an invisibly the framework of the Phenomenological Supersymmetric decaying Higgs, nature has not obliged us with a positive Standard Model (PMSSM). Due to certain symmetries that the signal for such a decay mode. SUSY theories possess, the lightest supersymmetric particle ̃ 0 ≅ is a perfect candidate for the DM particle, with EW Complementary ways of probing the invisible Higgs couplings with the SM particles as well as the Higgs. The mass using colliders and DD experiments and the couplings of the are determined completely by the PMSSM model parameters. The absence of any signal for production of SUSY particles at the LHC, means that some regions of this parameter space are already ruled out. The plot in the top panel of Fig. 7 shows the distributions of all the LHC allowed points in the parameter space with respect to invisible branching ratio and the reach of the various current and future DD experiments. We have restricted the scan to a region where M < MH/2. Further we ensure that the theoretically computed relic density is ≤ 0.1220 as given by the PLANCK [17] measurement. The cross-section for the observed Higgs boson into various final states also constrains the parameter space further. Only the blue coloured regions are still allowed by all the current constraints and will be probed by future DD experiments. The horizontal lines indicate capability of the LHC and future electron-positron linear colliders, for the invisible branching ratio of the Higgs. Thus we see that the regions of SUSY parameter space, where the DM particle has mass smaller than 65 GeV, will be completely probed by these different experiments. However, the cosmological considerations used while computing the relic density assume that the DM was in thermal equilibrium with radiation. If the cosmology is non-canonical and the DM is non-thermal, then as one can see from the plot in the bottom panel, a light DM candidate will be still allowed even after the future DD experiments have taken place. In this case the couplings will be such that at low values of the DM masses, the currently planned DD experiments will have no sensitivity. Thus in this case the invisible branching ratio of the Higgs might be the only probe for a light DM. Conclusions Observation of the Higgs boson has proved the correctness of the SM. However, need for BSM physics is indicated by a number of observational puzzles which have no explanation in the SM. Understanding the character of the DM in the universe is a problem in need of solution. Searches for the 'invisible' decay of the Higgs at the LHC and at the future e+e- Figure 7: The top panel shows the distribution of the PMSSM colliders and the experiments looking for the DM via its direct parameter space, with mass of the thermal DM particle, M̃ 0 detection are complementary ways of looking for the DM. In < 65 GeV, where the LHC constraints on Sparticle searches the framework of the PMSSM the parameter space where are implemented such that the relic is at most equal to the M < MH/2 will be completely probed by these experiments PLANCK value, with respect to the invisible branching ratio for a thermal DM. However, such is not the case for of the Higgs and the DD detections cross-sections. Both the nonstandard cosmology and hence non thermal DM. Even in current and future experimental reaches are shown. The this situation the invisible decay of the Higgs bosons offers bottom panel shows the expected scaled DD cross-sections as possible experimental probes of this parameter space of the a function of the DM mass along with the invisible branching PMSSM. ratio of the Higgs on the right axis. Both the plots are from [15]. Acknowledgement The discussion in the above section was almost model This work is supported by the Department of Science and independent. In this part I will give a flavour of the results of Technology, India under Grant No. SR/S2/JCB-64/2007.
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References 8. G. Bertone and D. Hooper, Rev. Mod. Phys. 90, no. 4, 045002 (2018), [arXiv:1605.04909 [astro- ph.CO]] 1. G. Aad et al. [ATLAS Collaboration], Phys. Lett. B 716, 1 9. The figure is from Wikipedia Commons, (2012). S. Chatrchyan et al., [CMS Collaboration], Phys. Lett. https://commons.wikimedia.org/wiki/File:Rotation_curve_of_s B 716, 30 (2012) piral_galaxy_Messier_33_(Triangulum).png 2. G. Aad et al. [ATLAS and CMS Collaborations], JHEP 1608, 10. M. Schumann, Braz. J. Phys. 44, 483 (2014) [arXiv:1310.5217 045 (2016) [arXiv:1606.02266 [hep-ex]] [astro-ph.CO]] 3. S. L. Glashow, Nucl. Phys. 22, 579 (1961). S.Weinberg, Phys. 11. P. A. R. Ade et al. [Planck Collaboration], Astron. Astrophys. Rev. Lett. 19, 1264 (1967). A. Salam, Weak and 571, A16 (2014) [arXiv:1303.5076 [astro-ph.CO]] Electromagnetic Interactions," Conf. Proc. C 680519 , 367 12. R. M. Godbole, M. Guchait, K. Mazumdar, S. Moretti and D. P. (1968) Roy, Phys. Lett. B 571, 184 (2003) [hep ph/0304137] 4. P. W. Higgs, Phys. Lett. 12, 132 (1964). P. W. Higgs, Phys. Rev. 13. O. J. P. Eboli and D. Zeppenfeld, Phys. Lett. B 495, 147 (2000) Lett. 13, 508 (1964). F. Englert and R. Brout, Phys. Rev. Lett. [hep-ph/0009158] 13, 321 (1964) 14. D. Ghosh, R. Godbole, M. Guchait, K. Mohan and D. Sengupta, 5. G. Aad et al., [ATLAS Collaboration], Phys. Rev. D 101 no.1, Phys. Lett. B 725, 344 (2013) [arXiv:1211.7015 [hep-ph]] 012002 (2020) 15. M. Tanabashi et al. (Particle Data Group), Phys. Rev. 6. Y. Gershtein et al., Working Group Report: New Particles, D 98, 030001 (2018) and 2019 update Forces, and Dimensions," arXiv:1311. 0299 [hep-ex]. 16. R. K. Barman, G. Belanger, B. Bhattacherjee, R. Godbole, G. 7. See for example, M. Drees, R. Godbole and P. Roy, Theory and Mendiratta and D. Sengupta, Phys. Rev. D 95, no. 9, 095018 phenomenology of sparticles: An account of four-dimensional (2017) [arXiv:1703.03838 [hep-ph]] N=1 supersymmetry in high energy physics," Hackensack, 17. P. A. R. Ade et al., [Planck Collaboration], Astron. Astrophys. USA: World Scientific (2004) 555 p 594 A, 13 (2016) [arXiv:1502.01589 [astro-ph.CO]]
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Quest for the “Top”
Naba K. Mondal Raja Ramanna Fellow, Saha Institute of Nuclear Physics, Kolkata Former Senior Professor, Tata Institute of Fundamental Research, Mumbai E-mail: [email protected] Prof. Naba K Mondal was a senior Professor at Tata Institute of Fundamental Research (TIFR), Mumbai and Project Director of India –based Neutrino Observatory (INO). He is now INSA Senior Scientist at Saha Institute of Nuclear Physics (SINP), Kolkata. He played a key role in the Kolar Gold Field (KGF) Necleon Decay Experiment. He was the leader of the TIFR group in the DØ experiment at Fermilab and led the Indian group in designing and building the Outer Hadron Calorimeter for the CMS experiment at CERN. He is a fellow of all the three science academies of India as well as The World Academy of Sciences for the Advancement of Science in Developing Countries (TWAS).
Abstract Existence of a third quark doublet was proposed by Kobayashi & Maskawa in 1973 to accommodate CP violation in K-meson decay. The lighter partner of this third generation -“bottom” quark was discovered in the form of 푏푏̅ bound state upsilon in the summer of 1977 at Fermilab, USA. A close look at its behaviour soon revealed that it indeed belongs to a doublet and needed a “top” partner. Thus began the search for top quark. The author, who was part of the team involved in this landmark discovery, recalls the top quark discovery after 25 years.
Early searches characteristic of a 40 GeV top. They saw 12 events with 3.5 expected background! Both turned out to be false hope as it Everybody knew that it would be heavier than bottom quark. was realised soon that the W+ jets background was initially But how heavy? Since mass of charm quark is about three underestimated by UA1 and by 1988, this turned into another times the mass of its lighter partner strange quark, some limit of Mt > 44 GeV. physicists naively suggested top quark mass to be around three times of bottom quark mass i.e. around 15 GeV. A bound state Meanwhile, the Large Electron Positron collider (LEP) at of 푡푡 ̅ might then be expected as a heavy hadron of mass around CERN started operation in 1989 with a centre of mass energy + − 30 GeV. The newly constructed 푒 푒 collider at DESY raced of 90 GeV and by 1990, LEP experiments set limits Mt > 45.8 for the prize catch, but found no hint of it up to a mass of about GeV. In the same year the UA2 experiment sets a limit 23.3 GeV. Thereafter, a new 푒+푒−collider, Tristan, with 60 (W→ tb) at 69 GeV, effectively closing the search channel of GeV center of mass energy, was specifically built in Japan to W decaying to top. find it. Tristan also failed in its discovery and by late 80’s, a lower limit on top quark mass of 30.2 GeV was set. The Tevatron collider at Fermilab in United States became operational in the fall of 1985. It was a proton-antiproton Meanwhile in 1981 a proton-antiproton collider with 630 GeV collider with a centre of mass energy of 1.8 TeV (900 GeV centre of mass energy, known as 푆푝푝̅푆, became operational at protons colliding with 900 GeV antiprotons) and opened up CERN. Two experiments, UA1 and UA2, operating at 푆푝푝̅푆, the possibility to extend the top quark search range up to discovered the carriers of the unified electroweak force, the W several hundred GeV mass. and Z bosons at masses of ~80 and ~90 GeV respectively in 1983. One would expect to see a top quark in W decay, if it’s The first detector to be operational at Tevatron was CDF in mass is less than 75 GeV. A good channel for the search is 1987. It was followed by DØ in 1992. At Tevatron, both CDF ̅ W→ tb→(eb) b. The main background for this channel is and DØ were looking for pair production of 푡푡. In Standard Model, heavy top decays 100% of time to W and b. The W in QCD production of W(e)+jets. In the same year, based upon UA1 published data of about 11 excess e+ isolated jet events, turn decays 33% of time into leptons ( e, µ, ) and 66% of ̅ ̅ Godbole, Pakvasa & Roy1 gave a possible interpretation in time into hadrons (푢푑, 푐푠̅). Final states reached from 푡푡 then characterised by (a) either W decays into leptons (all jets), (b) terms of top production with Mt ~ 35 GeV and suggested further test. In 1984, UA1 reported preliminary evidence for one W decays into leptons and the other to hadrons (lepton + jets), (c) both W decay to leptons (dilepton) an excess of events at low Mt (e) when jets were present,
1 Phys. Rev. Lett. 50, 1539 (1983)
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By 1992, CDF, searching for top quark above the W mass, By January 1995, after a significant improvement of the pushed the top quark mass limit to 91 GeV. In 1994, DØ Tevatron accelerator (fixing a rotated magnet) both pushed it further to 131 GeV. collaborations had collected over 50 pb-1 of data each. At Aspen Conference, held in Jan. 1995, DØ reported its result Moving toward discovery based upon 25 pb-1 analysed data, from which it was clear that While pushing the top mass to higher limits, CDF and DØ with double the data set available, either collaboration could were also seeing interesting individual events, but at low achieve the ~5 level needed for discovery. In both CDF and statistical sensitivity. For example, in October 1992, an event DØ, activities ramped up to fever pitch to analyze the full data with 2 energetic jets (one is b-tagged), one each of isolated set, and to finalize selection cuts, mass measurement moderate PT electron and muon with substantial Missing techniques, cross checks and systematic uncertainties. transverse energy was found by CDF. This event created lot The discovery of excitement in CDF. Some even regarded this event by itself as worthy of publication as a top quark event. But the The two collaborations although were working independently collaboration was simply not prepared to base a discovery as with no formal communications, had an agreement with important as the top quark on a single event. Similarly, a Fermilab Director on the process to be followed for top striking dilepton (e and 휇) event with substantial missing discovery. According to this agreement either collaboration transverse energy and two jets was presented by DØ in its final could trigger the end game by submitting the discovery paper limit paper. If hypothesize to be from top pair production to him. On receipt, a one week holding period would 푡푡→(̅ 푒 푗) (µj), the estimated top mass would fall in the commence during which the other collaboration could finalise range of 145-200 GeV. In early 1994, CDF published an its result, if desired, before final submission for publication. analysis based on 19 pb-1 data in which they found 2 events This agreement was useful for both as there was no fear of any + with 푒휇 2 jets and missing transverse energy ( ET), and 10 scoop at the final stage of discovery. On Feb 17, 1995, CDF + events with 푒 or µ ≥3 jets and missing ET, in which at least delivered its discovery paper to Fermilab director. DØ choose one of the jets was b-tagged either by silicon vertex detector to take advantage of this one week to refine their analysis and or by semileptonic decay. The estimated background (W+ jets, for a final cross-checks. On Feb 24, CDF & DØ submitted QCD multijets) was 6.0 ± 0.5 events, giving a probability for their top discovery papers simultaneously to Physical Review the background-only hypothesis of 0.26% (2.8 Gaussian Letters. CDF’s selection followed its 1995 ‘evidence’ paper equivalent). strategy with an improved b-tagging algorithm. They found 6 dilepton events and 43 lepton+ jets events (50 b-tags), with On the basis of its earlier 131 GeV limit, and the estimated background of 22.1 ± 2.9. Mt = 176 ± 13 GeV, understanding that CDF was preparing its ‘evidence’ paper, 휎 = 8+3.6 pb. Background-only hypothesis excluded at 4.8. DØ optimized its selection for higher mass top. Unlike CDF, 푡푡̅ −2.4 DØ’s selection refined the topological (A, HT) cuts to improve DØ at that time had limited b-tagging capability, so developed signal to background by a factor of 2.6. With tight cuts, they a selection criteria based on event topology variables, A found 3 dilepton events, 8 lepton+ jets events (topological (aplanarity = smallest eigenvalue of momentum tensor) and selection) and 6 lepton+ jets events (µ tag). Estimated HT (scalar sum of jet ET’s). DØ preliminary result with -1 + background to these 17 events was 3.8 ± 0.6 events. 13.5 pb data, had 7 events (1 푒휇, 4 푒 표푟 휇 jets with M = 199 ± 30 GeV, 휎 = 6.4 ± 2.2 pb. Background-only topological tag and 2 푒 표푟 휇+ jets events with 휇- tag) with t 푡푡̅ hypothesis rejected at 4.6. expected background of 3.2 ± 1.1 events (7.2% probability for background only hypothesis). This was presented at the Both collaborations announced their results in a joint public 27th International Conference on High Energy Physics seminar at Fermilab on 2nd March and the discovery papers (ICHEP) held at Glasgow. With no significant excess, DØ did were published in Physical Review Letters on April 3, 1995 not estimate a mass, but showed a cross section for its excess and thus came to an end a twenty years long quest to locate for a range of possible masses, in agreement with theory. the last member of the quark family.
CDF detector (left panel) & DØ detector (right panel) (Image credit : Fermi Lab )
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Resources for Quantum Technologies
Aditi Sen (De) Harish-Chandra Institute & HBNI Chhatang Road, Jhunsi, Prayagraj (Allahabad) -211019, India E-mail: [email protected]
Aditi Sen (De) is currently a Professor at the Harish-Chandra Research Institute (HRI), Prayagraj. She has started the quantum information and computation group (QIC) at HRI in 2009 along with two other faculty members, For the recognition of her pioneering work in this field, she was awarded Shanti Swarup Bhatnagar award in 2018. She is the first woman physicist to get this award.
Abstract Development of technologies based on principles of quantum mechanics is one of the challenging tasks around the world. For this purpose, identifying proper quantum mechanical state which can be used to build quantum technology is important. I will review the characteristic of such resources, possible generation and their applications in quantum information processing tasks.
Introduction qubits [12], atoms in a cavity [13] or in an optical lattice [14,15], nuclear magnetic resonances [16]. Based on the performance, technologies exploiting quantum mechanical principles can broadly be classified into two In this article, we will discuss the concept of entanglement categories — communication protocols in which capacities in including detection as well as quantification methods. We sending information can be increased by using quantum states show a possible way to generate entangled states. We then and quantum operations, and computational schemes which describe a communication protocol where entangled state are can complete certain assignments in polynomial time while essential ingredient. classical computational schemes cannot [1–3]. The former Resource States scenario includes quantum state transfer, classical information transmission, cryptographic schemes, while the latter one Depending on quantum information processing tasks, considers solving mathematical problems like finding prime classification of resource states has been made [4, 5]. In this factors of an integer. section, we will define one of the most important resource in quantum information science, namely entanglement [4]. We A set of states without which quantum protocols cannot be first define entangled states shared by at least two spatially accomplished is called quantum resources and is important to separated observers. We will then go beyond a two-party identify. At the same time, it is also interesting to find out the scenario. Finally, we will show a way to identify and quantify specific characteristics of these states, which lead to the entanglement. improvement. For example, in communication schemes, the quantum property called entanglement [4] shared between the A. Definition of Entanglement parties involved are shown to be essential. Other non-classical In entanglement paradigm, useless states are the resources, independent of entanglement, have also been found unentangled (also called the product or separable states) which and have attracted lots of attention in recent times [5]. can be prepared by two parties using local operations and Apart from theoretical predictions establishing superiority classical communication (LOCC). In this scenario, LOCC can of quantum mechanics towards building technologies, be called free operations. More specifically, two parties, Alia tremendous experimental achievements have been reported, and Brato, can prepare these useless states even when they are especially in last ten years [6, 7]. For example, polarization of situated in two distant locations. In case of pure states, product photons [8] and internal levels of ions [9, 10] can act as two states can mathematically be written as dimensional quantum systems, namely qubit [11]. These |⟩퐴퐵 = |⟩퐴 |⟩퐵 , (1) degrees of freedom of a particular physical system can be exploited to produce entangled states. Currently, two-party while entangled states are those states, which cannot be highly entangled states are routinely prepared in laboratories written in the above form in any basis. In a density matrix with these physical systems. Other potential candidate for level, a state, 퐴퐵, is said to be entangled if 푑 푖 푖 implementing quantum protocols are superconducting 퐴퐵 ≠ 푖=1푝푖 퐴 푩 (2)
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푖 푖 푖 Detecting entanglement: Partial Transposition Criteria where 푝푖 ≥ 0, ∀ 푖, 푖푝푖 = 1, 퐴 = |퐴⟩⟨퐴| (푖 = 1, . . 푑) and 푖 푖 푖 similarly 퐵 = |퐵⟩⟨퐵| (푖 = 1, . . 푑) while separable state Among several entanglement detection methods can be prepared by LOCC by Alia and Brato. An example of proposed [28, 29], over the years partial transposition criteria 1 an entangled pure state is the single state, |− ⟩ = (|01⟩ − turns out to be the efficient mathematical method to detect √2 entanglement. For two qubits, it is necessary and sufficient to |10⟩) where |0⟩ and |1⟩ represents eigenvectors of z , with prove whether a state is entangled or not. , = x,y,z being the Pauli spin matrices. The singlet admixed with white noise represented by the identity operator, I, in the Let CA and CB be the complex Hilbert spaces of Alia and 4-dimensional Hilbert space, can be written as Brato and their corresponding dimensions be N and M 퐼 = 푝 |− ⟩⟨− | + (1 − 푝) . (3) respectively. If a bipartite density matrix can be represented as 푊 4 = 푁 푀 푎푘푙(|푖⟩⟨푗|) (|푘⟩⟨푙|) (8) It is shown to be entangled for certain value of mixing 퐴퐵 푖,푗=1 푘,푙=1 푖푗 퐴 퐵 parameter p, specifically when p > 1/3. In 1989, R.F. Werner where {|푖⟩} (i = 1,2,….N; N ≤ dim CA) ({|푘⟩} (k = 1,2,….M; [17] possibly discussed about this state for the first time and M ≤ dim CB)) is a set of real orthonormal vectors in CA (CB), show that there exists a range of p, namely p ≤ 1/√2, in which The partial transposition of 퐴퐵 with respect to subsystem A, 푇퐴 푊 is entangled but does not violate Bell inequality [18, 19] denoted by 퐴퐵 , is defined as with two-settings. 푇퐴 푁 푀 푘푙 = 푖,푗=1푘,푙=1푎푖푗 (|푗⟩⟨푖|)퐴(|푘⟩⟨푙|)퐵 (9) It is important to note here that given a state, , it is not 퐴퐵 퐴퐵 If 푇퐴 has any negative eigenvalue, we can then infer to easy to prove whether the state is entangled or not. In case of 퐴퐵 퐴퐵 bipartite pure states in smaller dimensions like for two spin- be entangled. For two-qubit entangled states, it can be shown 1/2 particles, a state can be shown to be entangled from the that the partial transposed state has a single negative definition only. However, such a simple procedure may not eigenvalue. work for arbitrary density matrix. Therefore, it is important to B. Quantifying Entanglement find entanglement detection methods. Over last 25 years, several mathematical as well as experimentally testable Entanglement detection procedures can only give qualitative procedures have been found. Prominent entanglement answers. Given a state, it is also essential to find the amount detection methods [3, 4, 20] include partial transposition, of resource that the state possess. For bipartite pure states, the majorization, covariance matrix criteria, and entanglement von-Neumann entropy of local density matrices of a given witness. Among these, we will discuss about the partial state, |⟩퐴퐵, can faithfully quantify the entanglement, i.e. transposition criteria later. 퐸(|⟩퐴퐵) = 푆( 퐴) (10)
Let us now move to a scenario where we want to describe Where 푆 () = − 푡푟 ( 푙표𝑔2) denotes the von Neumann entanglement properties of a state, 12….푁, consisting of N entropy and 퐴 = 푡푟퐵(|⟩퐴퐵⟨|) is the local density matrix parties [21] situated in separate places. If a state can be written of |⟩퐴퐵[4]. Exploring convexity, we can now define as entanglement for an arbitrary state, 퐴퐵 as 푖 푖 푖 12……푁 = 푖 푝푖 1 2 … . 푁 (4) 퐸퐹( 퐴퐵) = 푚푖푛 푖푝푖 (|푖⟩⟨푖|) (11) it is called fully separable (useless). As one expects, N-party with the minimization being performed over all possible pure states can be classified in different ways. According to their state decomposition of [30]. Since infinite number of entanglement content. A pure state is said to be k- separable 퐴퐵 such decompositions for exist, it is, in general, not easy [21] if it can be expressed as 퐴퐵 to compute. A compact form of 퐸퐹 for two qubits was found |⟩12…..푁 = |⟩1…..|⟩푘−1|⟩푘+1……푁 (5) in Ref. [31]. while a pure state is genuinely multiparty entangled if it is not Another important entanglement measure which can be product in any bipartition. Following this definition, in case of computed in arbitrary dimension is logarithmic negativity three parties, there exist three types of entangled states – fully based on partial transposition criteria [32]. Specifically, for an separable, 2-separable or biseparable and genuinely arbitrary density matrix, 퐴퐵, it is defined as multiparty entangled states. Two prominent examples of genuinely multipartite entangled states read as 퐿( 퐴퐵) = 푙표𝑔2(2푖|푖 | + 1) (12)
1 where 푖 푠 are negative eigenvalues of partial transposed state |⟩퐺퐻푍 = ((|000⟩ + (|111⟩) (6) √2 of 퐴퐵 with respect to any party, say A. which is known in the literature as Greenberger-Horne- Although there are several entanglement measures Zeilinger state [22] and the W state [21,23,24], given by 1 introduced for two-qubit states, only a few multipartite |⟩ = ((|001⟩ + (|010⟩ + |100⟩) (7) 푊 √3 computable entanglement measures are known in literature. It was shown that genuine multiparty entangled states can be For pure multipartite states, we introduce a measure which can resource for several quantum information processing tasks quantify genuine multipartite entanglement content of the like quantum secret sharing [25], multisite dense coding [26], state [33, 34], known as generalized geometric measure measurement -based quantum computation [27]. (GGM). Let us define it for a three-party state,|⟩퐴퐵퐶, which
25 Physics News can easily be generalized to an arbitrary number of parties. It First note that the nearest-neighbor two-party density matrix is defined as the minimum distance between a given state and of the zero-temperature state which is obtained by tracing out non-genuinely multipartite entangled state. Mathematically, N-2-parties are all equal due to the periodic boundary condition. For example, to obtain , as depicted in Fig. 1, 퐺(|⟩ ) = 1 − 푚푎푥 |⟨│⟩ |2 , (13) 23 퐴퐵퐶 |⟩ 푛퐺 퐴퐵퐶 one should trace out all the other sites except site 2 and 3 from where the maximization is performed over the set of all a six-party state. We then can calculate its entanglement nongenuinely multipartite entangled states, denoted by nG. It content (logarithmic negativity) in terms of system parameter, can be shown that this measure can be computed for arbitrary = h/J, as shown in Fig. 2 (see Ref. [39] for infinite spin chain number of parties because it can be simplified in terms of and scaling of entanglement with N). One can check easily Schmidt coefficients of |⟩퐴퐵퐶 in different bipartitions. that next-nearest neighbor and other long-range entanglement Specifically, is almost negligible or vanishing with the variation of . 푚 푚 푚 퐺(|⟩퐴퐵퐶) = 1 − max (퐴 , 푩 , 퐶 ) 푚 where 푖 , i = A,B,C represent the maximum eigenvalues of ⟩ local density matrices of 푖, i = A,B,C of | 퐴퐵퐶. Notice that i = A,B,C s are the squares of the Schmidt coefficients of |⟩퐴퐵퐶in A : BC, B :CA and C : AB bipartitions. For example, the GGM for the GHZ state, in Eq. (6), can be computed to be 0.5 since all the reduced local density matrices of the GHZ state is same due to symmetry and is I/2, having equal eigenvalues, 0.5. It is important to note here that such a simplification does not hold if we extend it to a mixed state. However, it was recently found that for a certain classes of mixed states, GGM can be computed [35]. A compact form of GGM for arbitrary density matrices with arbitrary number of parties is still an open question.
Generation of Resource States Figure 2: Nearest neighbor entanglement of the zero Let us discuss a physically realizable model in which temperature state of the Ising model (ordinate) vs. = h/J resource states, i.e., entangled states can be created. To (abscissa). The numerical simulation and the figure is demonstrate that, let us consider a system in which N spin- 1/2 produced by L.L. Ganesh Chandra. particles interact according to the Hamiltonian, given by [36]
푁 푖 푖+1 푖 퐻퐼푠푖푛푔 = −퐽 푖=1푥푥 + ℎ푧, (14) where J and h represent respectively coupling constant and strength of the magnetic field, and periodic boundary condition is also considered, i.e., 푁+1 = 1 . The above model is known as the Ising model which is one of the simplest model to undergo quantum phase transition at zero temperature and hence this model plays an important role in condensed matter physics. With the advancements of cold atoms, such a system can be prepared by using currently available technologies [15,37,38].
Figure 3: Nearest neighbor entanglement of the evolved state of the Ising model (ordinate) against t (abscissa). The initial Figure 1: Schematic diagram of a spin chain consisting of N state is the canonical equilibrium state of the Ising spins. 23 denotes the density matrix of the nearest neighbor Hamiltonian in Eq. (14) with a= 0.5 and N = 6. The numerical sites, 2 and 3 which is obtained after tracing out N -2-parties simulation and the figure is produced by L.L. Ganesh of the ground or thermal or evolved state of the system. It can Chandra. be shown that for the quantum Ising model in Eq. (14), it can Let us now move to a situation where the system is initially be entangled for certain values of system parameters. prepared at the canonical equilibrium state with = 29.5 of We will show that proper tuning of system parameters lead to the Ising Hamiltonian. Then at t > 0, magnetic field, h is turned an entangled state in this model. Let us now consider the zero off, i.e., temperature state [44] of the above Hamiltonian with N = 6. h = a, t ≤0, h = 0, t > 0.
26 Physics News
The system evolves according to the Ising Hamiltonian in Fig. 4 for pictorial illustration of the protocol) in the Eq. (14) and can also generate nonvanishing nearest neighbor following: entanglement with the increase of time if one properly tunes Step 1 (Entangling measurement): Alia measures both her the system parameter. Production of entanglement in this 1 qubits in the Bell basis, {| ⟩ = (|01⟩ ± |10⟩), | ⟩ = dynamical process with a = 0.5 is depicted in Fig. 3 with √2 1 N = 6. In thermodynamic limit, i.e. for N→∞, one can find the (|00⟩ ± |11⟩)}. behavior of bipartite entanglement for large time and can √2 Step 2 (2 bits of classical communication): Alia investigate the statistical mechanical properties like ergodicity communicates measurement results having 4 outcomes to in this model [40, 41]. It is important to note here that creating Brato, i.e. she sends 2 bits of classical information to Brato. isolated system is almost impossible in laboratories – system Step 3 (Decoding): Depending on the measurement inevitably interacts with the environment which can cause outcomes, Brato performs an unitary operation from the set, decay of entanglement. Currently, one of the active directions of research is to study the effects of environment on {I, x, y,z}, on his qubit, as shown in Table below. entanglement in these spin systems (see e.g. [5, 42, 43, 45]). Quantum Protocol with Entanglement as Resource Table 1: Measurement outcomes and corresponding unitaries Outcome |−⟩ |+⟩ |−⟩ |+⟩
Unitary I z x y
Remark: If Alia does not communicate about her measurement outcomes, the state at Brato’s side can be found to be an Identity operator of that space, having no information about the unknown qubit. It shows that there is no faster than light communication in this protocol. The above protocol establishes that the singlet state is a resource in quantum teleportation. In laboratories, the shared state is in general mixed which have the fidelity with the singlet ranging from 80% to 99% depending on the physical Figure 4: Quantum teleportation protocol. Steps involved in systems used for implementations. The teleportation scheme this protocol are schematically depicted where A is the sender with arbitrary mixed shared state has been addressed and the having an unknown qubit A’ and B is the receiver. optimal teleportation fidelity is found [48, 49]. It has also been As discussed before, entanglement is the necessary realized by using different physical systems like photons, ion ingredient for several quantum communication protocols traps [8–10, 47]. Point to point communication has very involving two or multiple parties. In this section, we illustrate limited use. Therefore, considerable efforts have been put to one of them in which arbitrary quantum state is transferred extend such scheme in a multipartite setting [2,33]. from a sender to a receiver, known as quantum Conclusion teleportation [46, 47]. Summarizing, we introduced one of the main resources Let us consider a scenario associating two parties, a sender available to build quantum technologies. A possible method to called Alia (A) and a receiver called Brato (B) who are generate such resources was also shown. We then discussed a situated in different laboratories. Alia possesses another communication protocol which can only be successfully unknown qubit, A’, represented as |⟩ = 푎|0⟩ + 푏|1⟩ where a completed, if the resource state is distributed among the and b are arbitrary complex number, satisfying the parties. normalization condition, |a|2+|b|2 =1. Alia wants to send A’ to Brato [46] and she is not allowed to send it by using quantum Acknowledgment channel. Therefore, we ask the following question: Is there a process by which a qubit can be transferred from I thank Prof. Vandana Nanal for inviting me to write this Alia to Brato without sending it physically? article. I acknowledge L.L. Ganesh Chandra for numerical Let us suppose that Alia and Brato share an unentangled simulation to produce Figs. 2 and 3. I also thank state. Since an arbitrary pure qubit can be any point in the Amit Kumar Pal and Shiladitya Mal for reading the surface of the Bloch sphere, Alia requires infinite amount of manuscript. classical communication to encode such an arbitrary qubit, i.e. References any complex pair. 1. M. A. Nielsen and I. Chuang, Quantum Computation and Let us now consider a situation where Alia and Brato apriori Quantum Information, Cambridge University Press, Cambridge share a singlet state, |− ⟩, and she now wants to send A’ to (2000) Brato. Initially, Alia possesses two qubits – one part of a 2. A. Sen (De) and U. Sen, Physics News 40, 17 (2010) singlet and an unknown qubit. We describe the process (see (arXiv:1105.2412 [quant-ph])
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3. S. Das, T. Chanda, M. Lewenstein, A. Sanpera, A. Sen (De), and 24. A. Sen (De), U. Sen, M. Wiesniak, D. Kaszlikowski, and U. Sen, in Lectures on Quantum Information, edited by D. Bru M.Zukowski, Multiqubit W, Phys. Rev. A 68, 062314 (2003) and G. Leuchs (Wiley, Weinheim, 2006); and reference therein 25. M. Hillery, V. Buzek, and A.Berthiaume, Phys. Rev. A 59, 1829 4. R. Horodecki, P. Horodecki, M. Horodecki, and K. Horodecki, (1999) Rev. Mod. Phys. 81, 865 (2009) and reference therein 26. D. Bru, G. M. D'Ariano, M. Lewenstein, C. Macchiavello, 5. K. Modi, A. Brodutch, H. Cable, T. Patrek, and V. Vedral, Rev. A. Sen (De), and U. Sen, Phys. Rev. Lett. 93 210501 (2004) Mod. Phys 84, 1655 (2012); A. Bera, T. Das, D.s Sadhukhan, S. 27. H. J. Briegel, D. Browne, W. Dur, R. Raussendorf, and M. van Singha Roy, A. Sen (De), and U. Sen, Rep. Prog. Phys. 81, den Nest, Nat. Phys. 5, 19 (2009) 024001 (2018) and reference therein 28. A. Peres, Phys. Rev. Lett 77, 1413 (1996) 6. F. Arute et.al., Nature 574, 505 (2019) 29. M. Horodecki, P. Horodecki, and R. Horodecki, Phys. Lett. A 7. J. Yin et. al., Science 356, 1140 (2017) 223, 1 (1996) 8. J. W. Pan, Z. B. Chen, C. -Y. Lu, H. Weinfurter, A. Zeilinger, 30. C. H. Bennett, D. P. DiVincenzo, J. A. Smolin, and W. and M. Zukowski, Rev. Mod. Phys. 84, 777 (2012) K.Wootters, Phys. Rev. A 54, 3824 (1996) 9. D. Leibfried, R. Blatt, C. Monroe, and D. Wineland, Rev. Mod. 31. W. K. Wootters Phys. Rev. Lett. 80, 2245 (1998) Phys. 75, 281 (2003) 32. G. Vidal and R. F. Werner, Phys. Rev. A 65, 032314 (2002) 10. H. Hafner, C. F. Roose, and R. Blatt, Phys. Rep. 469, 155 (2008) 33. A. Sen (De) and U. Sen, Phys. Rev. A 81, 012308 (2010) 11. The smallest unit in a classical computer which can take a binary 34. T.-C. Wei and P. M. Goldbart, Phys. Rev. A 68, 042307 value {0,1}, is called a bit (binary digit). Computation in a (2003) classical computer has been performed by using bits. On the 35. T. Das, S.S. Roy, S. Bagchi, A. Misra, A. Sen (De) and U. Sen, other hand, in quantum computer, smallest unit is a two- Phys. Rev. A 94, 022336 (2016) dimensional quantum system which is called quantum bit, in 36. S. Sachdev, Cambridge University Press (13 January 2000) short qubit. 37. M. Lewenstein, A. Sanpera, V. Ahufinger, B. Damski, 12. R. Barends, J. Kelly, A. Megrant, A. Veitia, D. Sank, E. Jeffrey, A. Sen (De), and U. Sen, Adv. Phys. 56, 243 (2007) T. C. White, J. Mutus, A. G. Fowler, B. Campbell, Y. Chen, Z. 38. L. Amico, R. Fazio, A. Osterloh, and V. Vedral, Rev. Mod. Chen, B. Chiaro, A. Dunsworth, C. Neill, P. OMalley, P. Phys. 80, 517 (2008) Roushan, A. Vainsencher, J. Wenner, A. N. Korotkov, A. N. 39. A. Osterloh, L. Amico, G. Falci and R. Fazio, Nature 416, 608 Cleland, and J. M. Martinis, Nature 508, 500 (2014); C.P. Yang, (2002) Q.-P. Su, S.-B. Zheng, and F. Nori, New J. Phys. 18, 013025 40. A. Sen (De), U. Sen, and M. Lewenstein, Phys. Rev. A 70, (2016), and references therein 060304(R) (2004) and reference therein 13. J. M. Raimond, M. Brune, and S. Haroche, Rev. Mod. Phys. 73, 41. T. Chanda, T. Das, D. Sadhukhan, A.K. Pal, A. Sen (De), and 565 (2001) U. Sen, Phys. Rev. A 94, 042310 (2016) and reference therein 14. O. Mandel, M. Greiner, A. Widera, T. Rom, H T. W. ansch and 42. T.Chanda, T. Das, D.Sadhukhan, A.K. Pal, A. Sen (De), and U. I. Bloch, Nature 425, 937 (2003) Sen, Phys. Rev. A 97, 062324 (2018) 15. I. Bloch, J. Dalibard, and W. Zwerger, Rev. Mod. Phys. 80, 885 43. G. De Chiara and A. Sanpera, Rep. Prog. Phys. 81, 074002 (2008) (2018) exp (퐻 ) 16. L. M. K. Vandersypen and I. L Chuang, Rev. Mod. Phys. 76, 44. We find the canonical equilibrium state, 퐼푠푖푛푔 , with 1037 (2005); C. Negrevergne, T. S. Mahesh, C. A. Ryan, M. 푍 Ditty, F. Cyr-Racine, W. Power, N. Boulant, T. Havel, D. G. Z = tr(exp(H)) being the partition function Here = 1/푘퐵푇 Cory, and R. Laflamme, Phys. Rev. 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Subsurface detections by SHAllow RADar (SHARAD) on MARS
S. K. Mishra and Rajiv R. Bharti Physical Research Laboratory, Ahmedabad 380009, India E-mail: [email protected]
Dr. Sanjay K. Mishra is a PRL Physicist, having diverse research experience at various levels. His research interest is interdisciplinary plasma physics, including space and dusty (complex) plasma, laser-plasma interaction, sheath physics, and electronic transport processes. He is Young Associate of IASc, Bangalore, and recipient of the IPA Buti Foundation Award 2018.
Rajiv R. Bharti is a Scientist in the Planetary Sciences Division, PRL. His research area is the study of surface and subsurface of Mars using remote sensing data of various missions. Currently, he is working on the data of SHARAD to investigate the geophysical properties of Martian subsurface.
Abstract Orbiter sounding radars have been proven a vital tool in deriving the stratigraphic and geological information of the planetary bodies in the planetary exploration. SHAllow RADar (SHARAD) onboard Mars Reconnaissance Orbiter (MRO) spacecraft, orbiting Mars, is considered one of the very successful sounding radar mission detecting the subsurface interfaces beneath the Martian surface. The prime objective of such sounders is the search for water ice and aquifers within the Martian surface. Due to high-frequency operation mode, SHARAD has detected the ice layers around the polar caps and mid-latitude regions of Mars within few 100 meters beneath the top surface. In this article, we briefly present the concept, operation, and science objectives of SHARAD, alongwith a plausible approach to derive the dielectric properties of the subsurface materials from the SHARAD observations.
Introduction however, due to the coarse resolution, the instrument was not compatible with providing the stratigraphy description within Planetary exploration via in situ measurements and orbiter the first km below surface [4]. This is where the requirement remote sensing has brought out significant information that of another device with the lower vertical resolution was has advanced our understanding of the Martian surface and realized, and the SHAllow RADar (SHARAD) instrument climate history [1-3]. Particularly, due to global coverage of was conceptualized [7]. the planet, the orbiter radars are proven important instruments in identifying the adequate habitable zone for the forthcoming SHARAD is the instrument from the Italian Space Agency robotic and human explorations [3]. The search for the quest (ASI) operating at center frequency of 20 MHz (with 10 MHz of subsurface water deposits has been the prime focus of the bandwidth) and 10 – 20 m vertical resolution – SHARAD is campaigns so far to the Mars, and the low-frequency orbiter flown with NASA’s Mars Reconnaissance Orbiter (MRO) in radars with high penetration capabilities have been successful 2005 [8]. The operational orbit of MRO spacecraft is nearly in depicting such regions [4]. For instance, the Mars Advanced polar, quasi-circular, and sun-synchronous – it secures optimal Radar for Subsurface and Ionosphere Sounding (MARSIS), in illumination conditions for optical instruments. The science 2012, first depicted the evidence of lower-density material that objective of SHARAD is to map the dielectric interfaces to a fills the northern basin, indirect evidence of an ancient few hundred meters depth on selected sites, and interpret the northern ocean [5]. The discovery of the first water reservoir radar data in terms of the occurrence and distribution of the in the form of the subglacial lake of 20 km extension on Mars expected materials within the subsurface layers – this includes below the southern polar ice cap [6] has been another notable the water, ice, soil, and rocky materials. The observations from breakthrough by MARSIS. MARSIS operates at frequencies SHARAD are anticipated to complement the MARSIS between 1.8 and 5.0 MHz in subsurface sounding mode, with findings by refining the near-surface composition, a 1 MHz bandwidth, that corresponds to the vertical resolution stratigraphy, and structure for the first few 100’s m beneath the of 150 m – with this configuration, MARSIS was able to detect surface – refined structures of the internal layers of the polar the subsurface soundings from the ~1 – 5 km vertical extent, caps were of prime interest [8]. Among numerous science
29 Physics News results, the detection of layered ice deposits around north pole the form of off-nadir echoes alongwith the nadir surface and of Mars [9], and the underground ice (i.e., equivalent to the subsurface echoes [8]. As shown in Figure 1, the radar receives volume of the water in Lake Superior, North America) in the reflections from the off-nadir locations as well, in addition to Utopia Planitia region of Mars [10], are a couple of nadir echoes. Depending on the topography and roughness of outstanding discoveries from SHARAD. the target surface, the off-nadir clutter echoes may be significant to disguise it as an actual subsurface echo. In order In this article, we particularly focus on the SHARAD to overcome this, the idea is to reduce the radar footprint. In operation and describe the inherent physics principle, analysis this course, SHARAD works on the principle of Synthetic method, scientific objectives, and significance. For a detailed Aperture Radar (SAR) to achieve a fine along-track resolution description of the technical and scientific challenges in the [4] – depending on the orbiter location and region of interest instrument design and development, data acquisition and on the Mars, SHARAD may achieve the horizontal resolution processing, and overall instrument operation, the readers are of 0.3 – 3 km along and 3 – 6 km across the track [8]. This referred to Ref. [7-8], and the references therein. technique, however, reduces the footprint of antenna radar, but Operational principle and specifications could not completely diminish the effect of clutter originated from nearby off-nadir irregularities. This can be overcome by The working principle of the SHARAD instrument is quite simulating the Mars surface and irregularities at the scale of simple and is based on the radio wave transmission through a SHARAD interest – Mars Orbiter Laser Altimeter (MOLA) medium. Due to large wavelength, the radio wave possesses [11] topography data is used to construe the off-nadir echoes the capability to penetrate the surface, which is frequently and delineate the adequate subsurface echoes from SHARAD used in the ground-penetrating radar systems for archeological radargram. With this approach, one can distinguish between surveys and terrestrial applications. As radio wave falls on the the surface and subsurface signals. A complete description of target, a large fraction of energy reflected from the surface, clutter mitigation can be found in Refs. [11-12]. while it partially transmits into the surface depending on the dielectric properties of the medium. The transmitted wave Table 1: SHARAD parameters may exhibit further reflections as signal encounters dielectric Parameter Magnitude discontinuities – these signals, which carry the dielectric info of the geological layers in terms of power, are gathered by Frequency band 15 – 25 MHz antenna receiver for further data processing. Intuitively, the Mean frequency 20 MHz subsurface reflections can be much weaker than those from the Vertical resolution 15 m (in a vacuum) surface because of medium losses. SHARAD produces data Penetration depth 0.1 – 1 km for reflections from surface and subsurface layers in terms of twt delay, and respective power – this graphical representation Horizontal resolution 0.1 – 3 km (along track) is termed as ‘radargram’ [8] and infers the dielectric contrast 3 – 6 km (across track) encountered by a signal in its transmission. Such discontinuity Peak signal power 10 W on Mars could arise from many combinations of surface and subsurface materials. This may include the contacts of Another concern is the temporal resolution of the signal. To sediments with basaltic cover, ice with solid rock, icy porous optimize the instrument performance, it should be equivalent with porous/ solid rocks, volcanic flows, etc. – these to the vertical space resolution (~ 15 m) – in terms of two-way stratigraphic features deduce the physical evolution of the time (twt) delay. This is equal to 100 ns or the bandwidth of Martian surface and climate [8]. 10 MHz [8]. To achieve such resolution, a linear chirped-pulse has been opted as the radar transmission signal. The nominal physical parameters for SHARAD [8] are listed in Table 1. Subsurface detection With the help of an example, we show the detection of true subsurface echoes. As an illustration, consider the SHARAD track over an LDA (lobate debris apron) structure [13] located at coordinates (46.12N, 28.15E) around northern-mid latitude Deuteronilus Mensae region on Mars – this area is shown in the bottom panel of Figure 2, with radar track (yellow line). The LDAs are the geological features on Mars, comprising of piles of rock debris with a convex topography, connected from cliffs with a gentle slope. LDA’s are considered the signatures of the phase of glacial activities on Mars – the geomorphic and topographic studies of LDAs have suggested their formation by downflow of ice-debris mix away from the plateau during Figure 1: Schematic of the clutter generation through off-nadir the past ~ (1Ga – 100Ma). The geomorphic studies and limited surface echoes. occurrence give evidence of accumulation of km thick ice at a Moreover, the operation of orbiter radars from significantly certain depth in northern and southern mid-latitudes of Mars. large altitudes, and directivity constraints, generate clutter in Using SHARAD data, Plaut et al. [14] have shown the typical
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rate of decrease of ~2 dB/μs for the two-way echo attenuation magnitude of power (in dB) received by radar receiver after for a real dielectric constant of 3 (i.e., nearly pure water ice). reflection from the surface. Just below, the faint white lines of relatively lower intensity correspond to the reflected power (in dB) from the subsurface layers. Using these data set, the power associated with surface and subsurface reflections can be represented as a function of twt delay – this information we use to determine the dielectric properties of the subsurface material that we discuss next.
Dielectric permittivity of the subsurface material Radar echoes/ signals received after reflection/ scattering describes the characteristic dielectric features of the material of layers and sublayers [3]. As an electromagnetic (em) radio wave interacts with the surface, it exhibits partial reflection, scattering and transmittance. The transmitted wave propagates further within the surface and again exhibits partial reflection/ transmittance at any other dielectric discontinuity beneath. In this process, the waves also suffer the attenuation and phase change on account of its absorption within the medium. This process continues until the wave diminishes completely. These partial reflections from surface and sub-surfaces are usually recorded by the radar in terms of the power at different (two way) time delays, which eventually carries cumulative effects of the dielectric features and sublayer widths. To extract this information from the recorded (SHARAD) data, an inversion approach [15] of the reflection/ scattering analysis is usually adopted, and estimates are made on the basis of backward calculations. We herewith briefly describe the reflection model based on the linear theory of em wave propagation [16]; as SHARAD experiment is operating with low power signals, linear calculations are well applicable. The first reflection occurs as the signal wave interacts with vacuum – top surface interface. To model the radar observations, we consider the physical picture where the subsurface material is considered sandwiched between the successive flow deposits. Figure 2: Subsurface reflection: The top panel image refers to A schematic of this physical model is shown in Figure 3. In SHARAD radargram in terms of twt delay for track no. 871101. addition to top surface layer, we consider two main interfaces Middle panel image is the simulated cluttergram displaying off-nadir viz. (1) top surface layer – subsurface, and (2) subsurface – echoes. The bottom panel refers to the daytime IR THEMIS (Thermal base layer; both the interfaces are considered planar, and the emission imaging system) image of LDA. layers are assumed to be homogeneous and nondispersive. The The subsurface radar sounding of the apron extent indicates subsurface layer is further considered as a low-loss medium that these deposits are composed mainly of water ice, which is with the intensity attenuation coefficient (α). Considering the preserved at shallow depths beneath the LDAs [14]. large distant radar observations, the planar wavefront with a normal incidence of the wave has been modeled. The top panel of Figure 2 illustrates the reflection echoes received by SHARAD in the form of the radargram – this combines the information of reflected signals from the surface and subsurface layers along the track in the form of twt delay and signal intensity (in dB units). The middle panel of Figure 2 refers to the cluttergram generated using MOLA topography data for the same location. Comparing the two figures (top and middle panels), one may clearly distinguish the true subsurface reflections from those of off-nadir echoes; the subsurface features have been depicted by white arrows in the SHARAD radargram. To interpret the SHARAD data for mapping the distribution of the subsurface reflections and for exporting the relative subsurface reflection intensity or signal power, commercially available seismic software has been Figure 3: A schematic of the physical model considered for the used. The sharp-intense white line along the track refers to the calculation of the power reflectance (Eq.5).
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Following Fresnel’s formulation, the power reflected from calculated as a function of time delay (Δτ). The value of (Λm / the top surface into the vacuum (Λs) and the power transmitted Λs) at any given location may be used to obtain the subsurface (ΛT) into the surface, maybe given in terms of the incident dielectric constant (εml) by solving Eq.5 for the known εtl. This power (Λi) as model can be implemented with the measurements of (Λm / Λs) to constrain the possible values of εml. For dry materials, 2 2 s = tl i & T =(1 − tl ) i , (1 & 2) the dielectric permittivity is directly correlated with density – this empirical relationship may be presented as [17] 1/2 where tl=()/()k o − k tl k o + k tl , kktl= o tl , ko (= 2πν/c = ml =1 . 9 6 , (6) 2π/λo), ktl and εtl stand for the wavenumber and dielectric where ρ is the material density in gcm-3. constant of the top surface layer, ν , λo and c correspond to the mean frequency (20 MHz), wavelength (15 m) and light speed (i.e., 300 m/µs) in the free space for SHARAD operation. Table 2: SHARAD subsurface detections and corresponding dielectric properties The wave transmitted through the top layer (ΛT) acts as an incident wave for the 1st (i.e., surface–subsurface) interface. In Location Dielectric Loss Slope Material evaluating the reflectance from the subsurface layer, we take on Mars Constant Tangent DB/s account of the propagating solution of the wave equation in NW Dense the constituent (i.e., surface – subsurface – base layer) region. Ascraeus 7.6 2.3 0.006 -3.3 Low Ti By matching the continuity of the wave solutions and their [18] Basalt derivatives at layer interfaces (1st & 2nd), the coefficient of the Ascraeus Dense wave amplitude reflection and transmission may be obtained Mons Low Ti as a function of the medium variables [2]. With little algebra, 6.2-17.3 0.01-0.03 -7.9 st (Northern Basalt the reflection coefficient at 1 interface may be expressed as flow)[19] −2ikw ml Ascraeus Terrestrial/ ()()()()kkkkkkkketlmlmlblmlbltlml−++−+ ml = −2ikw , (3) Mons Lunar ()()()()kkkkkkkke+++−− ml 7.0-14.0 0.01-0.03 ----- tlmlmlblmlbltlml (Southern Basalt flow)[19] here kk= 1/2 and kk= 1/2 refer to the wave vectors mloml blobl Amazonis Moderate associated with the unknown (middle) subsurface layer (k ) ml Plantia ---- 0.009 -4.98 density and the base layer (kbl). The optimum value of γml corresponds [20] Sediments opt 22 to kmlw = π/2, as mltlblmltlblml=−+()k / kkk () kk . Moreover, LDA’s [9] ~3.0 ---- -2.0 Water Ice for kbl = ktl, Eq.3 reduces to a simpler form, viz., 22 Moreover, in the realistic scenario, the interfaces may not (ktl− k ml )[1 − exp ( − 2 ik ml w )] ml = 22 . (4) be exactly planar, and the width of the subsurface (and hence (ktl+ k ml ) − ( k tl − k ml ) exp ( − 2 ik ml w ) the two-way time delay) may vary along the track. Usually, the SHARAD data is graphically presented as a relation It should be noted that for known k and k , γ shows an tl ml ml between the power loss (Λ – Λ ) in dB and respective two- oscillatory dependence on k w and take a value between 0 to m s ml way time delay Δτ – the departure of the data from flatness, Δτ optimum depending on subsurface width w. The real part is a signature of the signal attenuation in the medium. Noted, becomes zero for w = nλ /2 while takes maximum value for ml the power reflectance (Λ / Λ ) in dB refers to the difference subsurface thickness w = (2n + 1) λ /4, where n refers an m s ml of the power reflected (in units of dB) from the two successive integer, and λ is the signal wavelength within the subsurface ml interfaces. In this context, rewriting Eq.5 in terms of power medium. As a simplification, let consider a linear attenuation loss in dB and Δτ can be expressed as of the wave within medium. It modifies the expression for power by a factor of exp(–2αw); here, 2w infers twt delay and −10c (1) 2 2 2 1/2 mtl= ml − + 10log . (7) equals ~ (/)cml . Thus, net em signal power approaching 1/22 s mltlln10 the radar in the after reflection may be expressed as dB
2−− 2ww 2 2 2 2 In view of Eq.5, an empirical linear fit between power loss m =( mlee ) T = (1 − tl ) ml i . (4) (Λm / Λs) in dB and Δτ through SHARAD data may provide a notion about the magnitude of the absorption coefficient. For Using Eq.1 and Eq.4, the ratio of the powers reflected from instance, the gradient of this linear relation refers to the loss the surface and sub-surface layers may thus be expressed as rate of the subsurface material in the units of dB/μs. Another (/) (1) = (/ −− )( 2)2 2 22 expw . (5) dielectric parameter that can be derived from the SHARAD mstlmltl data is the loss tangent. This physical quantity, by definition,
The power reflectance (Λm / Λs) derived in Eq.5 is, in fact, represents the ratio of imaginary and real parts of the complex an outcome of SHARAD data in the form of radargram, which dielectric constant of the subsurface material; physically, it illustrates the relationship between the power reflected along- represents the power loss of the em radio wave as it traverses track and round-trip time (twt) delay – from this, the ratio of through the medium. Following Campbell et al. [20], the loss reflected signal powers from surface and sub-surface, may be tangent consistent with the subsurface can be expressed as,
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2 1/2 plausible range of the dielectric constant, and hence the tanlnLc=+−2[() / 4]11 2 , (8) o density (Eq.6) of the subsurface material. As depicted in where L is the power loss per unit of time Δτ. For the sake of completeness and secure a notion of the subsurface characteristics, the dielectric constant and loss tangent of the SHARAD identified subsurface material on Mars are listed in Table 2; the data has been depicted from the various literature [9, 18-20].
Analytical illustrations Here we present a few calculations based on the analytical expression derived in the last section. The expression for the power reflectance (Eq.5) suggests that its magnitude depends on the dielectric constants of constituent layers (ε), attenuation factor (α), and width of the sandwiched subsurface layer (w). The basalt is prominent material and commonly dispersed on the Martian surface; for calculations, the dielectric constant of the top surface may be taken as εtl = 10. Further, considering the fact that the basaltic lava flow has been the dominant process in stratigraphy evolution [18], we assume εtl ≈ εbl = 10 in present calculations. Following the SHARAD detection of low-density subsurface deposits [20] around the region of Amazonis Planitia, the power loss between interfaces varies in the range (-17 dB to -5 dB) and corresponds to (Λm / Λs) ~ 0.02 – 0.3; higher the power reflectance corresponds to a significant contrast between the surface and subsurface materials. In other work by Carter et al. [19], high dielectric basaltic interfaces are detected beneath lava flow fields northwest of Ascraeus Mons. In this case, the power loss refers to a range (–90 dB to –9 –7.5 –75 dB) and corresponds to Λm/ Λs ~ 10 to 10 ; this infers weaker dielectric contrast between the surface and subsurface materials. Through SHARAD literature [8-15, 18-20], we note Figure 4: Density plot: Estimate of the dielectric constant of the subsurface material εml as a function of layer thickness w using Eq.5. a significant variation in (Λm / Λs) values, and wide diversity The computations correspond to ν = 20 MHz and εtl = εbl = 10.0; the in the subsurface deposits – we consider this (Λm / Λs) range top and bottom panels refer to the lossless medium and decay rate of as reference for our calculations. 5dB/µs, respectively, while the color bar represents the magnitude of In order to include the influence of the wave attenuation, (Λm / Λs). we consider two cases with decay rates 0 (lossless medium), Table 2, the large dielectric permittivity values correspond to and 5 dB/μs (low-density materials, [18]) in the calculations. the high-density basaltic subsurface deposits, while the With these known parameters, Eq.5 is used to obtain a moderate dielectric constant values may correspond to the plausible parametric space between εml and w, illustrating the subsurface deposits of low-density material. As depicted, power ratio (Λm / Λs) – this has been shown in Figure 4. This depending on the subsurface thickness, the reflectance may relation can be used to envisage the suitable range of the take an optimum value – this may provide the best possible subsurface dielectric constant (εml) and subsequent thickness scenario of efficient detection of the subsurface echoes. In (w) for given (Λm / Λs). Figure 4 indicates that depending on Figure 5, we have derived the dielectric permittivity of the w, the subsurface dielectric constant may vary over a wide subsurface material as a function of (Λm / Λs) optimum value; range, and multiple such combinations (εml, w) are plausible the computations correspond to the optimal condition w = (2n for a given value of (Λm / Λs); this behavior is in conformance + 1) λml /4 and n = 15. For the y-axis range, εml ϵ (1.0 – 10.0) with the analytical prediction of Eq.4. The power reflectance corresponds to the subsurface layer thickness w ϵ (35 – 120) is noticed large for the low dielectric values. It is also noticed meters for the typical SHARAD parameters and optimal that the inclusion of finite attenuation leads the solution to a conditions. The increase in εml with (Λm / Λs)opt is physically a lower dielectric permittivity value in comparison to a lossless consequence of a decrease in the dielectric contrast of the medium for a given (Λm / Λs). This suggests that the dielectric surface – subsurface interface. Another way around, the effect and attenuation effects on the wave propagation and the signal also can be understood from the expression in Eq.4 by power transmission are complementary. increasing denominator magnitude corresponding to optimal In the context of SHARAD, for the known values of condition. The dependence of the optimum power reflectance (Λm / Λs) and decay rate from the SHARAD data analysis, decreases with an increase in the attenuation factor – this is these variants derived in Figure 4 can be utilized to depict the consistent with the results shown in Figure 4. This figure also
33 Physics News suggests that the intense reflections may infer to the southern mid-latitude regions on Mars [14]. This instrument subsurface materials having (i) low dielectric permittivity complements its companion instrument MARSIS, and both (low-density) or (ii) low attenuation coefficient. together are capable of providing unique insight about the Mars evolution. Based on the present understanding of the orbiter radars, multi-frequency operation of the instrument may be of significant means to complement the high penetration with better resolution in forthcoming projects – such sounding radars are capable of probing the icy moons of Jupiter/ Saturn and the cometary atmosphere. Acknowledgments The authors acknowledge the support from the Department of Space, Government of India, in accomplishing this work. References 1. Spaceborne radar remote sensing: Applications and techniques by Elachi Charles (IEEE Press, NY, USA, 1988) 2. S. P. Kingsley et al., Adv. Space Res. 23, 1929 (1999) 3. Radar Remote Sensing of Planetary Surfaces by Bruce A. Campbell (Cambridge, NY, USA, 2002) 4. E. Flamini et al., 4th International Workshop on Advanced Ground Penetrating Radar, Aula Magna Partenope, p. 246 Figure 5: Optimum value of the power reflectance (Λm / Λs)opt as a (2007) function of the subsurface dielectric permittivity (εml) using Eq.5 for 5. J. Mouginot et al., 39, L02202 (2012) w = (2n + 1) λml /4, n = 15, ν = 20 MHz and εtl = εbl = 10.0. The 6. R. Orosei et al., Science 361, 490 (2018) colored lines refer to different values of α, as marked in the figure. 7. R. Seu et al., Planetary & Space Sci. 52, 157 (2004) 8. R. Seu et al., Jour. Geophys. Res. 112, E05S05 (2007) Summary 9. N. E. Putzig et al., Icarus 204, 4432 (2009) In this article, we have briefly portrayed an overview of the 10. C. M. Stuurman et al. 43, 9484 (2016) concept and operation of the SHAllow RADar (SHARAD) 11. P. Choudhary et al., IEEE Geosci. & Rem. Sens. Lett. 13, 1285 (2016) onboard MRO spacecraft – the instrument is a powerful tool 12. I. B. Smith, & J. W. Holt, J. Geophys. Res. Planets, 120, 362 to analyze and understand the Mars geology, stratigraphy, and (2015) geographical evolution. We have discussed the SHARAD data 13. M. H. Carr, & G. G. Schaber, J. Geophys. Res., 82, 4039 (1977) analysis, graphical representation, and a plausible approach to 14. Plaut et al., Geophys. Res. Lett. 36, L02203 (2009) determine the dielectric properties of the subsurface material. 15. G. Alberti et al., Jour. Geophys. Res. 117, E09008 (2012) The analysis suggests that the subsurface reflections are the 16. Propagation of Electromagnetic Waves in Plasma by V. L. impression of dielectric properties of the subsurface material, Ginzburg (Gordon & Breach Science, London, 1961) and dielectric permittivity may be obtained using the sounding 17. Microwave dielectric spectrum of rocks by F. T. Ulabay et al. radar observations of the subsurface reflectivity. The specs of (Rep. 23817‐1‐T, Univ. of Mich. Radiat. Lab., Ann Arbor, 1988) the SHARAD instrument make it appropriate to refine the 18. M. N. Simon et al., Jour. Geophys. Res.: Planets 119, 2291 stratigraphy for the first few 100 meters beneath the surface – (2014) SHARAD has demonstrated its significance by discovering 19. L. M. Carter et al., Geophys. Res. Lett. 36, L23204 (2009) the ice layers around polar caps [9], and in the northern and 20. B. Campbell et al., Jour. Geophys. Res. 113, E12010 (2008)
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ANN based identification of tsunamigenic earthquakes using seismic waves
Ajit Kundu1, Pratap Mane1 and Siddhartha Mukhopadhyay1,2 1 Seismology Division, BARC, Mumbai, India 2 Homi Bhabha National Institute, Mumbai, India E-mail: [email protected]
Dr. Ajit Kundu joined as station reactor physicist at Propulsion Reactor Plant Division of BARC in 2000 through 43rd batch BARC training school. In 2010, he joined Seismology Division of BARC. Since then he has been working in the field of seismology. He obtained his Ph.D. from Homi Bhabha National Institute in 2018. His research interests include identification of tsunamigenic earthquakes and generation of synthetic seismograms.
Shri Pratap Mane is working as Scientific Assistant in Seismology Division, BARC. He has been involved in analysis of seismic events.
Dr. Siddhartha Mukhopadhyay is presently holding the post of Head Seismology Division, BARC. He did his graduation in Electrical Engineering from Calcutta University, M Tech from IIT Bombay and PhD from HBNI, Mumbai. His area of research includes Seismic Monitoring, NDT signal processing, Modelling of Multiscale Systems and Control Design.
Abstract Tsunami is one of the most destructive events in nature triggered primarily by undersea earthquakes. It has caused catastrophic devastations all over the globe, several times in the history of mankind. Since tsunami occurs suddenly, without any prior warning, lives of the coastal communities are put to extreme danger. As a remedy to this danger, tsunami warning methods have been developed over the years. However, the current methods are prone to large computational delay involved in the source parameter estimation using multistation data as well as too expensive and face difficulty in maintaining the coastal gauges and deep-ocean gauges used in monitoring the sea level data. To circumvent these difficulties faced by the existing tsunami warning systems a novel method of tsunami identification has been developed using seismic data of a single 3- component broad band station. In this technique, the 3-component amplitudes of seismic waves, namely, P, S and LR have been mapped with the earthquake category (i.e., tsunamigenic or non-tsunamigenic) using an artificial neural network (ANN) for a large number of past occurred earthquakes. Moreover, to speed up the decision making, the spectral amplitudes of P wave alone have been mapped with earthquake category using artificial neural network. This method not only reduces the decision making time but also cut down the expenditure involved in establishing and maintaining multiple stations.
Introduction of 1 to 6 km [1] whereas the wind driven beach waves travel at speed of about 10 m/s and have wavelength around 100 m Tsunami is a series of shallow water waves which inundates and period near 10 s. Tsunami wave height may not be great large area of landmass and is generated by an impulsive in the open sea (e.g. < 1 m), but it turns into a giant, rising as disturbance inside the oceanic water volume. Normal ocean high as 30 m at the shore whereas the regular wind generated waves are caused by the wind, weather, tides, and currents, beach waves have height of about 3 m. whereas tsunamis are powered by a geological force. The ocean waves involve motion of the uppermost layer of the Tsunamis evolve through three quite distinct physical water only, but tsunami waves move the entire water column processes: (1) generation by a force (earthquake, volcano, from surface to seafloor. Tsunami waves travel with wave submarine landslide etc.) that disturbs the water column, (2) speeds of 0.1 to 0.24 km/s with wave periods of 100 to 2000 s propagation from deeper water near source to shallower and wavelengths of 10 to 500 km in water depths coastal areas, and (3) inundation of dry land. The present
35 Physics News article deals with identification of tsunamigenic earthquake. It parameter for tsunami identification. Subsequently, tsunami may be noted that the tsunamigenic earthquake refers to an warning systems are being developed which consider both earthquake which occurs at a fault located near or beneath the magnitude and focal mechanism along with sea level sea and has the potential to generate tsunami. Past records monitoring. Currently, there are a few such world-wide indicate that 75% of tsunamis are generated by shallow tsunami warning systems like Pacific Tsunami Warning undersea earthquakes. Tsunamigenic earthquakes are usually Centre (PTWC) and couple of regional warning centres like generated at the lithospheric plate boundaries which are of JMA, IOTWS etc. A typical tsunami warning procedure (with three types: divergent, strike-slip or transform and convergent. timeline) as followed by PTWC is shown in Figure 2. Depending on the type of boundary, two lithospheric plates can separate from each other, slip along each other or override each other. The subduction type of convergent plate boundary is known to have the largest potential of generating tsunamis. Tsunami generation is affected by the seismic moment, focal mechanism, focal depth and other factors. The earthquake size is expressed in terms of seismic moment, M0 = DA , where , D and A are rigidity of rocks, average slip and fault surface area, respectively. It is known that larger the earthquake’s seismic moment, the larger is the tsunami, if all other conditions remain same. Focal mechanism specifies the orientation of the earthquake fault and the direction of slip on the fault plane with faults idealized as rectangular planes.
Three angles the strike ( s ), the dip ( ) and rake ( ) determine the type of faulting and the direction of tsunami propagation (see Figure 1). Tsunami occurs mainly due to the thrust fault (at convergent boundary) that can cause vertical uplifting of the water column above the plate while the strike- slip faults, which involve no vertical displacement, are less likely to produce tsunamis.
Figure 2: A typical tsunami warning procedure with timeline When an earthquake occurs the preliminary parameters such as epicentral location, depth and magnitude are computed from the seismic data available at nearest stations. If depth of the earthquake is less than 100 km and magnitude exceeds 6.5 Figure 1: A thrust fault showing strike, dip and slip angles a preliminary tsunami threat message is issued typically 8 and the slip vector. minutes after its occurrence. To confirm the magnitude based When an earthquake takes place in the subduction zone warning, sea level data is monitored continuously and side by with thrust fault both seismic as well as tsunami waves are side W-phase Centroid Moment Tensor (WCMT) analysis is generated. The seismic waves travel through the earth whereas performed. Obtaining the more accurate earthquake the tsunami waves travel through the water body. Seismic parameters from the sophisticated moment tensor analysis a waves which travel much faster than tsunami waves are of two tsunami forecast model is run over the specific domain to types: 1) body waves (i.e., P, S, etc) and 2) surface waves (i.e., forecast the coastal amplitudes typically after 35 minutes after LR, etc). The fastest moving wave is P wave (6-8 km/s) the origin time. By that time, if one or two coastal and deep- followed by S (4-6 km/s) and LR (3-4 km/s). Since seismic ocean gauges measure the tsunami amplitudes, the forecast waves travel much faster than tsunami waves researchers have amplitudes produced by the models are adjusted with the been developing tsunami warning methods [2-14] based on observed data and the tsunami warning is upgraded or seismic waves. However, most of the methods depend heavily downgraded accordingly. However, this confirmation on magnitude estimation and are prone to false alarming procedure takes at least 35 minutes from the origin of because they do not consider focal mechanism, a key earthquake and may be prohibitively long for issuing alert at
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nearby coastal regions. Even though the more accurate Sumatra during 25 Oct 2000 to 23 Jan 2018 have been used. estimation of earthquake’s location, depth, magnitude and A typical record of 3-component seismic data is shown in focal parameters using WCMT analysis complements the Figure 4 where the onsets of prominent phases (P, S and LR) preliminary estimation of tsunami threats, it requires the have been marked by vertical lines. The basis for ANN based involvement of multiple stations which may not be accessible estimation of earthquake category using seismic data is stated by the poorly instrumented regions of the world. Moreover, as follows. coastal gauges and deep-ocean gauges which are used to monitor sea level data to measure tsunami amplitudes have their own limitations. For example, tide gauges which are generally installed near the coast and are used as confirmation of tsunami could detect the tsunami only after the wave reaches near the coast and no time is left for warning. Alteration of local seafloor bathymetry and harbour shapes could also affect the tide gauge performance. The open sea buoys which are being used to detect the passage of tsunamis through deep seas are too expensive and difficult to maintain. Furthermore, a tsunami wave in deep-ocean has very small wave amplitude and a long wavelength making its detection difficult.
The time delay of about 35 minutes in decision making by the existing tsunami warning method can be minimised by Figure 4: A typical waveform of 3-component seismic data adopting a novel method of tsunami warning which is recorded at PALK. E, N and Z corrrespond to East, North and presented in this article. vertical component of the seismogram. Method The far-field P wave displacement for a double-coupled point source at position ξ (ξ ,x ξ y ,z ξ ) with a moment rate function To identify tsunamigenic earthquakes a two-way approach has been studied as shown in Figure 3. In the first approach, Mt0 () in an infinite homogeneous elastic medium at receiver namely, temporal amplitude mapping, the rms amplitudes of position x (x, y, z) and time t is given by [15]
1 RrP ulP (r , tMt )()=− (1) X 3 0 4vP rvP
where is density of earth at source, vP is the P wave velocity, r =−X ξ is the source-receiver distance and RP is the P wave radiation pattern which is expressed as [15]
P2 Ri=cos sin sinξ sin 2( − s ) − cos cos sin 2iξ cos( − s ) (2) 2 2 2 + sin sin 2 (cosiiξξ − sin sin ( − s ))
+ sin cos 2 sin 2iξ sin( − s )
where , , , and i are strike, dip, rake, azimuth and s ξ Figure 3: A two-way approach for identifying tsunamigenic take-off angle at source position ξ . The P wave direction l is earthquakes using seismic data and ANN. given by [15] ˆˆˆ P, S and LR waves are fed to ANN-I along with distance, l =siniξ cos x + sin i ξ sin y + cos i ξ z (3) azimuth and magnitude parameters to estimate the earthquake category T/NT (i.e., Tsunamigenic/Non-tsunamigenic). On where xˆ , yˆ and zˆ are unit vectors which could be chosen the other hand, in the second approach, namely, spectral as North, East and Vertically upward directions respectively. amplitude mapping, the spectral amplitudes of first coming P The time depended moment function Mt0 () is written as [15] wave are fed to ANN-II to estimate the earthquake category. Mtu( t)(= A )( t ) (4) To demonstrate the efficacy of the methodology, broad-band 0 seismic data recorded at PALK (Pallekele, Sri Lanka) station where is rigidity of the earth at source, ut()is the slip provided by IRIS (Incorporated Research Institutes for Seismology) for earthquakes of magnitude 6 and above function and At()is fault area. The P wave displacement originating from one of most tsunami prone area, namely, amplitude can be obtained from Eq (1) as
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P Using this relation, Eq. (9) takes the form PP 1 Rr urtrtMtXX(,)(,)()==−u 0 (5) 3 P 4vP rvP CRL urtwutBtTP (,)()(,)= (11) X 3 R In reality seismic source is not a point one but has finite 4vP rTR dimension. In order to take the finiteness of the source, it can be considered as one dimensional Haskell source [16] as L where BtTHtHt(,)(()())R =−− is boxcar function of shown in Figure 5 where w is fault width and L is fault length. vR duration . If the slip is assumed as a ramp function as given below
=0;0t u uttT();0t== D (12) TD =utT; D
where u is the final slip and TD is the rise time, Eq. (11) can be rewritten as
CM RP BtTBtT(,)(,) urtP (,) = 0 DR X 3 (13) 4v rTT DR Figure 5: 1-D Haskell source model [16]. P
is considered to be consisting of number of fault segments where M w0 L(= u ) is seismic moment and B (t,T ) D is a each of length dx and fault rupture may be assumed to move boxcar function of duration TD . It may be noted here that with a constant velocity . Further, if the fault is assumed as vR moment magnitude Mw is related to M0 by [18] long and narrow one, it may be treated as a series of small segments that individually approximate point sources. 2 MMw =−log10.70 (14) Therefore, P wave displacement amplitude due to finite source 3 can be written using Eqs. (4) and (5) as Eq. (13) can be considered as the guiding equation for L identifying tsunamigenic earthquakes following the earlier 1 RxP urP ( tu , twdx )()=− mentioned two-way approach as follows. X 3 (6) 4v rv R P 0 Temporal amplitude mapping: The slip rate ut()can be written as Eq. (13) holds similar forms for S and LR waves with corresponding change in radiation pattern, wave velocity and xx u()( tu−=− )() tdt (7) time dependent boxcar functions. Thus, the rms velocity vvRR amplitude of a seismic wave can in general be computed using Eq. (13) as where d denotes Dirac delta function. Using Eq (7) and ii x v==rms [v ( r , t )]; i 1, 2, 3, ..., n (15) assuming zt=− , Eq.(6) can be expressed as X,rms X v R where the index i runs for number of seismic waves (n) and t v ( )r ,t is velocity amplitude corresponding to the 1 RP urtP (,)()()= wvut dzdz (8) displacement amplitude u(r,t) defined by Eq. (13) and (15) X 3 r R 4vP L t− indicate that rms velocity amplitudes of a seismic wave carry vR the combined influence of magnitude, rise time, rupture time and rupture orientation. As the earthquake category (T/NT) Since d() z dz is a Heaviside step finction Hz(), Eq.(8) can changes these amplitudes do not change linearly. To visualize this fact the 3-component rms velocity amplitudes of three be written as distinctly identified waves, namely, P, S and LR have been 1 RLP computed over time windows of 5, 10 and 120 sec [19] for uP (,) r t= wv u ()[()( t H t − H t − )] X 3 R (9) 120 events and plotted in Figure 6. 4vP rvR From this figure it is seen that the rms amplitudes of all the Rupture velocity is related to rupture time TR by [17] three waves for both tsuamigenic and non-tsuanmigenic earthquakes are not visually discriminated except for very few L vR events. This complex relationship between the seimic phase TCCR =; = 1 − cos (10) vvR P amplitudes and the earthquake category suggests the use of a
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non-linear mapping technique to discriminate the where F stands for Fourier transform. The attenuation tsunamigenic earthquake from non-tsuanmigenic one. spectrum can be taken as [20]
fr Bf()exp=− (18) vQfP () where Qf() is the attenuation factor along the path to station. If the instrument response is removed from the observed P displacement uX (,) r t , Eq. (16) can be expressed using Eqs. (17) and (18) as
P CM0 Rsin( fT ) sin( fT ) fr A( f )=−DR exp R ( f ) X 3 r fT fT v Q() f 4vP DRP (19) In practice, the displacement spectra defined by Eq. (19) can be obtained from the velocity seismogram by taking the Fourier transform of the output of the velocity sensor and Figure 6: Plot of 3-component amplitudes (rms) of (a) P, (b) subsequently dividing the spectra by angular frequency . S and (c) LR waves. Red solid circle corresponds to Eq. (19) essentially shows the combined effect of magnitude, tsunamigenic earthquake and blue hollow circle corresponds rise time, rupture time, rupture orientation, propagation path to non-tsunamigenic earthquake. and receiver site as well as azimuthal dependence on spectral amplitudes which in turn is governed by the earthquake In view of this, the rms velocity amplitudes (3-component) of category (T/NT). For visualization, the 3-component P wave P, S and LR waves have been mapped with the earthquake displacement spectral amplitudes of 120 earthquakes (both category using an ANN (say ANN-I in Figure 3) for past tsunamigenic and non-tsunamigenic) that occurred in Sumatra recorded earthquakes to estimate the category of new have been computed in the frequency range 0.006 Hz to 5 Hz earthquake originating from the same study region. During and are shown in Figure 7. It may be noted that the maximum this mapping location (i.e., epicentral distance and back- delay in computation time corresponds to the maximum delay azimuth) and magnitude parameters have been fed as in decision making. In this case decision making is as fast as additional inputs to ANN-I to ensure that mapping gets 3 minutes after the arrival of P wave at the recording station. explicit importance for the source-station path and magnitude. In Figure 7, red and blue spectra It may be noted here that the non-linear relationship between temporal amplitudes and earthqualke category can be captured more efficiently by ANN-I if more number of seismic phases are involved. However, this may involve more computational delay due to arrival delay of waves coming later. This constraint can be diluted to a greater extent if spectral amplitude mapping technique is used as stated in the following subsection. Spectral amplitude mapping: Though the temporal amplitude mapping eliminates the constraint of multistation data, it still consumes computation time of about 5 min after the arrival of LR phase. To speed up the decision making, use of only P wave has been considered as stated below. The 3-component displacement spectrum
AX (f) of P wave observed at far- field station is generally modelled as [20] Figure 7: Plot of 3-component P wave displacement spectral amplitudes. Red and blue spectra correspond to tsunamigenic AXX (f) = S (f)I(f)B(f)R(f) (20) and non-tsunamigenic earthquakes whereas green spectra indicates nontsunamigenic earthquakes with magnitude where S(f) , I ( f ) , B(f) and R(f)are far-field 3-component P X greater than 7.0. wave displacement source spectrum, the instrument response, the attenuation spectrum and the site response respectively. which correspond to tsunamigenic and non-tsunamigenic can be obtained by taking Fourier transform of Eq. (13) earthquakes respectively, are fairly distinguishable in all the three components except for few events whereas the green as spectra which correspond to non-tsunamigenic earthquakes CM RP sin(fT ) sin( fT ) with magnitude greater than 7.0 make the boundary between S( f )== F ( uP ( r , t )) 0 DR XX 3 (17) tsunamigenic and non-tsunamigenic events ambiguous. 4vP r fTDR fT Although grossly separable, this mixing nature in the spectra
39 Physics News of tsunmaigenic and non-tsunamigenic earthquakes indicates back-azimuth and magnitude and the number of hidden neuron that their discrimination by visual inspection in not possible. is 3. On the other hand, there are 27 input neurons However, it is perceived that, the discrimination can be made corresponding to 9 displacement spectral amplitudes for each possible if the spectral amplitudes are mapped with earthquake of 3-components and 2 hidden neurons in ANN-II. The dataset category training an ANN (say ANN-II in Figure 3) for past (of 120 earthquakes) has been divided into three parts with recorded earthquakes in a given source-station pair. A very standard data division ratio of 75:15:15 for training, validation brief introduction to ANNs as function mapping is given and testing respectively. The results obtained by ANN-I and before discussing the results. ANN-II using the same dataset are discussed below. Function mapping using ANN Results ANNs are computational systems consisting of many neurons Both ANN-I and ANN-II produce the same results as shown which are connected in parallel structure. The parallel in Figure 9. The results have been presented in the form of computational structure of an ANN, besides resembling the biological brain, has potential application in areas where the number of input parameters is often large and well defined analytical solutions are not available. The architecture of a three-layer feed forward neural network with I nodes in input layer, H nodes in hidden layer and Q nodes in the output layer has been shown in Figure 8. The function approximation by such an ANN in parametric form [19] can be expressed as HI fwgwxkQˆ ==,1,2,..., (20) kjkjiji ji==11
th Where the weights wij connect i neuron in the input layer to th j neuron in the hidden layer, the weights wjk connect neuron in the hidden layer to the kth neuron in the output layer. g (.) represents the activation function in the hidden j layer. It may be noted that although the decision is binary (i.e. T or NT) in the present context, ANN inherently establishes a non-linear map between temporal amplitudes/spectral amplitudes and earthquake category through function approximation as defined in Eq. (20).
Figure 8: Typical architecture of a three-layer feed forward neural network. The number of output neuron for both ANN-I and ANN-II is one. The activation function for output node is linear whereas that for hidden layer it is hyperbolic tangent sigmoid. The chosen training algorithm is Levenberg-Marquardt backpropagation algorithm. However, the number of inputs for the two ANNs are different. For ANN-I the number of Figure 9: Confusion matrix for (a) training (84 earthquakes), input is 12 which corresponds to 3-component rms velocity (b) validation (18 earthquakes) and (c) testing (18 amplitudes of P, S and LR along with epicentral distance, earthquakes) ANN-I and ANN-II.
40 Physics News confusion matrix. In Figure 9, 0 and 1 represent NT and T References category respectively, the rows correspond to the predicted 1. L. L. Noson et al., Washington Division of Geology and class (Output class) and the columns correspond to the Target Earth Resources Information Circular, 85, 77 (1988) class. The first two diagonal cells (green) show the number 2. Dziewonski et al., J. geophys. Res., 86, 2825 (1981) and percentage of true classifications (i.e. True Negative (TN) 3. Okal et al., J. geophys. Res., 94 4169 (1989) for class 0 or True Positive (TP) for class 1), the off-diagonal 4. Hiyoshi et al., Bulletin of the Seismological Society of cells (pink) correspond to number and percentage of false America, 82, 2213 (1992) classifications (i.e. False Negative (FN) for class 0 or False 5. Okal et al., Geophysical Journal International, 152, Positive (FP) for class 1). The far right two cells (light grey 416 (2003) coloured) represent negative predictive value 6. Lockwood et al., Geochemistry Geophysics Geo- (NPV = TN/(TN+FN))/false omission rate systems, 7 (2006) (FOR =FN/(TN+FN)) and positive predictive value 7. Chew et al., Journal of Asian Earth Sciences, 36, 84 (PPV =TP/(FP+TP))/false discovery rate (2009) (FDR =FP/(FP+TP)) respectively. The far bottom two cells 8. Chamoli et al., Geophysics, 17, 569 (2010) (light grey coloured) indicate true negative rate 9. Sipkin, S. A., Geophys. Res. Lett., 21, 1667 (1994) (TPR =TN/(TN+FP))/false positive rate (FPR =FP/(TN+FP)) 10. Tsuboi et al., Bulletin of the Seismological Society of and true positive rate (TPR =TP/(FN+TP))/false negative rate America, 85, 606 (1995) (FNR =FN/(FN+TP)) respectively. The diagonal bottom cell 11. Tsuboi et al., Bull. Seismol. Soc. Am, 89, 1345 (1999) (dark grey coloured) represent accuracy 12. Lomax et al., Geophys. J. Int., 170, 1195 (2007) (ACC =(TN+TP)/(TN+TP+FN+FP))/error rate 13. Lomax et al., Geophys. J. Int., 176, 200 (2009) (ERR =(FN+FP)/(TN+TP+FN+FP)). From all the plots 14. Lomax et al., Pure and Applied Geophysics, 22 May (corresponding to training, validation and testing set), it is (2012) observed that the negative predictive value, positive predictive 15. Quantitative Seismology by K. Aki and P. G. Richards value, true negative rate and true positive rate are 100% (W. H. Freeman and Company, USA, 1980) whereas the false omission rate, false discover rate, false 16. Modern global seismology by T. Lay and T. C. Wallace, positive rate and false negative rate are 0% each leading to (Academic Press Inc, USA, 1995) 100% accuracy and 0% error rate or misclassification rate. It 17. An introduction to seismology, earthquakes, and earth has been observed that both the networks yield the same structure by S. Stein and M Wysession (Blackwell results with 100% accuracy. Publishing Ltd, USA, 2003) 18. Hanks et al., Journal of Geophysical Research 84, 2348 Conclusions (1979) A novel method for prompt identification of tsunamigenic 19. Kundu et al., Physics of the Earth and Planetary earthquakes have been studied with two different approach Interiors, 259 10 (2016) yielding the same outcome. However, the spectral amplitude 20. Of poles and zeros, Fundamentals of digital seismology mapping technique is more appealing since the decision- by F. Scherbaum, (Kluwer Academic Publishers, USA, making time is less due to use of the first arriving P wave in 2001) the seismogram.
Acknowledgements Authors are thankful to United States Geological Survey (USGS) for providing earthquake parameters and IRIS for uploading the seismic data in their website.
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News & Events Celebrating Science… GIPWG Since 2016, United Nations (UN) has declared 11th February Black Holes’. The event was organized by the Science as “International Day of Women and Girls in Science”. This Association of the college and the Women Empowerment Cell year on this occasion, the UN Secretary-General António of the college and in collaboration with Jana Vijnana Vedika, Guterres, gave an important message - “To rise to the Vijayawada. Dr. S. Kalpana and Dr. G. Meenakshi as well as challenges of the 21st century, we need to harness our full Dr. T. Sri Kumar (Andhra Loyola College) and Shivaprasad potential. That requires dismantling gender stereotypes. On from Jana Vijnana Vedika deserve a special mention for this International Day of Women and Girls in Science, let’s organization and initiative. pledge to end the gender imbalance in science.” In India also, the theme on the science day 2020 was “Women in Science”. The GIPWG (Gender In Physics Working Group) of IPA has assisted in organization of special events at different places in the country to mark these special occasions. A brief report of some of these events is presented in the following. At TIFR, a special colloquium by Prof. Supraksh Roy, “Bibha Chowdhuri: A Scientist Incognito” was organized on 11th February. Prajval Shastri giving the talk titled ‘The Thrill from Black Holes’. Science Day lecture at CUTN The event began with the seminar given by astrophysicist Prof. Prajval Shastri. She introduced the concept of black holes and Dr. V. Madhurima, Professor of Physics at the Central the story of their discovery in the cosmos. She described how University of Tamil Nadu (CUTN) and a member of IPA, gave the findings of Cecilia Payne Gaposhkin (what stars were a talk on “Women in Physics” in the Physics department on made of), Jocelyn Bell (discovery of the first compact dead the occasion of Science Day. The CUTN as a whole and the stars) and Ruby-Payne Scott (the technique of interferometric department of Physics in particular have a predominantly imaging) laid the foundation for building this story. The female student population, of over 65%. The talk focused not discovery of the giant black hole in the centre of the Milky only on the challenges faced by women in progressing in Way using the Keplerian orbits of its central stars was made Physics but also on the remedial measures and resources by the team of Andrea Ghez. In the discovery of the silhouette available for them in India. Post-talk, many of the faculty and of a giant black hole in the galaxy Messier 87 by the Event research scholars mentioned that they were unaware of the Horizon Telescope, one of the main algorithms was written by existence of forums such as GIPWG and WIS (Women In the team of computer scientist Katie Bowman. All the science Science) in India and also that there are ear-marked students, professors and members of the Jana Vijnana Vedika fellowships for women with career breaks. then set out on the walk for science (to increase public awareness) on the streets around the college campus with a banner and placards in Telugu that highlighted this year’s theme of Science Day of ‘Women in Science’. The walk was followed by a demo of an electronic device built by the students to send SOS messages in moments of risk using a simple Arduino Nano board, a GPS antenna, GPS receiver and a GMS modem. Science Day lecture at PRL Physical Research Laboratory arranged a talk titled "Women in Science: Challenges and way forward" by Prof. Srubabati Goswami. She discussed the status of women in science specially in Physics and pointed out the Science day Lecture at Dept. of physics, CUTN gender gap specially as one moves from PhD to Faculty positions to women in leadership roles. She discussed what Science Day event at Vijaywada can one do to address it at an individual level and also the The Sri Durga Malleswara Siddhartha Mahila Kalasala various schemes and initiatives by Government agencies. She women’s college, Vijayawada, celebrated National Science emphasized on role models for inspiration and discussed how Day with an astrophysics seminar titled ‘The Thrill from women in PRL are working in various fields.
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The Hyderabad Charter for Gender Equity in Physics
This Hyderabad Charter is the outcome of the work of the The key recommendations, listed below, include work-life GIPWG since its inception and of the deliberations of the balance policies that are gender neutral, transparency in Pressing for Progress 2019 conference by the IPA hosted by criteria for hiring with no hidden norms, self-declaration of the University of Hyderabad, which was a first-of-its-kind sexual misconduct indictments in applications, diversity inter-disciplinary national conference in which 240 physicists, experts as observers in committees and editorial boards, social scientists and educationists gathered for three days to mentoring mechanisms for young faculty, gender sensitization bridge disciplinary divides, and to deliberate upon topics in training for all, courses on the social processes in science both physics and social processes in physics practice. practice in physics curricula and adoption of the IUPAP guidelines for conferences.
For Institutions and Departments
1. Work-life balance policies, such as child-care leave and “mobility schemes” should be gender neutral 2. Criteria for hiring should be formulated beforehand, and no hidden norms or criteria should be used 3. The age-bar for hiring should be removed 4. The hiring process should provide full information on all the steps and time-line of hiring to all candidates 5. Status/position/background of a life-partner should not be criteria in hiring 6. Hiring processes should have a wait-list so that ‘likelihood of joining’ is eliminated as a criterion in selection 7. Self-declaration of sexual misconduct indictments should be mandatory for staff applications 8. Institutions should invest in diversity officers as observers on selection, hiring and promotion committees 9. Policies that facilitate spousal hiring, employment in the neighbourhood and/or transfer should be formulated 10. Gender sensitization training should be mandatory, especially for senior management, directors and deans 11. Mentoring mechanisms for young faculty must be made available within institutions 12. Child-care facilities must be mandatory in institutions and preferably subsidized 13. Grievance cells should include at least one external member who is a gender equality expert 14. Current and potential members of ICCs should undergo training in the legal aspects of sexual misconduct 15. Safeguards should be formalized to protect members of ICCs from intimidation and harassment 16. Action-taken reports and statistics of sexual misconduct enquiries should be filed mandatorily 17. The sexual harassment policy should include guidelines for ensuring awareness among all concerned 18. Do’s and Don’ts for a healthier environment should be publicized and also reviewed regularly 19. Mandatory gender audit of staff at all levels should be published on the organizational webpage 20. Institutions should adopt gender neutral language in forms, documents, publications and daily practice Additional Recommendations for Physics Teaching 21. A critical review of multiple-choice based tests to short list research scholar candidates should be done 22. A sociology course on social processes in science practice should be part of the graduate physics curriculum 23. Concerted efforts must gender-balance role models in physics text books and pedagogic multi-media material Additional Recommendations for Conferences 24. All physics conferences should adopt the IUPAP guidelines for conferences 25. Funding support for conferences should be contingent on adoption of the guidelines 26. Child-care facilities must be mandatory in conferences and preferably subsidized 27. A priori compliance with conference guidelines should be required from all conference host institutions Additional Recommendations for National Agencies 28. Diversity officers should be appointed as observers on editorial boards, nomination committees and funding agency committees 29. Self-declaration of sexual misconduct indictments should be mandatory for all positions of administrative responsibility and leadership, including academy fellows, editorial board memberships and project funding committee memberships
Visit https://www.tifr.res.in/~ipa1970/gipwg/index.php for more information
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News & Events: Book Review Dr. B.V. Sreekantan-A Pioneer Physicist The book on Professor read on, what emerges beyond Sreekantan’s science of this B.V. Sreekantan written by Professor phase, is the spirit of adventure, together with his can-do P.R. Vishwanath, carries the approach; and the hard work, always the hard work. He thus prominent sub-title “A Pioneer laid the foundations for the remarkable success of the TIFR- Physicist”, and elsewhere on the KGF group from the 1950s all the way until the mid-1990s, as cover describes the text as A they contributed to aspects as diverse as Fundamental Scientific Biography. The book Interactions at high energies, neutrino physics and baryon certainly succeeds in conveying the non-conservation. Through this period the KGF group pioneering aspect of Professor attracted and retained international collaborators (from Osaka, Sreekantan’s early work at TIFR, on Japan, and Durham, U.K.) – the physics was at the frontiers. various aspects of Cosmic Ray In the 1950s, soon after his Ph.D. (Bruno Rossi the external Physics and X-ray and Gamma-ray examiner), Sreekantan spent considerable time in Cosmic Ray Astronomy. On the other hand, it is Laboratories in Europe and the US. Besides building more than the biography of a single individual, becoming also confidence in his own (and TIFR’s) skills at the frontiers of a biography of other individuals and an institution. the field, this sojourn also gave Sreekantan scientific The book begins (as all good things must), before the comrades for life, impressed as they were with his abilities and beginning, taking the reader into the small town of work ethic. His next scientific frontier thus very naturally Nanjanagudu, to the south of Mysore city, in the early 1900s. became the newly emerging fields of “high-energy It was here that Badanaval Venkatasubba Sreekantan was born astronomy” – in the early 1960’s he worked with the MIT in 1925. His father, Shri B.V. Pundit, was a self-made man – group of Rossi, Giacconi and others, including on the first a well-known ayurvedic physician, and a very successful rocket-based detection of a celestial X-ray source. Upon return entrepreneur, with a best-selling tooth powder formulation to India, he started groups working on X-ray and Gamma-ray that brought substantial wealth for the family. The young Astronomy, first through balloon-borne payloads flown from Sreekantan thus grew up in that happiest of settings – freedom the TIFR Balloon Facility at Hyderabad, and subsequently from want, together with an early exposure to the highest through dedicated instruments on satellites. These efforts have levels of intellectualism through books, discussions and been attended by success, in some cases quite spectacular. discourses, within the family and with visiting scholars hosted The book dwells on yet another frontier that Sreekantan by the family. Professor Vishwanath, with roots in similar explored, and that was in his role as Director of TIFR – the settings, empathetically evokes the atmosphere of the Pundit builder of new facilities and new scientific horizons. Under his household, and the idealism of the India that was rising to watch TIFR grew in many different directions, some of them Independence. The formative years of the young Sreekantan, unconnected with Cosmic Ray Physics, High Energy while rooted in Hindu tradition, Hindu scripture and Hindu Interactions and High-energy Astronomy. Often, this was philosophy, yet created the self-confidence (and thirst) in him because of Sreekantan’s providing a gentle prod at the right to look beyond, and assimilate wider streams of human time, and at all times because of his ensuring that there was an thought. Foremost among these was the desire to pursue a enabling atmosphere. All his life, Sreekantan was a man with career in science, something he did with remarkable success. a gentle touch with humans, and this emerges vividly through a device extensively employed by Vishwanath through the Sreekantan joined TIFR as a PhD student with Bhabha in book – to allow the story to be filled in by other people in their 1948, after a post-M.Sc. year spent learning high-speed own words. Readers are thus acquainted with BVS’s qualities electronics in the Communications Engineering Department of character, his remarkable equanimity even during scientific of IISc. Bhabha must have gauged his abilities very quickly, failures, his warmth, kindness and concern for all who worked for he put him up to performing a state-of-the-art experiment with and for him. Professor Vishwanath speculates that much to measure the lifetime of the muon, with everything to be of this would have been seeded in BVS’s early life, living, built by him. The 1951 paper by Sreekantan (sole author) sharing and caring within a large family. would do credit to a graduate student even today; at the time, it was one of the most precise measurements of the muon Are there flaws in the book? It is didactic, perhaps overly so. lifetime, and did a lot to broadcast Sreekantan’s capabilities – As the author reveals in his Foreword, he wanted to connect skill with electronics, careful design and building of with young people of today with an interest in science, and experimental apparatus from scratch, and sophisticated data therefore it was his intention to write in an accessible way. analysis. In Vishwanath’s telling, Bhabha had found a young This does happen with flair in some of the “Boxes”, but not all “pioneer”, and BVS’s next frontier was to address the problem of the writing has this character. There are several instances of of the “hard component” in Cosmic Rays, by studying the abbreviations being used much before their full forms are intensity of muons underground in the mines of KGF. As we explained. There are long digressions into developments in
44 Physics News fields that Sreekantan had seeded in TIFR, but which he Yet, I would not dwell long on these flaws. There is a quality moved out of long ago too, the later developments therefore of warmth to the writing that arguably comes from the author’s out of place in the “biography” of an individual. Better personal connect with (and respect for) his subject and his editorial oversight would have also lifted the overall quality – work, and this is uplifting. His unconventional device of removal of typesetting inconsistencies, errors of spelling etc. inviting third party reminiscences adds further to the warm Some errors of a scientific nature also exist – for instance “family-feeling”, something that Sreekantan did so much to accelerators of Cockcroft-Walton type are not linear engender within the scientific groups that he created, as also accelerators. The description of Sreekantan’s work of his last in the Institute that he led with distinction. The pages of the two decades – on consciousness and his speculations thereof book are full of gems that a wide spectrum of people can learn – is inadequate. Two missing features, whose presence would from. And most especially, the book is the celebration of an have helped enormously for the mature scientific reader are an eminent man of science, one who was dedicated to working at Index, and numbered citations to specific references that the frontiers of human activity but always with an eye on support specific points made in the text. doing it all “the Indian way”. N. Krishnan [email protected]
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Meet the Physicists!
The January-March issue covers both the UN’s International Day for Women and Girls in Science (Feb. 11) and International Women’s Day (Mar. 8). We profile 4 enthusiastic physicists across the country who seem to be enjoying their diverse careers!
Sadaf Jethva Sadaf Jethva Asst. Professor, Dept. of Nanoscience and Advanced Materials, Saurashtra Univ., Rajkot, Gujarat Area: Materials Science (multiferroic oxides) Years doing physics: 12 What I like about physics: the beauty of experiments Beyond physics: cooking, listening to music, traveling
Joyee Ghosh Professor, Dept. of Physics, IIT-Delhi Area: Quantum Optics/Photonics Years doing physics: 22 What I like about physics: The power of light and optics – from single photon interactions to using squeezed light to look at objects in the universe light years away Beyond physics: too many to list! Aryan, my baby, travelling, driving, music, books, movies … Joyee Ghosh Food T.V. Banumathi Professor and Head, Dept. of Physics, Sri GVG Visalakshi College for Women, Udumalpet, Tamil Nadu Area: I’ve taught almost all UG and PG courses! Years doing physics: More than 40 What I like about physics: It’s a part of our life, experiments help better understand concepts Beyond physics: A Carnatic music singer and veena player, pencil sketching, enjoy badminton and basketball
T.V. Banumathi Moon Moon Devi
Moon Moon Devi Asst. Professor, Tezpur Central University, Assam Area: High Energy/Astroparticle physics, Instrumentation Years doing physics: 14 What I like about physics: It helps understand how the universe works on from the smallest (sub-atomic) to the largest (entire visible universe) scales Beyond physics: music, reading and travelling
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Pandemics, Public Health, and Physics
Arnab Bhattacharya Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India E-mail: [email protected]
Till a few weeks earlier, a physicist would have associated simple laws of physics, modelling how human beings behave the word “corona” to the outer layer of the sun, the high- necessitates inputs on social, cultural, and political factors and voltage discharge around a conductor, or even perhaps the their interactions where the underlying equations to solve beautiful crown formed during the splash of a liquid drop. aren’t simple, or often unknown! Of course, the increase in Astronomers may have extended it to the names of the two computing power, and the mind-blowing availability of data constellations that bear the name (though they look more like today about human interactions (often captured without our semi-circular arcs than any crown). Unfortunately, today the even knowing, through our smart phones and activity trackers) world knows this word thanks to the sinister coronavirus is making modelling of epidemics an increasingly tractable disease now called COVID-19, a global pandemic that has problem. caused a major international health crisis. The other aspect one cannot avoid noticing is the stunning In the 14th-century, the “Black Death” plague took three visual depictions of data, both actual and simulated, as the years to slowly spread from western Turkey to the rest of epidemic unfolds. In fact, historically, innovations in data Europe, killing more than a 100 million people. Modern visualization techniques were spurred by the need to display transportation methods radically speeded up the spread of mortality rates during annexations and epidemics. From worldwide pandemics – in 1918, influenza took about one year Charles Minard’s depiction of Napoleon’s disastrous Russian to spread from its European source to isolated Pacific islands, campaign to John Snow’s map of water pumps in a London infecting almost 500 million people across the world. In the cholera outbreak, these impactful “info-graphics” are now amazingly interconnected 21st century we are glued to online- considered classics. Closer home, the one person we have to trackers that show the rapid spread of the coronavirus outbreak acknowledge in setting up public-health infrastructure in India in near real-time – which in a matter of weeks has now is the doyen of statistics Florence Nightingale, who came up affected over 140 countries. with new ways of displaying data e.g. coxcomb or pie charts, much overused today. (In fact, the journal of the Data While any pandemic is a major public-health challenge, Visualization Society is aptly called Nightingale). More apart from impressive advances in medical science, we important, she was also persistent in obtaining and analysing additionally have an increasingly important tool in on-the-ground data from across India that was key to the first understanding and arrest the spread of an epidemic: large-scale and effective “Swachh Bharat” movement – Indian computational modelling. And a lot of this has its origin in sanitation reforms the late 19th century. techniques used by physicists – be it in computational fluid dynamics, lattice gauge theory, plasma physics, statistical As the COVID-19 outbreak brings Gaussians showing physics of reaction-diffusion processes, ab-initio materials “flattening the curve” and debates about power-law vs modelling etc. Of course, it isn’t a simple problem to tackle. exponential fits into everyday WhatsApp groups, it is a good Physicists might be familiar with the computational tricks time to wash-our-hands-with-soap-for-20-seconds but not needed when simulating say 7.5 billion atoms in solving a wash one’s hands of the prospects of computational physics molecular dynamics problem, however, simulating the methodologies being an ever-more-important tool in the fight behaviour 7.5 billion humans on this planet is an altogether against disease. different beast. Unlike atoms that are governed by relatively
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Remembering Prof. Sir C.V. Raman Some glimpses from the December 1970 Physics News commemorative issue
The cover page picture had a sketch of C.V. Raman by Homi Bhabha!
M.G.K. Menon K.R. Ramanathan
(The entire issue is available on the IPA website archives) Registered with the Registrar of Newspapers for India under R.N. 20754/71
Published by the Indian Physics Association c/o Dept. of Physics, I.I.T. Bombay, Powai, Mumbai 400076
Online version