AWAKE Accelerates Electrons in World First

CERN Courier October 2018 In 1 design, both News electrothermal

effects and structural A ccelerAtors deformation are at play. AWAKE accelerates electrons in world first Microwave transmitters rely on filters to maintain a desired frequency output, but The AWAKE experiment at CERN has x (mm) physics programme of its own. For eventual thermal drift can affect their operation. passed an important milestone towards 100 300 500 700 900 collider experiments, another hurdle is to compact, high-energy accelerators be able to accelerate positrons. In the longer In order to optimize the design of these ) for applications in future high-energy 50 0.5 –2 term, a global effort is under way to develop components, engineers need to predict their physics experiments. Reporting in Nature 0.4 wakefield-acceleration techniques for a performance under real-world conditions. on 29 August, the 18 institute-strong 30 0.3 multi-TeV linear collider (CERN Courier Multiphysics modeling can be used to evaluate international collaboration has for the y (mm) 0.2 December 2017 p31). 0.1 Q/dxdy (fC mm

first time demonstrated the acceleration of 10 2 Although still at an early stage of the electrothermal and structural effects of ) 0.0 d microwave filters — simultaneously. electrons in a plasma wakefield generated by –1 development, the use of plasma wakefields a proton beam. The AWAKE team injected 2 could drastically reduce the size and The COMSOL Multiphysics® software is used electrons into plasma at an energy of around 1 therefore cost of accelerators. Edda Visualization of the for simulating designs, devices, and processes 19 MeV and, after travelling a distance of 0 Gschwendtner, technical coordinator and thermal expansion, in all fields of engineering, manufacturing, and 10 m, the electrons emerged with an energy dQ/dx (fC mm 0.2 0.3 0.4 0.5 0.7 1.0 2.0 4.0 CERN project leader for AWAKE, says electric field, and surface scientific research. See how you can apply it to of about 2 GeV – representing an average E (GeV) that the ultimate aim is to attain an average acceleration gradient of around 200 MV/m. acceleration gradient of around 1 GV/m current patterns in a microwave designs. microwave cavity filter. For comparison, radio-frequency (RF) The signal in the AWAKE electron spectrometer so that electrons can be accelerated to the comsol.blog/microwave-filters cavities in high-energy linear accelerators (top) and a vertical integration over the TeV scale in a single stage. “We are looking used for X-ray free-electron lasers achieve observed charge in the central region of the forward to obtaining more results from typical gradients of a few tens of MV/m. image (bottom), showing electron acceleration our experiment to demonstrate the scope Plasma-wakefield acceleration still has far in a proton-driven plasma wakefield. of plasma wakefields as the basis for future to go before it can rival the performance of particle accelerators.” conventional RF technology, however. First process called self-modulation, generating proposed in the late 1970s, the technique a strong wakefield as they move. A bunch ● Further reading accelerates charged particles by forcing of witness electrons is then injected at AWAKE Collaboration 2018 Nature 561 363. Design, build and test components them to “surf” atop a longitudinal plasma an angle into this oscillating plasma at AG wave that contains regions of positive and relatively low energies and rides the plasma FL-9496 BALZERS and systems for scientific research negative charges. Two beams are required: a wave to get accelerated. At the other end Sommaire en français “witness” beam, which is to be accelerated, of the plasma, a dipole magnet bends the Première mondiale pour l’accélération 7 www.hsr.li and a “drive” beam that generates the incoming electrons onto a scintillator to d’électrons par AWAKE wakefield. Initial experiments took place allow the energy of the outgoing particles to Leading with laser and electron drive beams at SLAC be measured (see figure). L’expérience ALPHA fait passer 9 and elsewhere in the 1990s, and the advent AWAKE has made rapid progress since its l’antihydrogène au niveau supérieur of high-power lasers as wakefield drivers inception in 2013. Following the installation Détecteurs et imagerie de pointe : 9 vacuum technology led to increased activity. Such techniques of the plasma cell in early 2016, in the tunnel l’UE lance un appel à projets are now capable of bringing electrons to formerly used by part of the CNGS facility for new technologies energies of a few GeV over a distance of a at CERN, a proton-driven wakefield in a Un projet américain pour développer 11 few centimetres. plasma was observed for the first time by le traitement des données du HL-LHC AWAKE (the Advanced Wakefield the end of the year (CERN Courier January/ Le détecteur Hyper-K en construction 11 Experiment) is a proof-of-principle R&D February 2017 p8). The electron source, dès 2020 project that is the first to use protons for electron beam line and electron spectrometer Quelle reconnaissance individuelle 12 the drive beam. Since protons penetrate were installed during 2017, completing the deeper into the plasma than electrons and preparatory phase beginning in 2018, and the dans les grandes collaborations ? lasers, thereby accelerating witness beams first electron acceleration was recorded early Du silicium ultrafin pour l’imagerie de STAR 13 for a greater distance, they potentially in the morning of 26 May. Premiers résultats sur les diélectrons 13 can accelerate electrons to much higher So far, the AWAKE demonstration de masse faible energies in a single plasma stage. The involves low-intensity electron bunches; the experiment is driven by a bunch of 400 GeV next steps include plans to create an electron Sous-structures des jets : dix ans d’avancées 14 protons from the Super Proton Synchrotron, beam at high energy with sufficient quality LHCb : la physique à cible fixe en mode 15 which is injected into a plasma cell to be useful for applications, although tests « collisions » containing rubidium gas at a temperature will pause at the end of the year when the of around 200ºC. An accompanying laser CERN accelerator complex shuts down for La désintégration du boson de Higgs 16 pulse is used to ionise the rubidium gas two years for upgrades and maintenance. en quarks b observée ALCA TECHNOLOGY S.r.l. and transform it into a plasma. As the A first application of AWAKE is to deliver L’énigme d’une ancienne explosion 19 Via Lago di Garda, proton bunch travels through the plasma, it accelerated electrons to an experiment and 130 - 36015 Schio (VI) - Italia stellaire peut-être résolue Tel. +39 0445 500064 splits into a series of smaller bunches via a extending the project with a fully-fledged E-Mail: [email protected]

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A n t i m A t t e r ALPHA takes antihydrogen to the next level

The ALPHA experiment at CERN’s represents a pivotal technological step 0.08 Antiproton Decelerator (AD) has made data towards laser cooling of antihydrogen and yet another seminal measurement of simulation 1 the extension of antimatter spectroscopy to the properties of antiatoms. Following 0.04 simulation 2 quantum states possessing orbital angular its determination last year of both the momentum. Simulations indicate that probability ground-state hyperfine and the 1S–2S 0.00 cooling to about 20 mK is possible with the transitions in antihydrogen, the latter –3 –2 –1 0 1 2 3 current ALPHA set-up, which, combined representing the most precise measurement detuning (GHz) with other planned improvements, would of antimatter ever made (CERN Courier reduce the 1S–2S transition line width May 2018 p7), the collaboration has reported The antihydrogen 1S–2P spectral line shape (see figure) by more than an order of in Nature the first measurement of the next showing detected events (black), with the magnitude. At such levels of precision, fundamental energy level: the Lyman-alpha error bars representing the statistical says the team, antihydrogen spectroscopy transition. The result demonstrates that counting uncertainties, and simulated line will have an impact on the determination ALPHA is quickly and steadily paving the shapes for different initial conditions. of fundamental constants, in addition to way for precision experiments that could providing elegant tests of CPT symmetry. uncover as yet unseen differences between and binding them with positrons from a Laser cooling will also allow precision the behaviour of matter and antimatter sodium-22 source, confining the resulting tests of the weak equivalence principle (CERN Courier March 2018 p30). antihydrogen atoms in a magnetic trap. via antihydrogen free-fall or antiatom- The Lyman-alpha (or 1S–2P) transition is A laser is used to measure the antiatoms’ interferometry experiments. one of several in the Lyman series that were spectral response, requiring a range of “The Lyman-alpha transition is discovered in atomic hydrogen just over a laser frequencies and the ability to count notoriously difficult to probe – even century ago. It corresponds to a wavelength the number of atoms that drop out of the in normal hydrogen”, says ALPHA of 121.6 nm and is a special transition in trap as a result of interactions between spokesperson Jeffrey Hangst. “But by astronomy because it allows researchers to the laser and the trapped atoms. Having exploiting our ability to trap and hold probe the state of the intergalactic medium. successfully employed this technique to large numbers of antihydrogen atoms for Finding any slight difference between such measure the 1S–2S transition, ALPHA has several hours, and using a pulsed source transitions in antimatter and matter would now measured the Lyman-alpha transition of Lyman-alpha laser light, we were able shake one of the foundations of quantum frequency with a precision of a few parts in to observe this transition. Next up is laser field theory, charge–parity–time (CPT) a hundred million: 2,466,051.7 ± 0.12 GHz. cooling, which will be a game-changer symmetry, and perhaps cast light on the The result agrees with the prediction for for precision spectroscopy and observed cosmic imbalance of matter the equivalent transition hydrogen to a gravitational measurements.” and antimatter. precision of 5 × 10 –8. The ALPHA team makes antihydrogen Although the precision is not as high ● Further reading atoms by taking antiprotons from the AD as that achieved in hydrogen, the finding M Ahmadi et al. 2018 Nature 561 211.

P o l i c y mechanisms or resources available to Europe calls for follow these ideas further or to make a case,” he says. “ATTRACT builds upon advanced detector the collaborative spirit of open science and co-innovation, where the experience and available infrastructure at laboratories such and imaging ideas as CERN could turn out to be useful.” sensor and imaging technologies both for The ATTRACT seed fund (www. The European Union (EU) has committed scientific purposes and to address broader attract-eu.com) is open to researchers €17 million to help bring a total of 170 challenges in the domains of health, and entrepreneurs from organisations all PUSHING SCIENCE TO ITS LIMITS breakthrough detection and imaging ideas sustainable materials and information, and over Europe. The call for proposals for to market. Led by CERN and funded by communication technologies. CERN users and other outside laboratories the EU’s Horizon 2020 programme, the Markus Nordberg of the CERN-IPT working on detection and imaging ATTRACT initiative involves several development and innovation unit laid technologies will close on 31 October, and other European research infrastructures the foundations for ATTRACT back the successful proposals will be announced and institutes: the European Molecular in 2013, observing then how detector in early 2019. The 170 projects funded by Biology Laboratory, European Southern developers found it difficult to find suitable ATTRACT will have one year to develop Observatory, European Synchrotron programmes to facilitate the wider use their ideas, during which business and Radiation Facility, European XFEL, Institut of generic detector R&D. “The detector innovation experts from Aalto University, Laue-Langevin, Aalto University, the R&D community, for example regarding EIRMA and ESADE Business School European Industrial Research Management the LHC upgrades and beyond, has ideas of will help project teams transform their For more information, please visit cosylab.com or Association (EIRMA) and ESADE. It will the potential suitability of its technologies technology into products, services, see our career opportunities on cosylab.com/jobs focus on the development of new radiation in other fields, but limited contacts, companies and jobs.

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A n t i m A t t e r ALPHA takes antihydrogen to the next level

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c o m p u t i n g US initiative to tackle data demands of HL-LHC CERN The US National Science Foundation (NSF) has launched a $25 million effort to help tackle the torrent of data from the High-Luminosity Large Hadron Collider (HL-LHC). The Institute for Research and Innovation in Software for High-Energy Physics (IRIS-HEP), announced on 4 September, brings together multidisciplinary teams of researchers and educators from 17 universities in the US. It will receive $5 million per year for a period of five years, with a focus on developing new software tools, algorithms, system designs and training the next generation of users. Construction for the HL-LHC upgrade is already under way (CERN Courier July/ August 2018 p7) and the machine is expected to reach full capability in the mid-2020s. Boosting the LHC’s luminosity by a factor of almost 10, HL-LHC will collect around 25 times more data than the LHC has produced up to now and push data processing and storage to the limit. How to address the immense computing challenges ahead was the subject of a recent community A simulation of a possible signal for new physics in the CMS detector at the HL-LHC, where white paper published by the HEP Software analyses will be complicated by the simultaneous presence of up to 200 background events. Foundation (CERN Courier April 2018 p38). In 2016, the NSF convened a project to collaboration. “The institute will be virtual, Cyberinfrastructure (OAC) and the NSF gauge the LHC data challenge, bringing with a core at Princeton, but coordinated division of physics, IRIS-HEP is the third together representatives from the as a single distributed collaborative project OAC software institute, following the high-energy physics and computer-science involving the participating universities Molecular Sciences Software Institute and communities to review two decades of similar to many activities in high-energy the Science Gateways Community Institute. successful LHC data-processing approaches physics,” he says. “High-energy physics had “Our US colleagues worked with us very and discuss ways to address the obstacles a rush of discoveries in the 1960s and 1970s closely preparing the community white ® that lay ahead. The new software institute that led to the Standard Model of particle paper last year, which was then used as one of Mileon – the longest hollow emerged from that effort. physics, and the Higgs boson was the last the significant inputs into the NSF proposal,” The institute is primarily about people, missing piece of that puzzle. We are now says Graeme Stewart of CERN and the HEP rather than computing hardware, explains searching for the next layer of physics beyond Software Foundation. “So we’re really happy conductor in the world IRIS-HEP principal investigator and the Standard Model. The software institute about the funding announcement and very executive director Peter Elmer of Princeton will be key to getting us there.” much looking forward to working together Developed in response to the customer demand University, who is also a member of the CMS Co-funded by NSF’s Office of Advanced with them.” for seamless magnetic windings. F a c i l i t i e s fiscal year, which will enable progress in Hyper-Kamiokande (Hyper-K) is a Hyper-Kamiokande preparatory work for construction and efforts water Cherenkov detector centered on to secure international collaboration. a huge underground tank containing construction to Coinciding with the MEXT 300,000 tonnes of water, with a sensitive announcement, the University of Tokyo volume about a factor of 10 larger than its pledged to ensure that construction of the predecessor Super-Kamiokande (Super-K). start in 2020 Hyper-Kamiokande detector commences in Like Super-K, Hyper-K will be located in April 2020. According to a statement from Kamioka on the west coast of Japan directly On 12 September, the Japanese government university president Makoto Gonokami: in the path of a neutrino beam generated www.luvata.com granted seed funding towards the “The University of Tokyo has made this 295 km away at the J-PARC facility in construction of the Hyper-Kamiokande decision in recognition of both the project’s Tokai, allowing it to make high-statistics experiment, a next-generation detector for importance and value both nationally and measurements of neutrino oscillations. the study of neutrinos. Japan’s Ministry of internationally. … Seed fundings in the Together with a near-detector located close Education, Culture, Sports, Science and past projects usually lead to full funding in to J-PARC, Super-K formed the “T2K” Technology (MEXT) allocated $700,000 the following year, as was the case for the long-baseline neutrino programme. An s within its budget request for the 2019 Super-Kamiokande project.” order of magnitude bigger than Super-K,

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D e t e c t o r s Hyper-K will serve as the next far-detector Hyper-K proto-collaboration was formed in at T2K, with a rich physics portfolio. This 2015 and currently comprises around 300 ranges from the study of the CP violation members from 73 institutes in 15 countries. Thin silicon sharpens STAR imaging

in the leptonic sector and measurements of Many European institutes are involved, collaboration BNL/STAR neutrino-mixing parameters, to studies of including the CERN neutrino group, which A new technology has enabled the STAR Hyper-K collaboration Hyper-K proton decay, atmospheric neutrinos and is already participating in the upgrade of the collaboration at Brookhaven National neutrinos from astrophysical sources. T2K near detector to serve Hyper-K. To this Laboratory’s Relativistic Heavy-Ion Collider It was at Super-K in 1998 that researchers end, in the summer of (RHIC) to greatly expand its ability to discovered neutrino oscillations, proving last year a detector called Baby MIND reconstruct short-lived charm hadron decays, that neutrinos are massive and leading that was designed and built at CERN was even in collisions containing thousands of to the award of the 2015 Nobel Prize in shipped to J-PARC (CERN Courier July/ tracks. A group of STAR collaborators, led Physics to Takaaki Kajita of the University August 2017 p12). by Lawrence Berkeley National Laboratory, of Tokyo and Arthur McDonald of Queen’s “Hyper-K is the next step in the Japanese used 400 Monolithic Active Pixel Sensor University in Canada. The Japanese Hyper-K’s giant tank will take neutrino neutrino adventure,” says Baby MIND (MAPS) chips in its new vertex detector, neutrino programme has progressed science into uncharted waters. spokesperson and Hyper-K collaborator called the heavy-flavour tracker (HFT), steadily since the 1998 discovery (CERN Alain Blondel of the University of Geneva. representing the first application of this Courier July/August 2016 p29). Hyper-K report was published earlier this year “This success comes from wise choices and technology in a collider experiment. was discussed as long ago as 2002 and (see further reading). intelligent planning. The increase in the The HFT reconstructs charmed hadrons a letter of intent was published in 2011, “Hyper-Kamiokande now moves from far-detector mass is exciting: demonstration over a broad momentum range by identifying following the first measurement of the planning to construction,” said Hyper-K of an asymmetry between neutrinos their secondary decay vertices, which are a neutrino mixing angle θ13 at T2K, which project co-leader Francesca Di Lodovico and antineutrinos was identified as the few tens to hundreds of micrometres away boosted the expectation of a discovery of of Queen Mary University of London, in ‘great discovery’ goal as soon as neutrino from the collision vertex. The charmed leptonic CP violation by Hyper-K. The a statement released by the Kavli Institute oscillations were discovered, although it hadrons are used to study heavy-quark experiment was placed in Japan’s list for the Physics and Mathematics of the presents a challenge regarding systematics. energy loss in a quark–gluon plasma (QGP) of priority projects in 2014 but was not Universe in Japan on behalf of the Hyper-K And if a proton decay is detected or a and to determine emergent QGP-medium A gold–gold collision recorded by STAR with the MAPS pixel detector (two most inner layers) short-listed. The project was proposed collaboration. “The collaboration will now supernova strikes, it will be fireworks!” transport parameters. and one layer of silicon-strip detector (outer layer). The white points show the measured hits again in 2017, this time making the work on finalising designs, and is very open The MAPS sensor is based on the that were used to reconstruct charged particle tracks (red and green lines). short-list of seven projects to be funded by to more international partners joining this ● Further reading same commercial CMOS technology MEXT. The Hyper-K conceptual design exciting, far-reaching new experiment.” The K Abe et al. 2018 arXiv:1805.04163. that is widely used in digital cameras. It The heavy-flavour physics programme own MAPS-based vertex detector – the ITS comprises an array of 928 × 960 square enabled by the HFT has been one of the upgrade – and the sPHENIX collaboration P o l i c y pixels with a pitch of 20.7 × 20.7 μm2 to driving forces for RHIC runs from 2014 to at RHIC is also planning a MAPS-based provide a single-hit resolution of <6 μm. 2016. The first measurement with the HFT detector. These next-generation detectors will Survey addresses recognition in large collaborations The sensors are thinned to a thickness on the D0 elliptic collective flow shows that have much faster event readout, by a factor of 50 μm and mounted on a carbon-fibre D0 mesons have significant hydrodynamic of 20, to reduce event pileup and therefore

CMS-PHO-PUBLIC-2012-014-1 mechanical support, and their relatively flow in gold–gold collisions, and the HFT allow physicists to reconstruct bottom The European Committee for Future low power consumption (170 mW/cm 2) pointing resolution also enabled the first hadrons more efficiently in high-luminosity, Accelerators (ECFA) has created a working allows the detector to be air-cooled. measurement of charmed-baryon production heavy-ion collision environments. group to examine the recognition of The thinness is important to minimise in heavy-ion collisions. individual achievements in large scientific multiple scattering in the HFT, allowing Building on the success of the STAR ● Further reading collaborations. Based on feedback from an for good pointing resolution even for low HFT, the ALICE collaboration at CERN’s G Contin et al. 2018 Nucl. Instrum. Methods Phys. initial survey of the leaders of 29 CERN-based transverse-momentum charged tracks. Large Hadron Collider is now building its Res A doi:10.1016/j.nima.2018.03.003. or CERN-recognised experiments in particle, nuclear, astroparticle and astrophysics, ECFA L H c experiments found that the community is ready to engage in dialogue on this topic and receptive to First low-mass dielectron results ahead of LHC Run 3 potential recommendations. In response, ECFA has launched a community-wide survey to verify how One of the main objectives of upgrades to the ALICE detector system are distribution of dielectrons from decays of individual researchers perceive the systems the ALICE physics programme underway, most notably a new inner tracking light mesons and J/ψ, as well as semileptonic put in place to recognise their achievements. for future LHC runs is the system and a new readout system for the time decays of correlated heavy-flavour pairs. The The survey will be distributed widely, precise measurement of the projection chamber. Pb–Pb results, recorded at a centre-of-mass and can be found on the ECFA website e+e− (dielectron) invariant-mass Meanwhile, the ALICE collaboration has energy of 2.76 TeV per nucleon–nucleon (https://ecfa.web.cern.ch) with a deadline for CMS physicists in CERN’s Building 40. The ATLAS and CMS collaborations at the LHC each continuum produced in heavy-ion also analysed the proton–proton (pp) and pair, are not yet sensitive enough to quantify responses by 26 October. number more than 3000 members from over 200 institutes. collisions. In contrast to strongly interacting lead–lead (Pb–Pb) collision data recorded the presence of thermal radiation and signs The results of the survey will be hadronic probes, dielectrons provide an so far during LHC Runs 1 and 2. The results, of chiral symmetry restoration on top of the disseminated and discussed at the upcoming (CERN Courier April 2018 p7). During the collaborations in high-energy physics,” says unperturbed view into the quark–gluon which have recently been submitted for vacuum expectation. plenary ECFA meeting at CERN on 15–16 remaining open sessions, comprehensive ECFA chairperson Jorgen D’Hondt. “On the plasma (QGP), a phase of deconfined quarks publication, provide new physics insights, The results obtained in pp collisions at November. An open session during the overviews of all major future collider eve of the update process of the European and gluons that is produced in such collisions. in particular into the production of heavy 13 TeV provide the first measurements of morning of 15 November, also to be webcast, projects in and beyond Europe, and related Strategy, it is an outstanding opportunity For example, they will allow physicists to quarks (charm and beauty) in pp collisions charm and beauty production cross sections will be devoted to the discussion of the accelerator technologies, will be given. for ECFA to take on its responsibility determine the initial temperature of the at centre-of-mass energies of 7 and 13 TeV. at mid-rapidity integrated over all transverse outcomes of the survey, and aims to gather “Visibility and promotion of young for informing the community about the QGP and to study the effects of the predicted The measured invariant-mass spectrum momenta at the current highest LHC energy. input to be submitted to the update of the scientists is of utmost importance in opportunities and challenges ahead of us. restoration of chiral symmetry. In order to of dielectrons (see figure) has been found Fitting the data with two different models s European Strategy for Particle Physics science and in particular also for the large Everybody is welcome.” perform these measurements, important to be in good agreement with the expected of heavy-flavour production (PYTHIA

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Dielectron cross section as a function of 5 results were among the many topics mass scale in certain regions of parameter generators, and are stable in the face of the invariant mass measured in pp collisions at a 10 ALICE discussed at the 10th BOOST workshop space also reaches the percent level, challenging high pile-up environment of centre-of-mass energy of 7 TeV (red) and pp: p > 0.2 GeV/c, |η | < 0.8 light meson decays in Paris this July, where the ATLAS which is unprecedented for substructure Run 2 at the LHC. 13 TeV (blue), as well as Pb–Pb collisions at a 104 T,e e Pb–Pb: p > 0.4 GeV/c, |η | < 0.8 + – collaboration presented new results observables. In two ATLAS publications, the early centre-of-mass energy of 2.76 TeV per nucleon– T,e e cc → e e accentuating the advances in Meanwhile, the ongoing revolution in Run-2 data have allowed for rapid progress B = 0.5T nucleon pair (green) scaled by the number of 103 bb → e+e– jet substructure. machine learning has directly intersected by enabling powerful in situ techniques. binary collisions (Ncoll). The data are compared Recent focus on measuring Standard with jet-substructure studies. Techniques The collaboration is now looking forward to a cocktail of known hadronic sources. total cocktail 102 Model properties using jet substructure has such as boosted decision trees and deep to the possibilities offered by the larger motivated ATLAS to measure the energy neural networks have been studied by full Run-2 dataset, where such data-driven ))

6.4 and POWHEG), ALICE observes 2 pp √ s = 13 TeV × 3000 and mass response of large-radius jets with ATLAS to identify W bosons and top calibrations will bring precision to an significant differences in the obtained 10 the highest possible precision [7]. A new quarks with high transverse momenta [8]. increasing number of observables. charm cross sections at both investigated in situ (that is, data-driven) calibration for These approaches allow several This will improve the quality of both collision energies. The difference arises 1 large-radius jets provides percent-level high-level substructure observables searches and measurements exploring the from different rapidity correlations between (mb/(GeV/c uncertainties by combining several such as the mass, or low-level information energy frontier. ee σ

charm and anti-charm quarks in the two d

dm measurements of the jet energy scale in such as measured energy depositions –1 pp √ s = 7 TeV × 30 calculations. Hence, the data provide . 10 events where the jet is balanced from the calorimeter, to be utilised ● Further reading coll 1 crucial input to improve models of charm N by a well-measured reference object simultaneously using their complex [1] J Butterworth et al. 2008 Phys. Rev. Lett. 100 production that is complementary to single 10–2 such as a leptonically decaying Z boson, correlation pattern to gain information. 242001. [2] ATLAS Collaboration 2018 Phys. Rev. Lett. 121 charmed-hadron measurements. Pb–Pb √ sNN = 2.76 TeV, Cent. 0–10% a photon or a system of well-calibrated Such techniques achieve improvements of In addition, the distance of the closest jets with lower momenta (figure, left). The more than 100% in terms of background 092001. 10–3 approach (dca) of the electrons to the collision mass scale of these jets is also measured rejection for top quark identification over [3] CMS Collaboration 2018 arXiv:1807.05974. vertex has been successfully used in the using fits to the jet mass distribution previous results (figure, right). [4] ALICE Collaboration 2018 arXiv:1807.06854. analysis of pp collisions at 7 TeV to distinguish 10–4 obtained from hadronically decaying In situ measurements of tagging and the [5] CMS Collaboration 2018 arXiv:1805.05145. displaced dielectrons from open-heavy W bosons and top quarks in data, and background efficiencies of these algorithms [6] ATLAS Collaboration 2018 flavour decays and prompt decays of light 10–5 by combining information from the robustly demonstrate that they are well ATLAS-CONF-2018-014. hadrons. This is an important test as the dca 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 ATLAS inner tracking detector and understood in terms of the QCD-based [7] ATLAS Collaboration 2018 arXiv:1807.09477. 2 calorimeters. The precision for the jet models implemented in Monte Carlo [8] ATLAS Collaboration 2018 arXiv:1808.07858. will be a crucial tool to isolate a thermal mee (GeV/c ) signal in the mass region 1–3 GeV/c2 in the data that will be collected in LHC Runs 3 and 4 (starting in 2021). Part of the data 0.5 to 0.2 T in order to further increase the ● Further reading Fixed-target physics in collider mode at LHCb will be recorded with the magnetic field of acceptance of dielectrons with low mass and ALICE Collaboration 2018 arXiv:1805.0439, the central barrel solenoid reduced from transverse momentum. arXiv:1805.04407 and arXiv:1807.00923. This year, the LHCb of 87 hours in 2016 with a 4 TeV beam and a collaboration reached an LHCb helium target (√sNN = 86.6 GeV). important milestone in Thanks to the high-precision tracking and A decade of advances in jet substructure its fixed-target physics 103 advanced particle-identification capabilities programme, publishing two key results of the LHCb detector, the production rate on the production rates of particles in of J/ψ and D0 mesons were measured with Ten years ago, 4 1.04 10 proton–ion collisions: measurements of 102 a very good precision (see figure). Taking the first in a ATLAS DNN top ATLAS simulation –1

σ (J/ Ψ ) (nb/nucleon) NLO NRQCD √s = 13 TeV, 36.2 fb BDT top √s = 13 TeV the cross section of antiprotons that advantage of the forward geometry of

series of annual trimmed R = 1.0 anti-kt (LCW + JES + JMS) ) shower trimmed anti-kt R = 1.0 jets constrain models of cosmic rays, and of the detector and the boost induced by the MC

bkg deconstruction true meetings devoted 3 |η | < 2.0 2 /R ∈ 10 2-var optimised true charmonium and open-charm cross sections 10 multi-TeV proton beam, the detector also to the theoretical and experimental / pT = [1500, 2000] GeV data 1.00 tagger top tagging (see further reading). √s NN (GeV) measures very backward particles in the understanding of massive hadronically TopoDNN HEPTopTagger v1 The LHCb fixed-target system, known centre-of-mass frame of the collision, decaying particles with high transverse t , mcomb > 60 GeV 102 32 as SMOG (System for Measuring Overlap The J/ψ cross-section, measured with the giving access to the large Bjorken-x region momenta took place at SLAC. These with Gas), injects a small amount of noble 4 TeV LHC proton beam hitting gaseous in the target nucleon. In this kinematic “BOOST” workshops coincided with 0.96 gas inside the LHC beam pipe, at a helium (red point) compared to previous region, no significant contribution from an influential publications on the subject –7 – response ratio, R γ + jet 10 experimental results (black) and a fit based T pressure of the order 10 mbar, within intrinsic c c component within the nucleon

p of reconstructing such Lorentz-boosted total uncertainty Z + jet on theoretical calculations (yellow). background rejection (1 the LHCb vertex detector region (CERN structure was observed. decays as single jets with large radius statistical component multi-jet Courier January/February 2016 p10). Building on the success of these analyses parameters [1], which kick-started the field 0.92 1 This system was initially designed LHCb has just taken the first step towards of the 2015 and 2016 data, LHCb plans 2 3 of jet substructure. Such techniques have 2 × 10 103 2 × 10 0.3 0.5 0.7 0.9 to improve the determination of the the use of charmonium and open-charm to carry out studies of charmonium become a critical aspect of the ATLAS and signal efficiency (∈ ) large-R jet pT (GeV) sig luminosity via beam-profile measurements, hadrons as probes of the QGP by measuring suppression with the large sample of CMS experimental programmes searching and can produce hundreds of millions their cross-sections in proton–nucleus proton–neon collisions collected in 2017, for new physics at the highest scales Left: the data-to-simulation ratio of the average large-radius jet transverse momentum of beam–gas collisions per hour. This collisions, where no QGP is expected to be and with samples of lead–neon collisions accessible with the LHC. response as a function of the large-radius jet transverse momentum, with the combined result provides a unique opportunity to exploit formed. The data for these measurements that will be taken in the upcoming LHC The understanding of large-radius jets based on three in situ techniques. The total uncertainty is shown as the green band, reaching the LHC proton and ion beams in a come from two SMOG data-taking heavy-ion run in November 2018. and their substructure has progressed percent level for jets with low-to-intermediate transverse momenta. Right: the background fixed-target mode, opening many campaigns with proton beams– one carried considerably. Analytical calculations have rejection versus signal efficiency for various algorithms, which identify hadronically physics opportunities such as a precise out over a period of 18 hours in 2015 with ● Further reading recently been published that predict the decaying top quarks with large transverse momenta. The application of machine learning study of the quark–gluon plasma (QGP) a beam of energy 6.5 TeV and an argon gas LHCb Collaboration 2018 arXiv:1808.06127. distribution of jet substructure observables (DNN top, BDT top, TopoDNN) leads to large improvements over traditional approaches. in the as-yet-unexplored energy target (meaning a centre-of-mass energy LHCb Collaboration 2018 at high accuracy, and these have been regime between existing fixed-target and per colliding nucleon–nucleon pair, √sNN, LHCb-PAPER-2018-023. F Maltoni et al. 2006 Phys. Lett. B 638 202.

compared to data by both ATLAS [2] Measurements of substructure observables ALICE, CMS and ATLAS collaborations s collider measurements. of 110.4 GeV), and the other over a period and CMS [3] in proton–proton collisions. have also recently been made by the in heavy ion collisions [4,5,6]. Such 15 14

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charm and anti-charm quarks in the two d

σ (J/ Ψ ) (nb/nucleon) NLO NRQCD √s = 13 TeV, 36.2 fb BDT top √s = 13 TeV the cross section of antiprotons that advantage of the forward geometry of

series of annual trimmed R = 1.0 anti-kt (LCW + JES + JMS) ) shower trimmed anti-kt R = 1.0 jets constrain models of cosmic rays, and of the detector and the boost induced by the MC

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Observation of Higgs-boson decay to bottom quarks

The observation of the 77.2 fb–1 (13 TeV) Higgs-boson decay to bottom – quark–antiquark (bb) by the CMS data CMS experiment is a seminal VH, H → bb achievement that sheds light VZ, Z → bb 1000 S + B uncertainty on one of the key missing pieces of the Higgs sector of the Standard Model (SM). Processes that include the Higgs boson’s favoured decay mode to b-quarks (with about 500 58% probability) have until now remained elusive because of the overwhelming

background of b-quark events produced via S/(S + B) weighted entries strong interactions. While the recent CMS 0 observation of Higgs-boson production in association with top quarks (ttH) constitutes 60 80 100 120 140 160 the first confirmation of the tree-level m(jj) [GeV] coupling of the Higgs boson to quarks (CERN Courier June 2018 p10), the Higgs-boson Left: a CMS candidate event for the Higgs boson decaying to two bottom quarks, in association – decay to bb tests directly its coupling to with a Z boson decaying to an electron and a positron. Right: weighted dijet invariant mass down-type quarks. Moreover, this decay is distribution comparing data with the VH and VZ processes, with all other background crucial for constraining, under fairly general processes subtracted. Weights are derived from the dijet invariant mass distribution. assumptions, the overall Higgs-boson decay width and thus reducing the uncertainty on control regions to monitor the different of the VH and VZ processes are separately the measurement of absolute couplings. This background processes. Then, a simultaneous visible, after all other background processes observation effectively narrows down the binned-likelihood fit of the signal and have been subtracted. remaining window available for exotic or control regions is performed to extract the The VH results are combined with CMS undetected decays. Higgs-boson signal. measurements in other production processes, At the LHC, the most effective strategy The score of the DNN separating signal including gluon-fusion, vector-boson – to observe the Higgs bb decay is to exploit from the background is used for the signal fusion, and associated production with top the associated production mechanism with extraction fit. Several observables are quarks, with data collected at 7, 8 and 13 TeV, an electroweak vector boson VH, where V combined and the most discriminating are: depending on the process. The observed corresponds to a W or Z boson. The leptons the angular separation between the two combined significance is raised to 5.6σ, and neutrinos arising from the V decay b-quarks and the b-tagging properties of the where the expectation from SM Higgs-boson provide large suppression of the multijet Higgs candidate jets. An event candidate for production is 5.5σ. The signal strength background, and further background the production of a Z boson in conjunction corresponding to this excess, relative to the reduction is achieved by requiring the with a Higgs boson is shown in the left figure. SM expectation, is 1.04 ± 0.20, in perfect Higgs-boson candidates to have large A clear excess of events is observed in the agreement with the latter. transverse momentum. combined 2016 and 2017 data, in comparison With the direct observation of the Advanced machine-learning techniques with the expectation in the absence of a Higgs-boson couplings to bottom quarks – (deep neural networks, DNN) are used in H → bb signal. The significance of this excess complementing those involving tau leptons different steps of the analysis including: the is 4.4σ, where the expectation from SM and top quarks (see further reading), b-jet identification, the measurement of the Higgs-boson production is 4.2σ. The signal the Yukawa couplings to all accessible b-jet energy, the classification of different strength corresponding to this excess, in third-generation fermions have now been backgrounds in control regions, and the final relation to the SM expectation, is 1.06 ± 0.26. firmly established. This opens a new era of signal extraction. When combined with the measurement precision studies in the Higgs sector that will This result uses LHC data collected in from LHC Run 1 at 7 and 8 TeV, the signal fully benefit from the larger dataset that will 2016 and 2017 at an energy of 13 TeV and has significance increases to 4.8σ, while 4.9σ is be available by the end of Run 2. benefited from the recent CMS pixel tracker expected. The corresponding signal strength upgrade with further improved b-quark is 1.01 ± 0.22. ● Further reading identification performance. The dijet invariant mass distribution (figure, CMS Collaboration 2018 Phys. Rev. Lett. 121 121801. A signal region enriched in VH events right) allows for a more direct visualisation CMS Collaboration 2018 Phys. Lett. B 779 283. is selected together with several dedicated of the Higgs-boson signal. The contributions CMS Collaboration 2018 Phys. Rev. Lett. 120 231801.

Les physiciens des particules du monde entier sont invités à apporter leurs CERN Courier welcomes contributions from the international contributions au CERN Courier, en français ou en anglais. Les articles retenus particle-physics community. These can be written in English or French, seront publiés dans la langue d’origine. Si vous souhaitez proposer un article, and will be published in the same language. If you have a suggestion for faites part de vos suggestions à la rédaction à l’adresse [email protected]. an article, please send proposals to the editor at [email protected].

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Observation of Higgs-boson decay to bottom quarks

CCOct18_News_v5.indd 16 21/09/2018 13:13 CERNCOURIER www. V o l u m e 5 8 N u m b e r 8 O c t o b e r 2 0 1 8 CERN Courier October 2018 Astrowatch High-Speed Cameras for C o m p i l e d b y m e r l i n K o l e , d e pa r t m e n t o f pa r t i C l e p h y s i C s , U n i v e r s i t y o f G e n e va Visible, Soft X-Ray, VUV Solving the mystery of a historic stellar blast NASA Some 180 years ago, a relatively normal star material as one star brightens followed by and EUV Applications called Eta Carinae suddenly brightened to more violent ejection from an explosion. become the second brightest star in the sky, Smith and collaborators claim that the before almost disappearing at the end of scenario which best matches the data, Princeton Instruments is the trusted choice for the 19th century. The sudden brightening including information about the age and and subsequent disappearance, recorded mass of the two remaining stars, is that the synchrotrons and laboratories worldwide! by astronomer John Herschel, suggested system originally consisted of three stars. that the star had undergone a supernova The two closest stars initially interacted to explosion, leaving behind a black hole. form one massive star, while the donor star More recent observations have shown, moved further away, losing mass and thereby however, that the star still exists – ruling increasing the radius of its orbit around the out the supernova hypothesis. Even more massive star. The gravitational field of the remarkably, what remains is a binary system far-away donor star would have caused the of two stars, the more massive of which is orbiting third star to dramatically change surrounded by a large nebula. orbit, forcing it to spiral into the massive Although supernovae imposters such as central star. In doing so, its gravitational Eta Carinea are now known to occur in other interactions with the massive star caused it galaxies, this event – known as the Great to shed large amounts of matter as it started Eruption – appeared relatively close to Earth A Hubble Space Telescope image of to burn brighter. Finally, the binary system at a distance of around 7500 light years. It Eta Carinae using a combination of merged, causing a violent explosion where is therefore a perfect laboratory in which image-processing techniques, showing a large amounts of stellar material were to study what exactly happens when stars huge pair of gas and dust clouds. ejected at large velocities towards the earlier appear to survive a supernova. ejected material. As the fast ejecta smashed The fate of Eta Carinae has remained the variability observed in the 19th century into the slower moving ejecta, a bright Soft X-Ray Visible mysterious, but since the turn of the with the variability of the light reflected object was formed on the night sky that was millennium clues have emerged in echoes of from a gas cloud, it can be determined how visible for many years during the 1850s. the light emitted during the Great Eruption. far in the past astronomers are observing The remaining binary system still lights up While the light observed in the 19th century the explosion. every few years as the old donor star moves ® ™ travelled directly from the system towards Now, a team led by Nathan Smith of through the nebula left over from the merger. PI-MTE3 SOPHIA -XO KURO Earth, other light initially travelled towards the University of Arizona in Tucson has The new details about the evolution of distant clouds surrounding the stars before studied the spectra of the light echo in more this complex and relatively nearby system Proven Back-illuminated, Back-illuminated being reflected in our direction. In 2003, detail using the 6.5 m Magellan telescopes not only teach us more about what was the light echoes from this event were and found that it matches observations observed by Herschel almost two centuries in-vacuum 4-port CCD scientific CMOS bright enough to be observed using the during the 1840s and 1850s, when the ago, but also provide valuable information cameras cameras cameras moderate-sized telescopes at the Cerro Great Eruption was at its peak. Spectral about the evolution of massive stars, binary Tololo Inter-American Observatory in Chile, analysis of the reflected light indicates that and triple systems, and the nature of the ► 2k x 2k and 4k x 4k ► 2k x 2k and 4k x 4k ► Excellent sensitivity in the while the different gas clouds reflecting the initially matter was ejected at relatively low supernovae imposters. 200nm - 1100nm range –1 ► Up to 3 fps ► Up to 3 fps light were observed more recently using the velocities of 150–200 km , while during ► 120 fps at 800 x 800 larger scale Magellan Observatory and the the 1850s some matter was travelling at ● Further reading ► ► Low outgassing 95% QE, back-illuminated ► 47 fps at 2k x 2k Gemini South Observatory, also located in speeds of 10,000–20,000 km–1. The data are N Smith et al. 2018 MNRAS 480 1457. ► Designed for SAXS, GIXD ► Low-noise, deep-cooled ► 100% fill factor Chile. By comparing historical records of compatible with a system that first ejects N Smith et al. 2018 MNRAS 480 1466. ► 95% quantum efficiency Fritz Helmut Hemmerich Fritz ► Large 11 x 11 um pixels - Picture of the month wide dynamic range This image of the Andromeda galaxy, the closest neighbouring major galaxy to our Milky Way, is disturbed by a bright green line. The line is a result of a meteor the size of a grain of sand entering the Earth’s atmosphere during the Perseid meteor shower visible every August. The meteor can be seen to flare several times as it travels through the atmosphere for only a fraction of a second. The green colour is the result of the meteor vaporising in the Earth’s NOTE: Princeton Instruments x-ray cameras are compatible with EPICS software. atmosphere. As both the atmosphere and the meteor itself glow, different colours can be created. The green colour observed here hints that the meteor contains large amounts of nickel. Visit www.princetoninstruments.com to learn more about these and other Princeton Instruments products

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