NEWS

N e u t r i n o s OPERA catches its first tau-neutrino

The OPERA collaboration has announced the 10 mm CS 5 100 μm observation of the first candidate tau-neutrino 1 1

(ντ) in the muon-neutrino (νμ) beam sent 2 mm 7 5 3 through the Earth from CERN to the INFN’s Gran 3 2 Sasso Laboratory 730 km away. The result is 4 parent an important final piece in a puzzle that has γ2 γ1 challenged science for almost half a century. 4 1 2 8 daughter γ The puzzle surrounding neutrinos originated 6 6 8 in the 1960s when the pioneering experiment daughter by Ray Davis detected fewer neutrinos arriving at the Earth from the Sun than solar models OPERA’s τ– candidate event. The longitudinal view (left) shows the tracks of charged particles detected predicted (CERN Courier December 2002 in the emulsion layers, together with the showers indicative of two photons (γ1 and γ2). A closer p15). A possible solution, proposed in 1969 transverse view (right) reveals the kink characteristic of a decay with track 4 (the parent) leading to by Bruno Pontecorvo and Vladimir Gribov, was track 8 (the daughter product). Track 4 is about 1.3 mm long. Both the kink angle and the path length that oscillatory changes between different satisfy the selection criteria for the decay of a τ–. types of neutrinos could be responsible for the apparent neutrino deficit. Conclusive demanding particle tracking at micrometre The collaboration has identified the first evidence that electron-neutrinos, νe, from the resolution to reconstruct the topology: candidate ντ in a sample of events from Sun change type en route to the Earth came either a kink – a sharp change (>20 mrad) data taken in 2008–2009, corresponding to from the Sudbury Neutrino Observatory in in direction occurring after about 1 mm – as 1.89 × 1019 protons on the target at CERN. 2002, a few years after the Super-Kamiokande the original τ decays into a charged particle The sample contains 1088 events, including experiment found the first evidence for together with one or more neutrinos, or the 901 that appear to be charged-current oscillations in νμ created by cosmic rays in the vertex for the decay mode into three charged interactions. The search through these has atmosphere. Accelerator-based experiments particles plus a neutrino. yielded one candidate with the characteristics – have since observed the disappearance of νμ, The OPERA apparatus has two identical expected for the decay of a τ into a charged – 0 confirming the oscillation hypothesis, but until Super Modules, each containing a target hadron (h ), neutral pions (π ) and a ντ. now there have been no observations of the section and a large-aperture muon Indeed, the kinematical analysis suggests the – – appearance of a ντ in a νμ beam. spectrometer. The target consists of decay τ → ρ ντ. The event has a significance OPERA’s result follows seven years of alternate walls of lead/emulsion bricks of 2.36 σ of not being a background – – 0 preparation and more than three years of – 150 000 bricks in total – and modules fluctuation for theτ decay to h (π )ντ. beam provided by CERN (CERN Courier of scintillator strips for the target tracker. This candidate event is an important first November 2006 p24 and p27). The neutrinos The nuclear-emulsion technique allows step towards the observation of ντ → νμ are generated at CERN when a proton beam the collaboration to measure the neutrino- oscillations through the direct appearance of from the Super Proton Synchrotron strikes interaction vertices with high precision. the ντ. That claim will require the detection of a target, producing pions and kaons. These The scintillators provide an electronic a few more events, but so far the collaboration quickly decay, giving rise mainly to νμ that pass trigger for neutrino interactions, localize the has analysed only 35% of the data taken in unhindered through the Earth’s crust towards particular brick in which the neutrino has 2008 and 2009 and ultimately should have Gran Sasso. The appearance and subsequent interacted, and perform a first tracking of five times as much data than as at present. decay of a τ in the OPERA experiment would muons within the target. The relevant bricks provide the telltale sign of νμ to ντ oscillation are then extracted from the walls so that the Further reading through a charged-current interaction. film can be developed and scanned using N Agafonova et al. OPERA collaboration 2010 Detecting the τ decay is a challenging task, computer-controlled scanning microscopes. arXiv:1006.1623v1, submitted Phys. Letts. B.

Sommaire OPERA capture son premier neutrino du tau 5 Communication des premiers résultats du LHC à Hambourg 9 DØ observe une asymétrie anormale dans les désintégrations Un linac à récupération d’énergie à deux orbites au laser à de mésons B 6 électrons libres de Novossibirsk 10 Le Conseil du CERN opte pour davantage d’intégration 7 De la vie artificielle créée pour la première fois en laboratoire 12 Poursuite de la mise en service: amélioration de la luminosité 8 Les rayons X détectent de la « matière manquante » dans des gaz 14

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NEWS

FERMILAB DØ sees anomalous asymmetry in decays of B mesons

The DØ collaboration at has reported evidence of a violation of matter–antimatter DØ, 6.1 fb–1 symmetry (“CP symmetry”) in the behaviour of neutral mesons containing b quarks. 0.01 Studying collisions where B mesons decay semi-leptonically into muons, the team fi nds

sl 0 s

about 1% more collisions when two negatively a charged muons are produced than collisions with two positively charged muons. The –0.01 DØ → Ab 68% collisions in the DØ detector occur through a sl 95% symmetric proton–antiproton state and the 99% C.L. –0.02 expected CP asymmetry from the Standard B factory W.A.

Model is predicted to be much smaller than DØ Bs→Ds μ X what has been observed. An asymmetry of 1% –0.03 preliminary is therefore completely unexpected. combination The properties of B mesons, created in – collisions where a bb quark pair is produced, –0.04 –0.02 0 d are assumed to be responsible for this asl asymmetry. Mesons containing b quarks are known to oscillate between their particle The semileptonic dimuon B meson charge – – – – – b (B=bd or bs) and anti-particle (B=bd or bs) asymmetry, Asl, obtained by the DØ collaboration, state before they decay into a positively compared with the Standard Model prediction and s d charged muon (for the B) or a negatively with the measured muon asymmetries asl and asl – 0 0 charged muon (for the B). If a B meson for Bs and Bd mesons. oscillates before its decay, its decay muon has the “wrong sign”, i.e. its charge is contains strange quarks, than to the K+, identical to the charge of the muon from the which contain strange antiquarks. In detailed other b decay. Having 1% more negatively studies the collaboration has derived the charged muon pairs therefore implies that the contribution of this effect almost entirely B meson decays slightly more often into its from the data, making the measurement of matter state than into its antimatter state. the asymmetry in B-meson decays largely The DØ detector has two magnets, independent of external assumptions and a central solenoid and a muon-system simulation. toroid, which determine the curvature and The fi nal result is 3.2 σ from the Standard charge of muons. By regularly reversing the Model expectation, corresponding to a polarities of these magnets the collaboration probability of less than 0.1 per cent that can eliminate most effects coming from this measurement is consistent with any asymmetries in the detection of positively known effect. The analysis was based on and negatively charged muons. This feature is an integrated luminosity of 6.1 fb–1 and crucial for reducing systematic effects in this the plan is to increase the accuracy of this measurement. measurement by adding signifi cantly more Another known source of asymmetry arises data and improving future analysis methods. in muons produced in the decays of charged kaons. Kaons contain strange quarks and the Further reading interaction cross-sections of positively and V M Abazov et al. 2010 DØ collaboration,

Mega Industries, LLC negatively charged kaons with the matter arxiv:1005.2757, submitted to Phys. Rev. D.; 28 Sanford Drive, Gorham, ME 04038 USA making up the DØ detector differ signifi cantly: www-D0.fnal.gov/Run2Physics/WWW/ Phone: 207-854-1700 Fax: 207-854-2287 more interactions are open to the K–, which results/fi nal/B/B10A/. E-Mail: [email protected] www.megaind.com

6 CERN Courier July/August 2010

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CERN CERN Council opens the door to greater integration

At its 155th session, on 18 June, the CERN through a period of fi nancial austerity, and document CERN/2918, which is available at Council opened the door to greater integration endorsed CERN’s new code of conduct. http://indico.cern.ch/getFile.py/access?resI in particle when it unanimously first_draft.qxpFull details of the new 01/06/2010membership 15:06 d=1&materialId=0&contribId=35&sessionIdPage 1 adopted the recommendations of a working arrangements can be found in Council =0&subContId=0&confId=96020. group that was set up in 2008 to examine the role of the organization in the light of increasing globalization in . “This is a milestone in CERN’s history and HIGH ENERGY PHYSICS a giant leap for particle physics,” said Michel Spiro, president of the CERN Council. “It FROM OXFORD recognizes the increasing globalization of the fi eld, and the important role played by CERN on the world stage.” The key points agreed at the meeting were: A Zeptospace Odyssey Energy, the Subtle A Journey into the Physics of the ● All states shall be eligible for membership, GIAN FRANCESCO GIUDICE Concept LHC The discovery of Feynman's JENNIFER COOPERSMITH irrespective of their geographical location; '...engaging and frequently blocks from Leibniz to Einstein ● A new associate membership status is to witty, speckled with original Explains energy in accessible analogies and pithy quotations be introduced to allow non-member states to terms, using the history to from physicists and cultural explain the physics establish or strengthen their institutional links 2009 figures alike ... an excellent July 2010 with the organization; introduction to the LHC.' ● Associate membership shall also serve as - Physics World the obligatory pre-stage to full membership; 978-0-19-954650-3 | 416 pp | Hardback | £29.95 ● The existing observer status will be phased 978-0-19-958191-7 | 288 pp | Hardback | £25.00 out for states, but retained for international Measurements and their organizations; PATRICK PETER, AND Uncertainties PrimordialJEAN-PHILIPPE Cosmology UZAN A practical guide to modern IFAN HUGHES, AND International co-operation agreements and error analysis Provides an extensive survey THOMAS HASE protocols will be retained. “Particle physics is of all the physics necessary to A concise, easy-to-read becoming increasingly integrated at the global understand the current introduction to error analysis, level,” explained CERN’s director-general Rolf developments in the field of focusing on computer fundamental cosmology July 2010 analysis of experimental data Heuer. “The decision contributes towards 2009 creating the conditions that will enable CERN OXFORD GRADUATE TEXTS to play a full role in any future facility, wherever in the world it might be.” 978-0-19-956633-4 | 152 pp | Paperback | £19.95 CERN currently has 20 member states: 978-0-19-920991-0 | 856 pp | Hardback | £55.00 978-0-19-956632-7 | 152 pp | Hardback | £39.95 Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Discrete Symmetries and Quantum Germany, Greece, Hungary, Italy, Netherlands, MARCOCP Violation SOZZI Chromodynamics From Experiment to Theory High Energy Experiments and Norway, Poland, Portugal, Slovakia, Spain, 'A unique contribution to the GÜNTHER DISSERTORI, IANTheory G. KNOWLES, AND Sweden, Switzerland and the UK. India, Israel, available texts on these MICHAEL SCHMELLING topics.' Japan, the Russian Federation, the US, Turkey, - Aaron Roodman, Covers in full detail both the the European Commission and UNESCO have 2008 Stanford University 2009 theoretical foundations and observer status. Applications for membership the experimental tests of the OXFORD GRADUATE TEXTS theory from Cyprus, Israel, Serbia, Slovenia and INTERNATIONAL SERIES OF MONOGRAPHS Turkey have already been received by the ON PHYSICS NO. 115 CERN Council, and are currently undergoing 978-0-19-929666-8 | 568 pp | Hardback | £57.50 technical verifi cation. At future meetings, 24-HOUR CREDIT CARD HOTLINE: +44 (0)1536 454534 978-0-19-956641-9 | 856 pp | Paperback | £37.50 Council will determine how to apply the new PHONE: +44 (0) 01865 353250 arrangements to these states. EMAIL: [email protected] In other business, Council recognized Visit our website: www.oup.com/uk that further work is necessary on the 1 organization’s medium-term plan, in order to maintain a vibrant research programme

CERN Courier July/August 2010 7

CCJulAug10News.indd 7 05/07/2010 15:43 NEWS

L H C N E W S Further commissioning improves luminosity

By the end of June the LHC was making good variety of commissioning studies to establish are ramped to 3.5 TeV. progress towards delivering the fi rst 100 nb–1 the appropriate baseline for operating the By 26 June the teams were ready with a of integrated luminosity at an energy of LHC in these conditions. new sequence to ramp, squeeze and collapse 3.5 TeV per beam. This followed some two These studies involved establishing the the separation at the interaction points to weeks devoted to beam commissioning, optimal reference settings for both ramping bring three bunches per beam at nominal with the goal of achieving stable collisions at the energy and for a “squeeze” to β* of 3.5 m, intensity into collision at 3.5 TeV. With a 3.5 TeV with the design intensity of 8 × 1011 prior to bringing the beams into collision. physics run at an instantaneous luminosity protons per bunch. The fi rst days of July saw (The squeeze reduces the beam size at the of 5 × 1029 cm–2 s–1, the integrated luminosity machine fi lls for physics with six bunches interaction points and is described by the in the experiments since 30 March already per beam at this nominal intensity, providing parameter β*, which gives the distance from doubled, rising to more than 30 nb–1. A few further boosts to the goal of reaching 1 fb–1 the interaction point to the place where the days later, on 7 July, the machine ran with before the end of 2011. beam is twice the size.) The settings include seven bunches per beam at nominal intensity The fi rst collisions at 3.5 TeV between the all-important collimator positions, a key and achieved a new luminosity record of bunches at nominal intensity were achieved part of the machine protection system, and 1030 cm–2 s–1. This is one more step towards on 26 May, following earlier tests on ramping this alone involved 108 setup operations. the goal for 2010 of 1032 cm–2 s–1, which will the energy with bunches at this intensity The work also involved commissioning require 800 nominal bunches per beam. (CERN Courier June 2010 p5). However, to the transverse damper – basically, an ● Sign up to follow CERN’s Twitter feed for make progress towards further stable running, electrostatic defl ector – to subdue the latest LHC news. See http://twitter.com/ the accelerator team needed to perform a instabilities in the nominal bunches as they /.

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8 CERN Courier July/August 2010

CCJulAug10News.indd 8 05/07/2010 15:46 NEWS

First LHC results aired in Hamburg

“Physics at the LHC 2010”, which took place at DESY in Hamburg on 7–12 June, was the first large conference to discuss 7 TeV collision data from the LHC. Covering all fields of LHC physics, it attracted 270 participants, among them many young postdocs and students. On the opening day, Steve Myers, CERN’s director for accelerators and technology, gave an overview of the LHC’s status and the steps required to increase the luminosity. The spokespersons of the four big experiments, (ATLAS), Guido Tonelli (CMS), Jurgen Schukraft (ALICE) and Andrzej Golutvin (LHCb) summarized the commissioning David Gross presents his theoretical outlook. The poster session gives time for discussions. of their experiments and the progress in understanding the detectors, and also in the centre-of- with an integrated point of view, Mike Lamont of CERN sketched flashed up the first physics results. luminosity of about 16 nb–1 delivered to each the plans for the LHC and emphasized the The main message from these experiment, all four collaborations have goal of collecting 1 fb–1 proton-collision presentations was that the LHC is progressing rediscovered almost the full Standard Model data per experiment at 7 TeV before the end well and that the experiments are well particle spectrum – except for the top quark, of 2011 (plus two heavy-ion runs). With this prepared. Data-taking is going smoothly, which is just round the corner. integrated luminosity, the LHC will already triggers and reconstruction are working well Among the first LHC physics highlights compete with the Tevatron in a number of and detectors are rapidly being understood. are the observations of W and Z bosons, fields. It would be sensitive to W' and Z' Data processing on the LHC Computing Grid and of high-pT jets. In several presentations bosons with up to 1.9 TeV and 1.5 TeV, is also performing as expected. the audience was reminded of how long the respectively, and low-mass There was special emphasis at the community waited for single weak bosons to would also be in reach. However, the Higgs – if conference on the operation and performance be produced in the early days of the Spp–S. this is indeed nature’s choice – will most likely of detectors and in the afternoon sessions Now, dozens of W and Z bosons have already take longer to be discovered. young researchers reported on experiences been reported by ATLAS and CMS in different The last day of the conference was in all of the experiments. The reports showed decay channels. However, there is still a long dedicated to overview talks from other fields that many design performances have already way to go to match the excellent work done in (astroparticle physics, dark-matter physics) been achieved or are within close reach. One the electroweak sector by the experiments at and concluded with an excellent experimental by-product of understanding the detectors Fermilab’s Tevatron. summary by CERN’s and a is a “detector tomography”, which has been The political support for the LHC in visionary overview of theory by David Gross, performed using mainly photon conversions; Germany was touched upon on the third day the 2004 Nobel laureate in physics. Gross this has allowed several shortcomings of the of the conference. In their messages, Georg reflected on 20 predictions made in 1993 – detector simulations to be identified and Schütte, state secretary in the German a good fraction of which have already come removed. Ministry for Education and Research, and true. There is reason to hope that at least a The pay-off for the years of hard work Bernd Reinert, state secretary for science and few others (among them the discoveries of that led to this excellent knowledge of the research of the state of Hamburg, expressed the Higgs, supersymmetry and the origin of detectors has been a quick turnaround the keen interest of the funding bodies for dark matter, and the transformation of string time for physics results. After only a few further support and exploitation of the LHC. theory into a real predictive theory) will also weeks of high-energy data-taking at 7 TeV Looking at the prospects from the scientific come true. There are exciting times ahead.

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

CERN Courier July/August 2010 9

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accelerators Two-orbit energy recovery linac operates at Novosibirsk free-electron laser facility

Over the past 30 years, the Budker Institute of with dry nitrogen and separated from the First and second horizontal Nuclear Physics in Novosibirsk has developed tracks with second FEL at accelerator vacuum by a diamond window, many free-electron lasers (FELs). The most bypass (in operation) and from the air by polyethylene windows. recent one, which has been in operation Third and fourth tracks with Radiation is delivered to six stations, two IR FEL (under construction) since 2003, is a continuous-wave terahertz of which are used for the measurement of FEL based on a single-orbit energy-recovery radiation parameters, and the other four linac (ERL), which is the world’s most intense by users, typically biologists, chemists and Single track in vertical radiation source at terahertz wavelengths. plane with the THz FEL physicists. The laboratory is now making progress in (in operation) Common accelerating The other four orbits of the final ERL lie in RF structure constructing a four-orbit 40 MeV electron ERL the horizontal plane. The beam is directed to generate radiation in the range 5–250 μm. to these orbits by switching on two round Already operating with two orbits, this is the Fig. 1. The Novosibirsk ERL-based high power magnets. The electrons will pass through the world’s first multiturn ERL. FEL facility. The terahertz FEL and the FEL at the RF cavities four times, to reach 40 MeV. After FELs provide coherent radiation in the bypass are both in operation; the third and fourth the fourth orbit the beam will be used in an wavelength range from 0.14 nm to 1 mm. ERL tracks are under construction. FEL, before being decelerated four times. A They use the phenomenon of stimulated bypass with another FEL is installed at the radiation from relativistic electrons moving second orbit (20 MeV). When the bypass in an undulator – a special magnet that magnets are switched on, the beam passes creates a periodic alternating field such that through this FEL. The length of the bypass has the electron trajectory remains close to a been chosen to provide the delay necessary straight line (the undulator axis). Travelling in this case to give deceleration on the third through an undulator, electrons amplify a pass through the accelerating cavities. collinear electromagnetic wave if the last one Two of the four horizontal orbits were has wavelength λ = d/(2γ2), where d is the assembled and commissioned in 2008. The undulator period, and γ is the particle’s total electron beam was accelerated twice and energy divided by its mass. then decelerated down to the low injection Unfortunately, the maximum electron energy, thus successfully demonstrating the efficiency of FELs is only about 1%, which world’s first multiorbit ERL operation. The first makes energy recovery a desirable feature. lasing of the FEL at the bypass was achieved The simplest realization of energy recovery for in 2009, providing radiation in the wavelength an FEL is to install it into a straight section of range 40–80 μm. At first a significant (several a storage ring. Such storage-ring FEL facilities per cent) increase in beam loss occurred exist now but the power of their radiation during lasing. Sextupole correctors were does not exceed a few watts. The intrinsic The location of two FELs in the accelerator hall. therefore installed in some of quadrupoles to limitation of the power is caused by multiple The original terahertz FEL is at the lower right, with make the 180° bends achromatic to second interactions of the same electrons with light, the RF cavities of the ERL above it. Centre left is order. This increased the energy acceptance which increases the energy spread of the the new undulator for the bypass FEL. for the reused electron beam. The output beam. To achieve high light power, it is better power is about 0.5 kW at an ERL average to use a fresh beam, which is just what ERLs vertical plane. This FEL generates coherent current of 9 mA. The output of this new FEL is can do (CERN Courier June 2005 p26). radiation, tunable in the range 120–240 μm. near 70 μm, so the power obtained is also the The Novosibirsk ERL has a rather It produces a continuous train of 40–100 ps world record for this wavelength range. complicated magnetic system, which makes pulses at a repetition rate of 5.6–22.5 MHz. The beamline to deliver radiation from the use of a common accelerating structure The maximum average output power is 500 W, new FEL to existing user stations has been (figure 1). This differs from other ERL-based with a peak power of more than 1 MW. The assembled and commissioned. Thus, the FEL facilities in that it uses low-frequency minimum measured linewidth is 0.3%, which world’s first two-orbit ERL is now operating (180 MHz) non-superconducting RF cavities, is close to the Fourier-transform limit. A for a far infrared FEL. In the meantime, the with continuous-wave operation. The existing beamline directs the radiation from the FEL in assembly of the third and fourth ERL orbits is terahertz FEL uses one orbit, which lies in the the accelerator hall to the user hall. It is filled in progress.

10 CERN Courier July/August 2010

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