The ATLAS experiment, the world’s largest-volume detector.

14 | CERN “For the physics community, the LHC is a discovery machine, in that finding just the Higgs boson anticipated by the Standard Model would be almost a disappointment.” in the CERN Bulletin. Physics and Experiments

ALICE, probing the quark–gluon plasma continued during 2008 for detectors added later to the design ALICE is a heavy-ion experiment designed to study the physics (TRD, PHOS, and EMCAL). Thus, detector integration and of strongly interacting matter and the quark–gluon plasma in commissioning were the main activities in 2008. lead–lead collisions at the LHC. The ALICE Collaboration currently includes more than 1000 physicists and senior Several runs with cosmic rays were performed at the beginning engineers — from both nuclear and high-energy physics — of the year, and from May until mid-October ALICE was from about 100 institutions in some 30 countries. Some new operated continuously (24/7). As far as could be verified, the institutes from the US and South Korea joined ALICE in 2008, performance of all subsystems is very close to (or better than) while the associate members IPE Karlsruhe (Germany) and specification. BARC (Mumbai, India) left after completing their respective technical contributions to the experiment. During LHC commissioning in September, only a subset of detectors was switched on because the particle flux was ALICE consists of a central part, which measures hadrons, occasionally very high during beam tuning. Nevertheless, electrons, and photons, and a forward spectrometer to measure timing of most trigger detectors was verified and adjusted with muons. The central part is embedded in the large L3 solenoid beam. magnet and comprises an inner tracking system (ITS) of high- resolution detectors, a cylindrical time projection chamber The commissioning of ALICE required an extremely large (TPC), three particle identification arrays of time-of-flight effort in terms of manpower. Extrapolating from the experience (TOF), ring imaging Cherenkov (HMPID) and transition with operation 24 hours per day, a nominal year of data taking radiation (TRD) detectors, plus two single-arm electromagnetic would require the collaboration to provide about 17 000 shifts, calorimeters (the high-resolution photon spectrometer PHOS as each subsystem currently requires at least one person on shift and the large-acceptance jet calorimeter EMCAL). The forward in the ALICE control room in addition to experts being on call muon arm consists of a complex arrangement of absorbers, a at CERN. However, this need will be reduced in the course of large dipole magnet, and 14 planes of tracking and triggering 2009 by automating procedures and recovery operations and chambers. Several smaller detectors (ZDC, PMD, FMD, T0, by combining shifts for different detector systems. V0) used for global event characterization and triggering are located at forward angles. An array of scintillators (ACORDE) ATLAS, the largest volume on top of the L3 magnet is used to trigger on cosmic rays. ATLAS is a general-purpose experiment for recording proton– proton collisions at the LHC. The detector design has been Most of the ALICE detectors were installed, tested, and pre- optimized to cover the largest possible range of LHC physics. commissioned in situ during 2007. Construction and assembly This includes searches for Higgs bosons or alternative schemes

2008 | 15 Inner view of the ALICE detector; the red part is the gigantic magnet. to answer the puzzling question about the origin of mass, and A major challenge concerned the installation of over searches for supersymmetric particles, and other new physics 50 000 cables, more than 3000 km in length, and more than beyond the Standard Model. The ATLAS Collaboration 10 000 pipes for services. On 16 June an historic moment consists of 169 institutions from 37 countries with roughly occurred with the closure of the LHC beam pipe, followed in 2800 scientific participants. early August with the successful bake-out of the beam pipe. The latter operation was particularly critical because it required the The ATLAS detector has cylindrical symmetry around the evaporative cooling system to be in full working order to protect beam pipe, with increasingly large layers of subdetectors placed the pixel layers from overheating. The evaporative cooling plant around it and endcaps to ensure hermiticity. The inner detectors had suffered a major failure of its compressors at the beginning — a series of thin silicon and gas detectors immersed in a of May 2008, and the repair and cleaning of the plant dictated solenoidal magnetic field — are used for pattern recognition, the critical path for the closure of the detector. and for momentum and vertex measurements. In addition to the central solenoid, the magnet system also comprises a barrel In 2008 several dedicated running periods with cosmic rays were toroid and two endcap toroids. The high granularity liquid-argon used to test and calibrate detectors, including the trigger and electromagnetic calorimeters and the hadronic scintillator-tile data-acquisition systems. The detector was largely operational calorimeter are surrounded by the muon spectrometer, which defines the overall dimensions of the ATLAS detector. for the LHC start-up in September, as was the distributed computing infrastructure. The first beam-related events were Installation in the cavern 90 m underground began in summer successfully recorded and reconstructed and were used very 2003 and culminated in 2008 with completion of the initial efficiently for initial timing adjustments. ATLAS detector configuration. The muon chambers were the last component to be installed in July. In parallel with the Following the LHC incident on 19 September, the full detector installation process, testing and consolidation work for the on- has essentially been in continuous operation in cosmic-ray data and off-detector electronics and power supplies were important collection mode. These runs are very valuable for improving activities, and the detector systems were gradually brought into monitoring and data-quality procedures, as well as for initial operation, calibrated, and tested with cosmic data. global alignments and calibrations.

16 | CERN The central barrel and one endcap of the CMS experiment with the LHC pipe connecting the two. CMS, the heavy-weight detector The year began with the lowering of the last two of eleven CMS (), like ATLAS, is a general- massive iron disks and wheels, marking the completion of eight purpose detector used to study a large range of physical years of assembly in the SX5 surface building. Also, the magnet phenomena produced by particle collisions at the LHC. In a ancillaries were brought down from the surface, reinstalled, unique strategy, the detector was assembled above ground and commissioned. The cooldown of the CMS solenoid to the concurrently with the excavation of the underground cavern. nominal temperature of 4.5 K was achieved at the beginning The CMS Collaboration consists of over 2500 scientists and of August. Final closure and commissioning of the detector, engineers from over 180 institutes in 38 countries. beam pipe, and trigger electronics proceeded throughout the summer, including installation of the last major element, the The main volume of the CMS detector is a cylinder, 21 m long endcap electromagnetic calorimeter. Commissioning of all and 16 m in diameter, weighing in total 12 500 t. The tracking elements using the final data-acquisition system (with 1/8 of volume is defined by a cylinder of length 6 m and a diameter the ultimate online computing power) took place in parallel of 2.6 m. About 210 m2 of silicon microstrip detectors (around with extensive tests of reconstruction and physics analysis 10 million channels) provide the required granularity and software and of the Worldwide LHC Computing Grid. precision in the bulk of the tracking volume; pixel detectors placed close to the interaction region improve measurements of In early September, after almost 20 years of design and the track impact parameters and allow accurate reconstruction construction, CMS started taking data with LHC beams. of secondary vertices. The tracking system is placed inside the The solenoid and the inner tracking system were switched off huge superconducting magnet, 13 m long and 6 m in diameter, awaiting stable beams. The rest of the detector subsystems which will operate at 3.8 T. The magnet is used to determine took good-quality data and reacted quickly to changing beam the momentum of charged particles from the curved paths conditions. Measurements of the fringe fields in the cavern they follow in the magnetic field. The magnet return yoke acts showed them to be higher than expected. To understand fully as the principal support structure for all the detector elements. and mitigate the adverse effects, the collaboration decided Muons are identified and measured in four identical muon to slow the schedule for bringing the field to 3.8 T. This was stations inserted in the return yoke. Each muon station consists reached a few days after the incident on 19 September. This of many planes of aluminium drift tubes in the barrel region same magnetic field was then used for the one month of data and cathode-strip chambers in the endcap region. taking with cosmic rays that followed the failure in the LHC.

2008 | 17 The large cavern that hosts the LHCb detector 100 m underground.

The experiment was then shut down for annual maintenance consists of two coils, both weighing 27 t, mounted inside a of the cooling and other services, installation of the pre-shower 1 450 t steel frame. subdetector, and repairs of various elements. The calorimeter system — a sandwich-like structure, with LHCb, tracking down antimatter alternating layers of metal and polystyrene plates — is LHCb’s The main purpose of the beauty main way of identifying neutral particles, such as photons. (LHCb) experiment is to investigate the phenomenon known as CP violation in the decay of particles containing b and Muons are present in the final states of many B meson decays, so anti-b quarks, collectively known as ‘B mesons’. CP violation muon detection is vitally important for the LHCb experiment. is a necessary ingredient in explaining the total absence of Located at the far end of the detector, the muon system antimatter in the Universe. comprises five rectangular ‘stations’, gradually increasing in size and covering a combined area of 435 m² — about the same size Rather than flying out in all directions, B mesons formed by the as a basketball court. colliding proton beams (and the particles they decay into) stay close to the line of the beam pipe. This is reflected in the design Given the fact that the main parts of the detector had already of the detector, which stretches for 20 m along the beam pipe, been installed in 2007, the main activity for the LHCb with its subdetectors stacked behind each other like books on Collaboration in 2008 was to prepare for operation with cosmic a shelf. rays and real beams from the accelerator. The point where the beams collide, and B mesons are produced, is inside the VErtex LOcator (VELO) subdetector. With On 22 August, during the LHC final synchronization test, the its 84 half-moon-shaped silicon sensors, each connected to proton beam in the SPS accelerator was stopped at the beam electronics, the VELO can locate the position of B particles to dump before being injected into the transfer line that connects within 10 μm. to the LHC at a point next to LHCb. Particle tracks from the proton interaction in the line were seen in the VELO triggered Two ring-imaging Cherenkov detectors (RICH), as well as by the calorimeters and other tracking detectors, showing that tracking detectors, lie on either side of LHCb’s magnet, which the detector was ready to take data.

18 | CERN Detail of the assembly of the LHCf detector. Detector elements of the TOTEM experiment.

On 10 September, the first beam of protons made a full circuit LHC tunnel during January and February 2008. Control and of the LHC arriving at LHCb at 10.12 a.m. During these first data collection were successfully performed from the ATLAS manoeuvres, the experiment’s sensitive tracking detectors were counting room via signal cables and optical fibres 200 m long. not switched on, but the muon detectors, the calorimeters, and In May 2008, a dedicated control room for LHCf was prepared the RICH detectors all recorded the particles produced by the on the surface at Point 1, and all LHCf operation became beam. The tracking system was activated later in the day and available from the dedicated control room before the first beam recorded the tracks from the LHC proton-beam interactions. circulation in the LHC in September. The LHCb detectors, trigger system, and data-acquisition system again worked perfectly. During this first beam circulation, the LHCf main calorimeters were disconnected to avoid any accidental damage. However, The installation of the radiation shielding wall and all detector the LHCf front counters (subdetectors composed of thin services in the experimental area was also completed in 2008. plastic scintillators, located in front of the main calorimeters) were at the nominal position, allowing the monitoring of beam- LHCf, studying high-energy cosmic rays related effects. LHCf is an experiment dedicated to the measurement of neutral particles emitted in the very forward direction in LHC TOTEM, measuring the proton collisions. The physics motivation for LHCf is the calibration The TOTEM experiment studies the ‘forward particles’, that of the hadronic interaction models that are used in very high- is those particles travelling at small angles with respect to the energy cosmic-ray physics. Because the 14 TeV of proton– beams. The experiment will measure the total interaction cross- proton collisions at the LHC corresponds to 1017 eV equivalent section of protons at the LHC. The data collected will help to energy in the laboratory system, the LHCf measurements will gather information about the internal structure of protons and provide a crucial calibration point for studying the origin and the principles that determine their shape and form as a function composition of very high-energy cosmic rays. of their energy. Furthermore, TOTEM will also allow very precise measurements of the LHC luminosity and individual LHCf makes use of two independent detectors, installed on cross-sections to be used by the other LHC experiments. either side of the ATLAS interaction point, at a distance of 140 m away from it. Both detectors were installed in the The TOTEM apparatus includes two main types of detector:

2008 | 19 The NA48/2 apparatus on the right used for studying The ISOLTRAP Penning trap spectrometer at ISOLDE. rare particle decays. telescopes for the detection of particles scattered inelastically and focused at around 200 m downstream in front of the first and special detectors called ‘Roman pots’ for the detection spectrometer chamber. The main experimental components of particles scattered elastically. The Roman pots are devices used in this analysis are the magnetic spectrometer consisting mounted on the vacuum chamber of the accelerator that can of a dipole magnet surrounded by two sets of drift chambers; be moved towards the machine axis until the inner edge of the liquid krypton (LKr) calorimeter used to measure the detectors reaches a distance of a millimetre or so from the electromagnetic deposits and identify electrons through their beam. energy-to-momentum ratio; the hodoscope — a two-plane segmented scintillator detector — to trigger the detector During 2008, all the subdetectors were tested with the final readout on charged track topologies; and a neutral hodoscope electronics and a production line with final tests for each consisting of a plane of scintillating chambers inside the LKr subdetector was established. The installation of the detectors and a two-level trigger logic. close to the CMS experiment continued steadily as the team prepared for the first runs of the LHC. The aim of the TOTEM With the help of dedicated theoretical calculations developed Collaboration is to install the complete experiment before in close collaboration with theorists, the collaboration was able summer 2009, with the exception of the Roman Pots at the high to make new measurements on the ππ scattering lengths with radiation stations (147 m from the interaction point). These an experimental error of just a few per cent, a factor three times will be only partially equipped in order to gain preliminary better than before. The results confirmed with an unprecedented experience of the radiation levels. precision the predictions of chiral perturbation theory.

NA48/2, fine tuning QCD parameters ISOLDE and nuclear physics Thanks to the very large samples of charged kaon decays that The production, study, and use of exotic nuclei located far the NA48/2 experiment accumulated in 2003–2004, the from stability are the focus of the ISOLDE facility, which over collaboration was able to present the results of the data analysis the past 40 years has garnered unique expertise in radioactive in 2008. Two simultaneous K± beams were produced by beams. Several hundred scientists from around the world 400 GeV/c primary protons from the SPS, impinging on a make use of the broad range of isotopes available to study beryllium target. The beams were then deflected in a front- nuclear structure, nuclear astrophysics, atomic and solid-state end achromat to select momenta in the range (60±3) GeV/c physics, and applications to life sciences, currently within more

20 | CERN Animated discussions between theorists and experimentalists are a vital part of research at CERN. than 50 experiments. During 2008 an experiment using the were reviewed and endorsed by the CERN Scientific Policy ISOLTRAP Penning trap system discovered a new isotope of Committee. radon, 229Rn, and simultaneously measured its atomic mass with high precision. This is the first time that a new nucleus Members of the Unit worked hard in 2008 to be ready to interpret has been discovered in an ion trap. Another novel use of traps the first experimental results from the LHC experiments, and was demonstrated in the production of unstable iron nuclei of to compare them with predictions from scenarios beyond astrophysical importance, which were obtained by injecting the Standard Model. In supersymmetric theories, special copious amounts of radioactive manganese nuclei into the trap, attention has been devoted to possible signatures coming from where they decay into isotopes of iron. In the application of extra dimensions in the Universe. For this, models have been exotic nuclides to biophysics, nuclear hyperfine spectroscopy constructed to reproduce the mass spectrum and the mixing using radioactive beams of mercury and lead provided new angles of neutrinos. information about heavy-metal toxicity in proteins. THEORY, the Standard Model and beyond Research in the Theory Unit covers a wide spectrum of topics, including the phenomenology of the Standard Model of particles and forces, models of physics beyond the Standard Model, particle astrophysics, cosmology, and string theory. During 2008 the Unit produced some 250 publications.

Triggered by the imminent start of the LHC and some technically unjustified popular fears, members of the Unit reviewed existing safety reports dealing with the possible consequences of the production of new particles or states of matter at the LHC. The work focused on strangelets and stable microscopic black holes. The results confirm that the outcomes of LHC collisions do not pose any risk, and the final documents

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