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The ATLAS Experiment, the World's Largest-Volume Particle Physics The ATLAS experiment, the world’s largest-volume particle physics 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.” Sergio Bertolucci 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 particle detector 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 (Compact Muon Solenoid), 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 Large Hadron Collider beauty main way of identifying neutral particles, such as photons.
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