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Event Recorded by ATLAS When the LHC's Beam 2 Reached Closed

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Event Recorded by ATLAS When the LHC's Beam 2 Reached Closed A ‘beam splash’ event recorded by ATLAS when the LHC’s beam 2 reached closed collimators near the detector on 20 November. 12 | CERN “The imminent start-up of the LHC is an event that excites everyone who has an interest in the fundamental physics of the Universe.” Kaname Ikeda, ITER Director-General, CERN Bulletin, 19 October 2009. Physics and Experiments The moment that particle physicists around the world had been The EMCal is an upgrade for ALICE, having received full waiting for finally arrived on 20 November 2009 when protons approval and construction funds only in early 2008. It will detect circulated again round the LHC. Over the following days, the high-energy photons and neutral pions, as well as the neutral machine passed a number of milestones, from the first collisions component of ‘jets’ of particles as they emerge from quark� at 450 GeV per beam to collisions at a total energy of 2.36 TeV gluon plasma formed in collisions between heavy ions; most — a world record. At the same time, the LHC experiments importantly, it will provide the means to select these events on began to collect data, allowing the collaborations to calibrate line. The EMCal is basically a matrix of scintillator and lead, detectors and assess their performance prior to the real attack which is contained in ‘supermodules’ that weigh about eight tonnes. It will consist of ten full supermodules and two partial on high-energy physics in 2010. supermodules. The repairs to the LHC and subsequent consolidation work For the ATLAS Collaboration, crucial repair work included following the incident in September 2008 took approximately modifications to the cooling system for the inner detector. one year (see p. 21). The teams working on the six LHC The shutdown also allowed some schedules to advance. This experiments — ALICE, ATLAS, CMS, LHCb, LHCf and included the partial installation of muon chambers in the region TOTEM — put this time to good use, undertaking maintenance, between the central barrel of the experiment and the endcaps, repair and installation work, as well as major test runs with which had been originally deferred. As a result, by the autumn cosmic rays. the ATLAS experiment was even better prepared for collisions than it had been in 2008. Installation and maintenance Some of the most important work on the CMS experiment Consolidation work on the ALICE experiment began as soon involved the complete refurbishment of the cooling system for as the LHC stopped. A major operation to move the cabling of the tracker. Other items included some channels of a few muon the inner tracking system to allow more space and better access detectors, which were removed, repaired and reinstalled. The took until July 2009. Other maintenance included replacing CMS Collaboration also installed the final sub-detector, the a number of capacitors on the time projection chamber. The pre-shower, which sits in front of the endcap calorimeters. The shutdown also provided the opportunity for installation work pre-shower consists of a lead�silicon ‘sandwich’ with silicon that was originally scheduled for after the first LHC run. Further sensors comprising strips that are 2 mm wide. It can distinguish modules were installed in the transition radiation detector and between single photons and pairs of ‘overlapping’ photons in the photon multiplicity detector, but the biggest addition arising from the decays of neutral pions in the endcap region, was the new electromagnetic calorimeter (EMCal). where particularly large numbers of particles will be produced. 2009 | 13 Installation of the EMCal supermodules in ALICE ‘Edgeless’ silicon detectors in one of the Roman pots used in employs a special device to insert them between the magnet the TOTEM experiment. coil and the time-of-flight counters. This is a crucial task when trying to spot certain decays of which allow the detectors to come to within 1 mm of the particles such as the long-sought Higgs boson. beam. There are currently two Roman pot stations, each one at a distance of 220 m from the interaction point. Each station The LHCb Collaboration took the opportunity to finish their contains six pots. For the start-up in 2008 only two pots had detector completely, with the installation of the final muon the silicon detectors in place. The collaboration completed the chambers. Other improvements included modifications to installation of the other four pots in 2009. In the future, two reduce noise in the electromagnetic calorimeter to a negligible more stations will be installed at 147 m. level and upgrades to the computing network. Cosmic calibrations LHCf is a small experiment made up of two independent Cosmic rays provide a natural source of particles, in particular detectors located in the tunnel at 140 m to either side of the muons, that proved extremely valuable for checking and aligning ATLAS collision point. Both detectors were completed and detectors before the LHC restarted. In a five-week data-taking ready to take data in 2008. During the shutdown period the exercise in July–August, CRAFT (Cosmic Rays At Four Tesla), LHCf Collaboration worked mainly to optimize the data the CMS experiment recorded more than 300 million cosmic acquisition system. events with the magnetic field on. This large data-set was used to improve further the alignment, calibration and performance The TOTEM experiment is situated in the CMS cavern, with of the various sub-detectors prior to collisions in the LHC. detectors on both sides of the interaction point. On each side there are two charged-particle trackers, T1 and T2, at distances The full ALICE detector system (with nearly 40% of the of about 10.5 m and 13.5 m from the interaction point; indeed, EMCal installed) ran with cosmic rays from August to October. T1 is within the endcap of CMS. During the shutdown, the Altogether, the experiment accumulated more than 600 million remaining three quarters of the T2 detector were tested and cosmic events during the year, both with and without the installed. The T1 detector was also assembled and tested with magnets turned on. beam from the SPS. It will be installed at the next opportunity. During the shutdown, the ATLAS Collaboration continued In addition TOTEM has silicon detectors installed in ‘Roman the analysis of cosmic data collected in 2008. This allowed for pots’. These are movable devices inserted in to the beam pipe, detailed alignment and calibrations studies, reaching a precision 14 | CERN In June, tests of the beam transfer systems from the SPS A ‘splash’ event in CMS recorded at the beginning of to the LHC allowed the reconstruction of tracks through November when beam travelled as far as collimators just the Vertex Locator in LHCb. before Point 5. far beyond expectations for this stage of the experiment. Once beam entered the LHC just before Point 2, where the ALICE the detector was fully closed again in September 2009 cosmic- experiment is installed, and was dumped before Point 3. ray studies restarted, and from mid-October, with the control Several sub-detectors of ALICE saw their first beam. This room staffed round the clock, the experiment collected cosmic helped the collaboration to synchronize with the LHC clock data continuously until first beam appeared in the LHC. and test the capability to measure large numbers of particles simultaneously. LHCb differs in design from the other big experiments, as its detectors are aligned along the LHC beam line to observe the decay of B mesons. These particles, which are a major focus for During the following day, protons were injected into the LHCb’s studies, stay close to the beam pipe as they fly out from anticlockwise beam pipe of the LHC. They passed through the the collision zone. Near-horizontal cosmic-rays proved valuable LHCb experiment at Point 8 before being dumped just before for checking and aligning the outer parts of the LHCb detector, Point 7. Most of the LHCb sub-detectors were switched off to and allowed the observation of the first rings from one of the keep the experiment safe at this early stage, but a highlight was two ring-imaging Cherenkov detectors, RICH1. the switching-on of the LHCb magnet. This allowed the LHC operators to measure its effect on the LHC beam and to adjust The rate of horizontal cosmic rays is too low to align smaller the magnetic compensators around LHCb to keep the beam in components near the beam line in LHCb, but fortunately the orbit. experiment is located close to the point where the anticlockwise beam enters the LHC. During tests of the transfer line from the SPS to the LHC in June, the beam was directed into a beam The first weekend of November saw protons complete their stopper, which produced dense bursts of muons that crossed journey anticlockwise through three octants of the LHC before LHCb horizontally. This allowed the collaboration to align the being dumped in collimators just prior to entering the cavern vertex detector and the inner tracker to a few microns. of the CMS experiment at Point 5. The particles produced by the impact of the protons on the collimators left energy and First beam tracks in the experiment’s calorimeters and muon chambers, On 23 October, particles entered the LHC for the first time respectively. The more delicate inner detectors remained since September 2008, during tests to provide lead ions. The switched off for reasons of protection. 2009 | 15 A proton–proton collision at 900 GeV recorded by the Tracks in the ATLAS inner detectors show the decay of a ALICE experiment with all the sub-detectors switched on neutral kaon to two pions (red) in this event from stable during stable running on 6 December.
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