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The ATLAS Experiment Physics Department | Departments Physics Physics Department The Physics (PH) Department carries out basic research in the field of experimental and theoretical particle physics. It aims at providing a stimulating scientific atmosphere and remains an important reference centre for the world physics community. The Department also hosts the majority of the scientific visitors to CERN who are mostly members of one of the collaborations of experiments performed here. The Experimental Physics unit plays a leading role in the construction and running of experiments and in the physics analysis, at the same time ensuring the support of the collaborations in the domains of detector technologies includ- ’ ing mechanics, electronics, and computing. The Theoretical Physics unit s role is to develop new theoretical ideas aimed at understanding the fundamental constituents and forces of nature, to develop their relevance for the experimental programme of CERN, and generally to contribute to the scientific culture at CERN. The ATLAS experiment The largest detector ever constructed in high-energy physics is taking shape in its underground cavern. ATLAS (A large Toroidal LHC ground cavern. Also, between May ApparatuS) is a collaboration of 151 and November 2004, a complete institutions from 34 countries with ‘vertical slice’ of the ATLAS detector roughly 1770 scientific authors. The (containing all elements from the ATLAS detector, designed to cover interaction point outwards) was the widest possible range of physics tested on a CERN beam line using a at the LHC, will be 46 m long and variety of particle beams.For the first 25 m high — the largest-volume time, all sub-detectors were operat- The cryostat containing the collider detector ever constructed. ed together with a common data- central solenoid and the barrel acquisition and controls system and liquid argon calorimeter makes 2004 saw steady progress in the con- 4.5 TB of data were collected, corre- its slow descent into the ATLAS struction and integration of compo- sponding to 90 million events. cavern. nents for the detector, and, most importantly, the first active detector modules were installed in the under- | CERN 24 | Physics Departments ATLAS uses a superconducting mag- detector system of ATLAS to be The first two of the eight barrel net system consisting of a 2-tesla installed in the experimental hall. toroid coils in place in the ATLAS solenoid magnet covering the inner cavern. tracking detector and a very large ATLAS has paid particular attention air-toroid magnet to bend the tra- to its inner tracking and muon sys- jectories of the highly penetrating tems, essential parts of all LHC Although accumulated delays have muon particles. The toroid system detectors. For tracking, 2004 saw the eaten up some of the envisaged comprises eight 26-m long coils for series production for pixel and sili- contingency, the current schedule the barrel region and 16 smaller coils con strip modules and straw cham- foresees the completion of detector for the end-caps. By the end of 2004, bers in full swing. For the muon sys- installation by March 2007. four of the eight coils had been put tem, the major effort has been on into their cryostats and two were constructing and testing the large lowered into the cavern after having number of chamber stations at successfully undergone acceptance many sites all over the world. tests. Assembly and integration of the end-cap toroid is progressing ATLAS is currently engaged in the well with all coils already wound.The massive Data Challenge 2 (DC2) central solenoid has also been suc- computing campaign, which is a test cessfully tested. at the 10% level of the data rate expected at LHC start-up.The goal is On 28 October the complete barrel to gain experience with worldwide- liquid argon (LAr) calorimeter was distributed production and analysis lowered into the cavern, following environments. The data will be used successful cold tests and commis- for large-scale physics studies, and sioning at the surface. The tile also to test and shape the ATLAS calorimeter project has also pro- computing model. gressed well. The barrel has been assembled in the underground cav- In conclusion, the detector installa- ern and in situ commissioning has tion, despite its enormous complexi- started. The complete calorimeter ty,is progressing steadily.In a project barrel, consisting of the LAr electro- of this scale, there are inevitable magnetic calorimeter and the tile problems which, however, have hadronic calorimeter, was the first been successfully addressed. Annual report 2004 | 25 | Departments Physics The CMS experiment The CMS (Compact Muon Solenoid) detector is compact compared with ATLAS, the other general-purpose LHC detector, but still impressively big, being more than 20 m long and 15 m high. The CMS detector is built around a The first detector inside the solenoid 66 million pixels, each measuring large superconducting solenoid, is the HCAL, which measures the 100 µm by 150 µm, will give 3-D 12.5 m long and with an inner diam- energy of hadrons and is formed information on particle trajectories eter of 6.3 m, which will provide a from layers of brass embedded with where the density of tracks from the magnetic field of 4 tesla. This size plastic scintillator. In 2004 the two collisions is highest; where there are allows the inner tracking detector, as end-caps were assembled, comple- fewer tracks, silicon strip sensors, well as the electromagnetic and menting the barrel that had been which provide accurate information hadronic calorimeters (ECAL and finished in 2003. The HCAL is com- in two dimensions, will be used. HCAL, respectively), to be placed pleted by the HFs — two ‘forward’ More than 200 square metres of sili- inside the magnetic volume. Four hadron detectors — placed at the con sensors will be required and out of five segments of the rein- ends of the CMS detector outside mass production has begun. forced niobium–titanium solenoid the magnetic volume. These use coils, which will carry 20 kA at 4.2 K, quartz fibres embedded in wedges The complete muon system occu- were delivered to CERN from Italy of steel to measure high-energy for- pies the outer regions of the barrel during 2004. ward-going particles. Assembly for and the two end-caps. It consists of both HFs finished in 2004. four layers of chambers made up of three different types of detectors. The ECAL detects and measures the Installation advanced significantly energies of electrons and photons, during 2004 and commissioning for One of the discs of an end-cap of and fits within the HCAL. This will the installed parts is well under way. the CMS detector installed with ultimately contain some 76 000 The design of the level-1 trigger sys- muon chambers. lead-tungstate scintillating crystals. tem was completed in 2003 and pro- Around 33 000 crystals were deliv- duction started in 2004. In order to ered to CERN by the end of 2004.The test the algorithms to be imple- first ‘supermodule’ (1700 crystals mented in the higher-level trigger with their final readout electronics system, some 70 million simulated and environmental control system) events were produced early in 2004. was assembled and operated in a Experience gained during this ‘Data particle beam at CERN, with excel- Challenge 04’ will be invaluable for lent results. A ‘preshower’ detector the Computing Technical Design based on silicon sensors will be Report in mid-2005, while the data placed in the end-caps in front of the will be used for the analyses to be crystals. By the end of 2004 all the presented in the Physics Technical required 4300 sensors for this detec- Design Report in late 2005. tor had been produced. Last but not least, the two under- The heart of CMS is the inner track- ground caverns, one for the detector ing system,consisting of silicon pixel and one for off-detector electronics, and silicon strip sensors. More than were completed by the end of 2004. | CERN 26 | Physics Departments The ALICE experiment Unlike the other LHC detectors, ALICE will have to cope with the huge – number of particles produced in nucleus nucleus collisions at the LHC, and will unravel the mysteries of a very hot, very dense state of matter. ALICE (A Large Collider Experiment) at CERN and the production of the Two technical design reports (TDRs) is a detector specialized in analysing readout chamber in Darmstadt and were submitted by the ALICE heavy-ion collisions. It will study the Bratislava were completed during Collaboration in 2004: the Forward properties of quark–gluon plasma, a 2004. Detector TDR and the TDR for the state of matter where quarks and trigger, data acquisition, high-level gluons, under conditions of very ALICE incorporates several kinds of trigger, and control systems. Various high temperatures and densities, are detector to identify particles. The components of the trigger and data no longer confined inside hadrons. PHOS is a specialized detector to acquisition were tested during runs The detector consists of a central measure photons emerging directly with test beams of the ITS and the part, which measures electrons, pho- from the collisions. It consists of lead TPC systems. tons and hadrons, and a forward tungstate crystals that are being spectrometer that measures the manufactured north of the polar cir- The ALICE offline framework has more penetrating muons. cle in Apatity,Russia.More than 8500 been further developed and was crystals (about 50% of the total) used in the 2004 Physics Data The forward spectrometer is com- have been accepted at CERN. The Challenge in a distributed Grid envi- posed of absorbers, tracking and ring imaging Cherenkov detector ronment. More than 2 million jobs triggering chambers, and a large (HMPID), which identifies particles were run for simulation and recon- dipole magnet.
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