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ALICE (A Large Ion Collider Experiment) is one of the six detector experiments being constructed at the Large Hadron Collider at CERN. It is optimized to study heavy ion collisions. Pb-Pb nuclei collisions will be studied at a centre of mass energy of 5.5 TeV per nucleon. The resulting temperature and energy density are expected to be large enough to generate a quark-gluon plasma, a state of matter wherein quarks and gluons are deconfined.

Inner Tracking System

The Inner Tracking System (ITS) consists of six cylindrical layers of silicon detectors. The layers surround the collision point and measure the properties of the emerging particles, pin-pointing their positions to a fraction of a millimetre. The ITS will recognize particles containing heavy quarks by identifying the points at which they decay.

ITS layers (counting from the interaction point):

* 2 layers of SPD (Silicon Pixel Detector),

* 2 layers of SDD (Silicon Drift Detector),

* 2 layers of SSD (Silicon Strip Detector).

Time Projection Chamber

The ALICE Time Projection Chamber (TPC) is the main particle tracking device in ALICE. Charged particles crossing the gas of the TPC ionize the gas atoms along their path, liberating electrons that drift towards the end plates of the detector. An avalanche effect in the vicinity of the anode wires strung in the readout, will give the necessary signal amplification. The positive ions created in the avalanche will induce a positive current signal on the pad plane. The readout is done by the 570312 pads that form the cathode plane of the multi-wire proportional chambers (MWPC) located at the end plates. This give the r and φ coordinates. The last coordinate, z, is given by the drift time.

Transition Radiation Detector

Electrons and positrons can be discriminated from other charged particles using the emission of transition radiation, X-rays emitted when the particles cross many layers of thin materials. To develop such a Transition Radiation Detector (TRD) for ALICE many detector prototypes were tested in mixed beams of pions and electrons.

Time of Flight

Charged particles are identified in ALICE by Time-Of-Flight (TOF); heavier particles are slower and so take longer to reach the outer layers of the detector. For its TOF system ALICE uses detectors called Multigap Resistive Plate Chambers (MRPC). There are approximately 160 000 MRPC pads with time resolution of about 100 ps distributed over the large surface of 150 square meters. Using the tracking information from other detectors every track firing a sensor is identified.

Photon Spectrometer

The Photon Spectrometer (PHOS) is designed to measure the temperature of collisions by detecting photons emerging from them. It will be made of lead tungstate crystals. When high energy photons strike lead tungstate, they make it glow, or scintillate, and this glow can be measured. Lead tungstate is extremely dense (denser than iron), stopping most photons that reach it.

High Momentum Particle Identification Detector

The High Momentum Particle Identification Detector (HMPID) is a RICH detector to determine the speed of particles beyond the momentum range available through energy loss (in ITS and TPC, p = 600 MeV) and through time-of-flight measurements (in TOF, p = 1.2–1.4 GeV). Its momentum range is up to 3 GeV for pion/kaon discrimination and up to 5 GeV for kaon/proton discrimination. It is the world's largest caesium iodide RICH detector, with an active area of 11 m². A prototype was successfully tested at CERN in 1997 and currently takes data at the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory in the US.

Muon spectrometer

The muon spectrometer measures pairs of muons, in particular those coming from the decays of J/ψ and Upsilon particles. Tracking chambers to detect the muons and reconstruct their trajectories will be made from a special composite material, which is highly rigid but very thin. A set of resistive plate chambers (RPC) will act as a triggering device.

Forward Multiplicity Detector

The Forward Multiplicity Detector (FMD) consist of 5 large silicon discs with each 10 240 individual detector channels to measure the charged particles emitted at small angles relative to the beam. The forward detectors also comprise the main trigger detectors for timing (T0) and for collision centrality (V0). Another impostant forward detector in ALICE is the Photon Multiplicity Detector (PMD). This is a pre-shower detector which measures the multiplicity and spatial distribution of photons produced in the collisions.

Electro-Magnetic Calorimeter

The Electro-Magnetic Calorimeter (EM-Cal) will add greatly to the high momentum particle measurement capabilities of ALICE.

Links

* Official ALICE Public Webpage at CERN

* ALICE section on US/LHC Website

* ALICE photography panorama

* Photography panorama of ALICE detector center

Physics Encyclopedia

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