ALICE - A Large Ion Collider Experiment


*The ALICE collaboration is currently building a state of the art* detector optimized for heavy-ion physics.**
	Inner Tracking System The Inner Tracking System (ITS) will be made from six cylindrical layers of silicon wafers like this one. They will surround the collision point and measure the properties of particles emerging from collisions, pin-pointing their positions to a fraction of a millimetre. The ITS will look for particles containing strange and charm quarks by identifying the points at which they decay.
Time Projection Chamber Particle tracking will continue outside the ITS in a large detector called the Time Projection Chamber (TPC). The TPC will be full of gas with an electric field applied across it. When charged particles pass through, they will knock electrons out of atoms in the gas and these will drift in the electric field. By measuring the arrival of electrons at the end of the chamber, the TPC will reconstruct the path of the original charged particles.
Photon Spectrometer The photon spectrometer (PHOS) is designed to take the temperature of collisions by detecting photons emerging from them. It will be made of lead- tungstate crystals like these. When high energy photons strike lead tungstate, they cause it to glow, or scintillate, and this glow can be measured. Lead tungstate is extremely dense, which means that it stops most photons that reach it.
	Particle Identification A special task of the ALICE experiment is to identify the mass of the particles emitted. If the low energy particles may be identified by the loss of energy, the higher ones are detected measuring the time it takes for a particle to reach from the collision point to the detector barrel which is 3.5 meters away. The sensor for the arrival of the particles will be Parrallel Plate Counters - 160000 of them distributed over 150 square meters. Using the tracking information from other detectors every track firing a sensor is identified.
At even larger energies where the yield of particles is low, making it senseless to measure particles on an event-by-event basis, we have a smaller detector (14 square meters) called HMPID for identification of the mass of the particles. This detector is based on the detection of so-called Cherenkov photons emitted by the particles in a dielectric medium. Hence the detector is called a RICH (Ring Imaging CHernkov) because the pattern of the photons detected by a CsI photocatode is ring like. The two-third prototype successfully tested at CERN in November 1997 is shown. The large area CsI photocathodes are the result of a CERN R&D program.
	Muon Arm The tracking chambers of the ALICE muon arm will be made of 'sandwich composite' technology like this. Sandwich composites are highly rigid but use very little material. One job of the muon arm will be to measure J/Y particles. Since these decay into a pair of muons, two muons coming from the same place signal a possible J/Y decay.

CERN © 1998 - European Organisation for Nuclear Research - ALICE Experiment