Detectors of the Alice experiment
First one needs to know the initial conditions, namely how powerful the collision was: this is done by measuring the remnants of the colliding nuclei in detectors made of high density materials located about 110 meters on both sides of ALICE (the ZDC’s) and by measuring with the FMD, V0 and T0 the number of particles produced in the collision and their spatial distribution. T0 also measures with high precision the time when the event takes place.
An ensemble of cylindrical detectors (from inside out: ITS Pixels (ITS Drift, ITS Strips, TPC, TRD) measures at many points (over 100 just the TPC ) the passage of each particle carrying an electric charge, so that its trajectory is precisely known. The ALICE tracking detectors are embedded in a magnetic field (produced by the huge red magnet !) bending the trajectories of the particles: from the curvature of the tracks one can find their momentum. The ITS is so precise that particles which are generated by the decay of other particles with a very short life time can be identified by seeing that they do not originate from the point where the interaction has taken place (the “vertex” of the event) but rather from a point at a distance of as small as a tenth of a millimeter.
The particles' Identity
ALICE also wants to know the identity of each particle, whether it is an electron, or a proton, a kaon or a pion. In addition to the information given by ITS and TPC, more specialized detectors are needed: the TOF measures, with a precision better than a tenth of a billionth of a second, the time that each particle takes to travel from the vertex to reach it, so that one can measure its speed, while the HMPID measures the faint light patterns generated by fast particles and the TRD measures the special radiation very fast particles emit when crossing different materials, thus allowing to identify electrons. Muons are measured by exploiting the fact that they penetrate matter more easily than most other particles: in the forward region a very thick and complex absorber stops all other particles and muons are measured by a dedicated set of detectors: the muon spectrometer.
The photons (particles of light), like the light emitted from a hot object, tell us about the temperature of the system. To measure them, special detectors are necessary: the crystals of the PHOS, which are as dense as lead and as transparent as glass, will measure them with fantastic precision in a limited region, while the PMD and in particular the EMCal will measure them over a very wide area. The EMCal will also measure groups of close particles (called “jets”) which have a memory of the early phases of the event.