Relation of observables to experimental techniques

Every probe in the search for the QGP in relativistic heavy ion collisions tends to have a different experimental technique associated with it. In all cases the multiplicities in nuclear collisions are so large that all the detectors used are very highly segmented. For measuring the charged multiplicity or dn/dy,a segmented multiplicity detector, usually an array of proportional tubes with pad readout, or a silicon pad array were used in early experiments, while Time Projection drift Chambers (TPC) have become popular more recently. For measuring transverse energy flow, dET/dy, a hadron calorimeter is used. Some groups use an electromagnetic shower counter for this purpose. This has the advantage of being smaller, cheaper and higher in resolution than a full hadron calorimeter; but it has the disadvantage of being biased, since only π0 and η0 mesons are detected (via their two photon decay). Nuclear fragmentation products are detected by calorimeters in the projectile direction and by E, dE/dx scintillator arrays in the target fragmentation region. The particle composition and transverse momentum distributions are measured using magnetic spectrometers with particle identification. Typically, time-of-flight, gas and aerogel Cerenkov counters, and dE/dx are used to separate pions from kaons, protons, deuterons, etc. Drift chambers are generally utilized for charged particle tracking, although streamer chambers and TPCs are also in use. Lepton pair detectors are very specialized, and usually combine magnetic spectrometers with lepton identification (muons by penetration, and electrons by “gas” and “glass”).