Particle identification with the ALICE TOF detector at very high particle multiplicity

ALICE (A Large Ion Collider Experiment) [l], together with CMS, ATLAS and LHCb, will be one of the experiments at the CERN LHC (Large Hadron Collider). It will study Pb-Pb collisions with a centre-of-mass energy , , G = 5.5 TeV/nucleon pair, (-30 times higher than RHIC energies) in order to explore the behaviour of nuclear matter in extreme density and temperature conditions. As a matter of fact, such an energy should make nuclear matter undergo a QCD phase transition which, in turn, should result in the formation of a state where quarks and gluons are almost free, the so-called Quark-Gluon Plasma (QGP). This state is what is supposed to have existed very little after the Big Bang, and is thought t o be now present in the heart of Neutron stars. However, if the QGP is actually produced during Pb-Pb collisions, it will last only for a very small fraction of a second after which it will expand into a dilute hadronic state. Besides the QGP formation, another important goal of the ALICE experiment is to study whether a partial restauration of the chiral symmetry, with quark masses reduced to the bare ones, will occur. In order to face the extremely compelling task of understanding and analyzing such a system, ALICE will concentrate on the study of a large number of observables [Z], such as particle multiplicities, particle spectra, the production of jets and direct photons, and many others, relying especially on an excellent power of separation among pions, kaons and protons in the momentum range from 0.5 GeV/c up to a few GeV/c, where a large fraction of the primary charged hadrons are expected to be produced. For this reason, an effective Particle IDentification (PID) based on a powerful Time Of Flight (TOF) system, is of undoubtful importance.