Identification of particles and hard processes with the spectrometer phos of the alice experiment

Resum

Heavy-ion collisions are a unique tool to study nuclear matter under extreme conditions of density and temperature as those existing a few moments after the Big Bang, in which is thought that nuclear matter was a plasma of almost free quarks and gluons. The production of the Quark-Gluon Plasma is the main goal of the ALICE experiment at the LHC collider, where Pb-Pb collisions at 5.5A TeV will be made. High energy photons are an interesting probe to investigate produced in these collisions because they give unperturbed information on their creation conditions. In this thesis, The PHOS detector in the ALICE experiment is devoted to their measurement with high accuracy in an energy domain ranging from 0.5 GeV to beyond 100 GeV. The PHOS performance is studied and particle and hard processes identification algorithms are developed. The PHOton Spectrometer PHOS consists of five modules, each composed of 56 x 64 PbWO4 crystals and a Charged Particle Veto detector. The parameters of the algorithms developed to simulate the response of PHOS and to perform the reconstruction of the signals collected by PHOS have been tuned to reproduce the data measured in test experiments performed with a prototype of PHOS. PHOS will be able to measure photons with a high energy resolution, ranging from 5% at 0.5 GeV to 1% at 120 GeV. Particle identification with PHOS relies on three identification criteria: time of flight measurement; charged particle matching between the Charged Particle Veto and the Electromagnetic Calorimeter; and shower shape analysis. Combining these criteria we are able to discriminate photons, electrons and hadrons with good accuracy. I find that the contamination of wrongly identified photons in a heavy-ion collision ranges from about 3% to 1%. Neutral pions are identified through an invariant-mass analysis in the energy range between 0 and 40 GeV, and by an event-by-event shower shape analysis for energies from 40 GeV to 120 GeV at