Détection de pions neutres dans les collisions proton-proton à l'aide du spectromètre de photons PHOS de l'expérience ALICE auprès du LHC

Ultra-relativistic Heavy-Ion Collisions (HIC) allow to create nuclear matter as Quark Gluon Plasma (QGP) and to study it through various observables proposed to highlight its formation, in particular, via measurements of neutral pions (0). The 0 measurement is an important probe for both proton and heavy ion physics. In the former case, it provides an important mean of testing pQCD as well as useful data to constrain current and future theoretical models. It will then serves as a baseline to detect abnormal behaviour in heavy ion collisions. Dedicated to heavy ion physics, the ALICE experiment at Large Hadron Collider, allows the identification of 0 mesons via their decay into two photons (0) using the high resolution photon spectrometer (PHOS). PHOS will measure over a wide range of transverse momentum, spanning from hundreds of MeV/c up to several tenths of GeV/c. In order to perform the first 0 spectrum measurement at LHC and compare it to theoretical predictions, an estimation of 0 production cross-section in proton-proton collisions is calculated in a leading order LO and a next-to-leading order (NLO) approximation and presented at a first place. We also discuss the Monte Carlo correction factors to apply for obtaining a realistic 0 spectrum as the geometrical acceptance, the identification efficiency and the trigger efficiency along with the analysis on the invariant mass. The quality of the measurement depends on the performance of PHOS detector. In particular calibration studies on the first PHOS module have also been performed yielding a map of bad channels