Production of Neutral Pions in Pb+Au collisions at 158 AGeV/c

The direct photons are a particularly useful probe to search for an evidence ofthe Quark-Gluon Plasma formation in ultra-relativistic heavy-ion collisions. Directphotons can be extracted experimentally by measuring the large background from π$^{0}$ and η meson decays. This thesis work represents the production of the neutral pion mesons measured with the CERES/NA45 experiment at the top SPS energy in 158 AGeV/cPb-Au collisions. The π$^{0}$−→$\gamma\gamma$(98.8%) is the decay channel used in the reconstruction scheme. The π$^{0}$ measurement is based on the data taken in the year 2000. The CERES experiment can measure the photons that convert shortly before the TPC by measuring the e$^{+}$e$^{-}$ pairs in the TPC. The RICH2 mirror is the main converter used for this analysis. The presented analysis method describes in more details the selection of the e$^{+}$ and e$^{−}$ tracks using only the TPC information to reconstruct the converted photon. A Secondary vertex technique was developed and used to select the photons converted in the RICH2 mirror area. The extraction of the π$^{0}$ needs a careful study of the combinatorial background determined using the mixed event technique. The obtained invariant mass distribution of the two photons defined well the π$^{0}$ peak. The neutral pion transverse spectra compared to phenomenological models and other experiments validate our analysis method.