Direct Photon and Neutral Mesons Measurements with the ALICE Detector

Abstract

The ALICE experiment at LHC is dedicated to studies of the QuarkGluon Plasma (QGP) state, which is going to be created in heavy-ion collisions. Both photons and neutral mesons are excellent probes for QGP formation. Photons are produced during the different stages of the expansion of the initial hot matter fireball. They do not interact strongly with the medium and passing through it, they carry information on their emission point. The prompt photons which are formed at the early stage of the collision enable us to test perturbative QCD constraining parton distributions and fragmentation functions. Looking into the regime of thermal photons, one can extract the temperature of the medium. The mediuminduced energy loss of particles can be investigated via the measurement of neutral meson spectra for different centrality classes as well as via neutral meson-hadron correlations. A decrease of the nuclear modification factor (R AA ) with centrality of the collision is observed. The suppression of the away side of the per-trigger yield modification factor (I AA ) shows in a similar way the evidence for energy loss in medium. Both direct photons and neutral mesons have been measured by the ALICE experiment. Photons are measured in ALICE directly in the two electromagnetic calorimeters (PHOS and EMCal), as well as via method of photon conversion (PCM) into electron-positon pairs in the inner tracking system (ITS) and the time projection chamber (TPC). Neutral mesons are combined from photon pairs via the invariant mass technique. Results obtained in EMCal, PHOS and PCM are consistent one to the other and allow to measure the spectra of particles with high precision over a wide kinematical range. An overview of the recent results on photon and meson physics from ALICE will be shown.