Nuclear Particle Astrophysics (NPA) Seminar: Friederike Bock, CERN, “Photon-related measurement in ALICE”

Direct photons are produced by various processes in pp, p-A and A-A collision and are sensitive to details of the space-time evolution of the medium produced in heavy ion collisions. The low p_T part of the direct photon spectrum is expected to be dominated by thermal direct photons–thermal radiation of hot matter, i.e., of the quark-gluon plasma and the hadron gas. At RHIC energies it is a challenge for hydrodynamical models to simultaneously describe the yield and the elliptic flow of direct photons in p-A and A-A collisions, which is often referred to as the direct-photon puzzle.
Within ALICE photons can be detected with either of the two electromagnetic calorimeters, EMCal and PHOS, and via reconstruction of e^+e^- pairs from conversions in the ALICE detector material using the central tracking system. An additional hybrid method, combining the conversion information with that of the calorimeters has been developed.
Wherever possible the results were combined to reflect our best estimate of the inclusive photon spectra and flow, R_gamma, as well as the extraction of direct photon spectra or their upper limits and the flow.
In this talk the recent results on direct photon production in pp, p-Pb and Pb-Pb collisions from the ALICE collaboration will be discussed. In addition the final results on the direct photon elliptic flow (v_2^gamma dir,SP) at sqrt s_NN = 2.76$ TeV from the Pb-Pb run in 2010 in the pt range of 0.9 < p_T < 6.2 GeV/c will be presented in this talk. Comparisons to PHENIX results and to predictions of several theoretical models will be presented in order to shed light on the status of the direct photon puzzle at LHC energies.