Seminar in Hadronic Physics

Measurement of Direct Photons in Pb-Pb Collisions at √s= 2.76 TeV: First Results from ALICE

Daniel Lohner

Alice collaboration & Heidelberg University

A unique tool for the study of the collision evolution in nucleus-nucleus collisions is the measurement of photons. Since photons do not interact with the medium they carry undistorted information of system at their production time. Besides photons from hadron decays also direct photons are emitted at every stage of the system evolution. Recently, the ALICE collaboration presented a first measurement of the direct-photon spectrum and elliptic flow in Pb-Pb collisions at √s= 2.76 TeV at the LHC. The observed direct photon spectrum can be described by prompt photon NLO (pQCD) predictions at momenta above 4 GeV/c while the spectrum at low p_T shows a clear excess above the NLO pQCD prediction, which is interpreted as a thermal photon signal. In order to describe the low p_T direct-photon spectrum, recent hydrodynamical calculations include a substantial portion of thermal photons from the hot plasma phase. As a consequence of the early production time, the elliptic flow of direct photons is expected to be small compared to the one of hadrons. However, the measurement of the direct photon azimuthal anisotropy provides evidence for a non-zero elliptic flow for 1 < p_T < 3 GeV/c with a magnitude similar to the observed charged pion elliptic flow. A large elliptic flow might lend support for a significant direct-photon emission from late stages of the system evolution where hadron ow has developed.