Dresden 2013 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 40: Schwerionenkollisionen und QCD Phasen

HK 40.6: Talk

Tuesday, March 5, 2013, 18:15–18:30, HSZ-204

Measurement of Direct Photon Elliptic Flow in Pb-Pb Collisions at √s = 2.76 TeV with ALICE — •Daniel Lohner — Physikalisches Institut, Ruprecht-Karls Universität Heidelberg

A unique tool for the study of the system evolution in nucleus-nucleus collisions is the measurement of photons. Since photons do not interact with the medium they carry undistorted information of the 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 v₂^γ,dir in Pb-Pb collisions at √s_NN=2.76 TeV at the LHC. The observed direct photon spectrum can be described by 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. 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, v₂^γ,dir is expected to be small compared to hadrons. However, the measurement of the direct photon azimuthal anisotropy provides evidence for a non-zero v₂^γ,dir for 1 < p_T < 3 GeV/c with a magnitude similar to the observed charged pion v₂^π±. A large v₂^γ,dir might lend support for a significant direct-photon emission from late stages of the system evolution where hadron flow has developed.