Heidelberg 2015 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 64: Heavy Ion Collisions and QCD Phases 8

HK 64.2: Group Report

Thursday, March 26, 2015, 17:30–18:00, T/HS1

Direct photon production in Pb-Pb collisions at the LHC with the ALICE experiment — Friederike Bock^1,2 and •Lucia Leardini¹ — ¹Physikalisches Institut, Heidelberg University — ²Lawrence Berkeley National Laboratory, Berkeley

Unlike hadrons, direct photons are produced in all stages of a nucleus-nucleus collision and therefore test our understanding of the space-time evolution of the produced medium. Of particular interest are so-called thermal photons expected to be produced in a quark-gluon plasma and the subsequent hadron gas. The transverse momentum spectrum of thermal photons carries information about the temperature of the emitting medium. In this presentation, direct-photon spectra from Pb-Pb collisions at √s_NN = 2.76 TeV and p-Pb collisions at √s_NN = 5.02 TeV will be shown. The results were obtained by measuring e⁺e⁻ pairs from external conversions of photons in the detector material. The measured direct-photon spectra will be compared with predictions from state-of-the-art hydrodynamic models. In the standard hydrodynamical modeling of nucleus-nucleus collisions, thermal photons mostly come from the early hot stage of the collision. As collective hydrodynamic flow needs time to build up, the azimuthal anisotropy of thermal photons quantified with the Fourier coefficient v₂ is expected to be smaller than the one for hadrons. However, the PHENIX experiment and ALICE experiment observed v₂ values of direct-photons similar in magnitude to the pion v₂. We will present the inclusive photon v₂ and v₃ in Pb-Pb collisions at √s_NN = 2.76 TeV and discuss implications for the v₂ and v₃ of direct-photons.