Frankfurt 2014 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 33: Schwerionenkollisionen und QCD Phasen

HK 33.5: Talk

Wednesday, March 19, 2014, 17:45–18:00, HZ 6

How robust is a thermal photon interpretation of the ALICE low-p_T data? — Michael Klasen¹, Christian Klein-Bösing^2,3, •Florian König¹, and Johannes Wessels² — ¹Institut für Theoretische Physik, Westfälische Wilhelms-Universität Münster — ²Institut für Kernphysik, Westfälische Wilhelms-Universität Münster — ³ExtreMe Matter Institute, GSI, Darmstadt

We present a rigorous theoretical analysis of the ALICE measurement of low-p_T direct-photon production in central lead-lead collisions at the LHC with a centre-of-mass energy of √s_NN=2.76 TeV. Using NLO QCD, we compute the relative contributions to prompt-photon production from different initial and final states and the theoretical uncertainties coming from independent variations of the renormalisation and factorisation scales, the nuclear parton densities and the fragmentation functions. Based on different fits to the unsubtracted and prompt-photon subtracted ALICE data, we consistently find T=304 ± 58 MeV and 309±64 MeV for the effective temperature of the quark-gluon plasma (or hot medium) at p_T∈[0.8;2.2] GeV and p_T∈[1.5;3.5] GeV as well as a power-law (p_T⁻⁴) behavior for p_T>4 GeV as predicted by QCD hard scattering.