How robust is a thermal photon interpretation of the ALICE low-pT data?
Abstract
We present a rigorous theoretical analysis of the ALICE measurement of low-pT direct-photon production in central lead-lead collisions at the LHC with a centre-of-mass energy of sNN=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 pT ∈ [0.8;2.2] GeV and pT ∈ [1.5;3.5] GeV as well as a power-law (pT-4) behavior for pT > 4 GeV as predicted by QCD hard scattering.
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