Measurement of Direct Photons in Au+Au Collisions at sqrt(sNN) = 200 GeV
Abstract
We report the measurement of direct photons at midrapidity in Au+Au collisions at sqrtsNN = 200 GeV. The direct photon signal was extracted for the transverse-momentum range of 4 GeV/c < pT < 22 GeV/c, using a statistical method to subtract decay photons from the inclusive-photon sample. The direct-photon nuclear-modification factor RAA was calculated as a function of pT for different Au+Au collision centralities using the measured p+p direct-photon spectrum and compared to theoretical predictions. RAA was found to be consistent with unity for all centralities over the entire measured pT range. Theoretical models that account for modifications of initial-direct-photon production due to modified-parton-distribution functions in Au and the different isospin composition of the nuclei, predict a modest change of RAA from unity and are consistent with the data. Models with compensating effects of the quark-gluon plasma on high-energy photons, such as suppression of jet-fragmentation photons and induced-photon bremsstrahlung from partons traversing the medium, are also consistent with this measurement.
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