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Measurement of Direct Photons in Au+Au Collisions at sNN=200  GeV

S. Afanasiev et al. (PHENIX Collaboration)
Phys. Rev. Lett. 109, 152302 – Published 9 October 2012
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    Abstract

    We report the measurement of direct photons at midrapidity in Au+Au collisions at sNN=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. They 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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    • Received 27 May 2012

    DOI:https://doi.org/10.1103/PhysRevLett.109.152302

    © 2012 American Physical Society

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    Vol. 109, Iss. 15 — 12 October 2012

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    • Figure 1
      Figure 1

      Ratio Rγ for different centrality selections, for the PbGl and the PbSc analysis. The error bars indicate point-to-point uncertainties, the boxes around the points indicate pT correlated uncertainties which are also mostly correlated between centralities.

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    • Figure 2
      Figure 2

      (a) Direct photon spectra for all centrality selections in Au+Au, and for p+p measured in [23]. The error bars indicate point-to-point uncertainties, the boxes around the points indicate pT-correlated uncertainties. The lines depict a TAA scaled fit for pT>8  GeV/c to the p+p cross section, they are dashed for the range where the fit is extrapolated to lower pT. (b) Comparison of the PbGl and PbSc results with the combined result for the 0–5% most central events. The error bars show the total uncertainties.

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    • Figure 3
      Figure 3

      Direct photon nuclear modification factor RAA for three different centrality selections. The error bars show point-to-point uncertainties, the boxes around the points depict pT correlated uncertainties. The boxes on the left show the uncertainty of the total inelastic p+p cross section, the boxes on the right show the uncertainty in Ncoll. Note that all errors from the p+p reference spectrum are correlated between the centralities.

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    • Figure 4
      Figure 4

      Direct photon nuclear modification factor RAA for 0%–5% most central events, compared with theoretical calculations [2, 3, 4, 5] for different scenarios. The boxes depict the same uncertainties as in Fig. 3. Note that the EPS09 PDF curve is calculated for minimum bias collisions.

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