Direct Photons at RHIC

Abstract

Direct photons are ideal tools to investigate kinematical and thermodynamical conditions of heavy ion collisions since they are emitted from all stages of the collision and once produced they leave the interaction region without further modification by the medium. The PHENIX experiment at RHIC has measured direct photon production in p+p and Au+Au collisions at 200 GeV over a wide transverse momentum ($p_T$) range. The $p$ + $p$ measurements allow a fundamental test of QCD, and serve as a baseline when we try to disentangle more complex mechanisms producing high $p_T$ direct photons in Au+Au. As for thermal photons in Au+Au we overcome the difficulties due to the large background from hadronic decays by measuring "almost real" virtual photons which appear as low invariant mass $e^{+}{e}^{-}$ pairs: a significant excess of direct photons is measured above the above next-to-leading order perturbative quantum chromodynamics calculations. Additional insights on the origin of direct photons can be gained with the study of the azimuthal anisotropy which benefits from the increased statistics and reaction plane resolution achieved in RHIC Year-7 data.

4 pages, 2 figures, parallel talk at ICHEP08; fixed Fig. 2 problem

References in corpus (7)

• Enhanced production of direct photons in Au+Au collisions at sqrt(s_NN)=200 GeV and implications for the initial temperature
• Centrality Dependence of Direct Photon Production in sqrt(s_NN) = 200 GeV Au+Au Collisions
• A new critical study of photon production in hadronic collisions
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• Electromagnetic radiation from nuclear collisions at RHIC energies
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