Centrality Dependence of Low Momentum Direct Photon Production in = 200GeV Au+Au Collisions in PHENIX

Abstract

Interest in the production of direct photons at low momentum in heavy ion collisions has increased recently with the rather puzzling recent PHENIX results on the observation of a large excess in the yield of direct photons in Au+Au collisions compared to the Ncou scaled p+p yield and the observation of a large direct photon v2 at pT below 3GeV/c. The two measurements are difficult to reconcile within the current understanding of the fireball evolution and the assumption that low momentum direct photons are dominantly emitted early in the collision. This has sparked much theoretical interest, with new calculations refining old works as well as calculations of newly proposed production mechanisms. Experimental constraints on newly proposed production mechanisms are crucial in understanding the evolution of the fireball. One would also desire experimental evidence whether the direct photons below pT of 3GeV/c are dominantly emitted early or late in the collision. In this proceedings, we show new PHENIX measurements of the detailed centrality dependence of direct photon production in the range of 0.4 < pT < 5GeV/c in Au+Au collisions at = 200GeV based on the high statistics of the combined 2007 and 2010 datasets. Photons are cleanly identified through their external conversion in the backplane of the Hadron Blind Detector to dielectron pairs. We observe the shape of the excess direct photon invariant yield in the range of 0.6 < pT < 2GeV/c is well described by an exponential fit with an inverse slope of 240MeV/c2 consistent with previous measurements. Additionally, the shape is seen to be the same for all centralities within uncertainties and the integrated yield follows a power law as a function of Npart. The new results will offer powerful constraints to current calculations which must predict the same centrality dependence.