Centrality dependence of low-momentum direct-photon production in Au + Au collisions at GeV

Adare, A.*; Imai, Kenichi; Tanida, Kiyoshi   ; PHENIX Collaboration*; 477 of others*

The PHENIX experiment at RHIC has measured the centrality dependence of the direct photon yield from Au+Au collisions at GeV down to . Photons are detected via photon conversions to pairs and an improved technique is applied that minimizes the systematic uncertainties that usually limit direct photon measurements, in particular at low . We find an excess of direct photons above the -scaled yield measured in collisions. This excess yield is well described by an exponential distribution with an inverse slope of about 240 MeV/ in the range 0.6-2.0 GeV/. While the shape of the distribution is independent of centrality within the experimental uncertainties, the yield increases rapidly with increasing centrality, scaling approximately with , where .