Centrality dependence of low-momentum direct-photon production in Au$+$Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV

Abstract

The PHENIX experiment at RHIC has measured the centrality dependence of the direct photon yield from Au$+$Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV down to $p_T=0.4$ GeV/$c$. Photons are detected via photon conversions to $e^{+}{e}^{-}$ pairs and an improved technique is applied that minimizes the systematic uncertainties that usually limit direct photon measurements, in particular at low $p_T$. We find an excess of direct photons above the $N_{\mathrm{coll}}$-scaled yield measured in $p$$+$$p$ collisions. This excess yield is well described by an exponential distribution with an inverse slope of about 240 MeV/$c$ in the $p_T$ range from 0.6--2.0 GeV/$c$. While the shape of the $p_T$ distribution is independent of centrality within the experimental uncertainties, the yield increases rapidly with increasing centrality, scaling approximately with $N_{\mathrm{part}}^{\alpha }$, where $\alpha =1.48\pm 0.08(\mathrm{stat})\pm 0.04(\mathrm{syst})$.

480 authors, pages, 9 figures, 3 tables, 2007 and 2010 data. Submitted to Phys. Rev. C. v2 is version accepted by PRC w/ updated figures. Plain text data tables for the points plotted in figures for this and previous PHENIX publications are publicly available at http://www.phenix.bnl.gov/papers.html

References in corpus (4)

• Glauber Modeling in High Energy Nuclear Collisions
• Measurement of direct photon production in p + p collisions at sqrt(s) = 200 GeV
• Centrality dependence of the direct photon yield and elliptic flow in heavy-ion collisions at sqrt(s)=200 GeV
• gamma-hadron correlations as a tool to trace the flow of energy lost from hard partons in heavy-ion collisions