Centrality dependence of soft photon production and its collective flow in Au+Au collisions at sNN=200 GeV

Abstract

Direct photons coming from heavy-ion collisions are of particular interest since once produced, they are unmodified, thus carrying information about the space–time evolution and properties of the hot and dense medium produced in such collisions. We present new results on the centrality dependence of soft single photon production in s N N = 200 GeV Au+Au collisions. Photons are measured down to p T = 400 GeV / c via conversion of real photons into e + e − pairs in the detector material. Our measurement of the photon p T spectra indicates that the spectral shape does not depend on centrality. In addition, the azimuthal emission patterns of photons carry additional information on their production mechanisms. Previously published PHENIX results [2] indicate that the 2nd order Fourier coefficient ( v 2 ) is positive for p T < 4 GeV / c , which is qualitatively explained by hydrodynamical model calculations, but not quantitatively. The 3rd order Fourier coefficient ( v 3 ) of photons has been proposed as a critical additional handle to understand the photon emission. We found that the v 3 of photons is positive for p T < 4 GeV / c , and both the strength and shape of v 3 are comparable to those of hadrons, similarly to the case of v 2 . We report the latest results on the centrality dependence of the soft photon production and the v 3 coefficients in Au+Au collisions at s N N = 200 GeV . These measurements provide stringent tests of the hydrodynamic space–time evolution as a function of the collision geometry.