Nonprompt direct-photon production in Au+Au collisions at √sNN = 200 GeV

Abstract

The measurement of the direct-photon spectrum from Au+Au collisions at √sNN=200 −−−√=200 GeV is presented by the PHENIX collaboration using the external-photon-conversion technique for 0\%--93\% central collisions in a transverse-momentum (pT) range of 0.8--10 GeV/c. An excess of direct photons, above prompt-photon production from hard-scattering processes, is observed for pT<6 GeV/c. Nonprompt direct photons are measured by subtracting the prompt component, which is estimated as Ncoll-scaled direct photons from p+p collisions at 200 GeV, from the direct-photon spectrum. Results are obtained for 0.8<6.0 GeV/c and suggest that the spectrum has an increasing inverse slope from ≈0.2 to 0.4 GeV/c with increasing pT, which indicates a possible sensitivity of the measurement to photons from earlier stages of the evolution of the collision. In addition, like the direct-photon production, the pT-integrated nonprompt direct-photon yields also follow a power-law scaling behavior as a function of collision-system size. The exponent, α, for the nonprompt component is found to be consistent with 1.1 with no apparent pT dependence.