Nonprompt Direct-Photon Production in Au+Au Collisions at Snn=200 GeV

The measurement of the direct-photon spectrum from Au+Au collisions at root s(NN) = 200 GeV is presented by the PHENIX Collaboration using the external-photon-conversion technique for 0%-93% central collisions in a transverse-momentum (p(T)) range of 0.8-10 GeV/c. An excess of direct photons, above prompt-photon production from hard-scattering processes, is observed for p(T) < 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 < p(T) < 6.0 GeV/c and suggest that the spectrum has an increasing inverse slope from approximate to 0.2 to 0.4 GeV/c with increasing p(T), 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 p(T) -integrated nonprompt direct-photon yields also follow a power-law scaling behavior as a function of collision-system size. The exponent, a, for the nonprompt component is found to be consistent with 1.1 with no apparent p(T) dependence.