Centrality dependence of the direct photon yield and elliptic flow in heavy-ion collisions at √SNN= 200 GeV.

We calculate the centrality dependence of direct photons produced in Au+Au collisions at the invariant collision energy √SNN = 200 GeV and their transverse momentum spectra within the parton-hadron-string dynamics transport approach. As sources for "direct" photons, we incorporate the interactions of quarks and gluons as well as hadronic interactions (π + π --> p + y, p + π -> π + y, meson-meson bremsstrahlung in + m --> m + m + y, and meson-baryon bremsstrahlung m + B --> m + B + y), the decays of ø and a1 mesons, and the photons produced in the initial hard collisions. We find that the transverse momentum pT spectra of the "thermal" photons (i.e., the direct photons after the pQCD contribution is subtracted) deviate from exponential distributions and, consequently, observe a strong dependence of the inverse slope parameter Teff on the fitting range in pT. On the other hand, all the obtained "effective temperatures" are well above the critical temperature for the deconfinement phase transition even for peripheral collisions, reflecting primarily a "blue shift" due to radial collective motion of hadrons. Our calculations suggest that the channel decomposition of the observed spectrum changes with centrality with an increasing (dominant) contribution of hadronic sources for more peripheral reactions. Furthermore, the thermal photon yield is found to scale roughly with the number of participant nucleons as Nαpart with α ≈ 1.5, whereas the partonic contribution scales with an exponent ap αp ∾ 1.75. Additionally, we provide predictions for the centrality dependence of the direct photon elliptic flow v2(PT). The photons from the hot deconfined matter in the early stages of the collision carry a much smaller elliptic flow than the final hadrons. Consequently, the direct photon v2 in the most central bin is of the order of a few percent. On the other hand, the elliptic flow of direct photons is considerably larger in more peripheral collisions, approaching that of hadrons. [ABSTRACT FROM AUTHOR]