Monday, April 29, 2013

1304.7030 (O. Linnyk et al.)

Photon elliptic flow in relativistic heavy-ion collisions: hadronic
versus partonic sources
   [PDF]

O. Linnyk, V. P. Konchakovski, W. Cassing, E. L. Bratkovskaya
We study the transverse momentum spectrum and the elliptic flow v_2 of photons produced in Au+Au collisions at sqrt{s}=200GeV using the Parton-Hadron-String Dynamics (PHSD) transport approach. As sources for photon production, we incorporate the interactions of off-shell quarks and gluons in the strongly interacting quark-gluon plasma (sQGP) (q+qbar->g+gamma and q(qbar)+g->q(qbar)+gamma), the decays of hadrons (pi->gamma+gamma, eta->gamma+gamma, omega->pi+gamma, eta'->rho+gamma, phi->eta+gamma, a_1->pi+gamma) as well as their interactions (pi+pi->rho+gamma, rho+pi->pi+gamma). The PHSD calculations reproduce the transverse momentum spectrum, the effective temperature T_eff and the elliptic flow v_2 of direct photons as measured by the PHENIX Collaboration. The photons produced in the QGP contribute about 50% to the observed spectrum, but have small v_2. We find that the large direct photon v_2 - comparable to that of hadrons - is attributed to the intermediate hadronic scattering channels and hadronic resonance decays not subtracted from the data. The QGP phase causes the strong elliptic flow of photons indirectly, by enhancing the v_2 of final hadrons due to the partonic interaction in terms of explicit parton collisions and the mean-field potentials. On the other hand, the presence of the QGP radiation is manifest in the slope of the direct photon spectrum, leading to T_eff far above the critical temperature for the deconfinement phase transition.
View original: http://arxiv.org/abs/1304.7030

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