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The photon transverse momentum spectrum and its anisotropy from Pb+Pbcollisions at the CERN Large Hadron Collider energy $\sqrt {s_{NN}}=2.76$ TeVare investigated within the integrated hydrokinetic model (iHKM). Photonproduction is accumulated from the different processes at the various stages ofrelativistic heavy ion collisions: from the primary hard photons of very earlystage of parton collisions to the thermal photons from equilibrated quark-gluonand hadron gas stages. Along the way a hadronic medium evolution is treated intwo distinct, in a sense opposite, approaches: chemically equilibrated andchemically frozen system expansion. Studying the centrality dependence of theresults obtained allows us to conclude that a relatively strong transversemomentum anisotropy of thermal radiation is suppressed by prompt photonemission which is an isotropic. We find out that this effect is gettingstronger as centrality increases because of the simultaneous increase in therelative contribution of prompt photons in the soft part of the spectra. Thesubstantial results obtained in iHKM with nonzero viscosity ($\eta/s=0.08$) forphoton spectra and $v_2$ coefficients are mostly within the error bars ofexperimental data, but there is some systematic underestimation of bothobservables for the near central events. We claim that a situation could besignificantly improved if an additional photon radiation that accompanies thepresence of a deconfined environment is included. Since a matter of aspace-time layer where hadronization takes place is actively involved inanisotropic transverse flow, both positive contributions to the spectra and$v_2$ are considerable, albeit such an argument needs further research andelaboration.

physical review. c

Direct photon spectrum and elliptic flow produced from Pb+Pb collisions  at $\sqrt{s_{NN}}=2.76$ TeV at the CERN Large Hadron Collider within an  integrated hydrokinetic model