Hadronic and partonic sources of direct photons in relativistic heavy-ion collisions

The direct photon spectra and flow (v(2), v(3)) in heavy-ion collisions at CERN Super Proton Synchrotron, BNL Relativistic Heavy Ion Collider, and CERN Large Hadron Collider energies are investigated within a relativistic transport approach incorporating both hadronic and partonic phases, the parton-hadron-string dynamics (PHSD). In the present work, four extensions are introduced compared to our previous calculations: (i) going beyond the soft-photon approximation (SPA) in the calculation of the bremsstrahlung processes meson + meson -> meson + meson + gamma, (ii) quantifying the suppression owing to the Landau-Pomeranchuk-Migdal (LPM) coherence effect, (iii) adding the additional channels V + N -> N + gamma and Delta -> N + gamma, and (iv) providing PHSD calculations for Pb + Pb collisions at root s(NN) = 2.76 TeV. The first issue extends the applicability of the bremsstrahlung calculations to higher photon energies to understand the relevant sources in the region p(T) = 0.5-1.5 GeV, while the LPM correction turns out to be important for p(T) < 0.4 GeV in the partonic phase. The results suggest that a large elliptic flow v(2) of the direct photons signals a significant contribution of photons produced in interactions of secondary mesons and baryons in the late (hadronic) stage of the heavy-ion collision. To further differentiate the origin of the direct photon azimuthal asymmetry (late hadron interactions vs electromagnetic fields in the initial stage), we provide predictions for the photon spectra, elliptic flow, and triangular flow v(3)(p(T)) of direct photons at different centralities to be tested by the experimental measurements at the LHC energies. Additionally, we illustrate the magnitude of the photon production in the partonic and hadronic phases as functions of time and local energy density. Finally, the "cocktail" method for an estimation of the background photon elliptic flow, which is widely used in the experimental works, is supported by the calculations within the PHSD transport approach.