Azimuthal flow of decay photons in relativistic nuclear collisions

Abstract

An overwhelming fraction of photons from relativistic heavy-ion collisions has its origin in the decay of π0 and η mesons. We calculate the azimuthal asymmetry of the decay photons for several azimuthally asymmetric pion distributions. We find that the kT dependence of the elliptic flow parameter v2 for the decay photons closely follows the elliptic flow parameter vπ02 evaluated at pT≈kT+δ, where δ≈0.1−0.2 GeV, for typical pion distributions measured in nucleus-nucleus collisions at relativistic energies. Similar results are obtained for photons from the 2−γ decay of η mesons. Assuming that the flow of π0 is similar to those for π+ and π− for which independent measurements would be generally available, this ansatz can help in identifying additional sources for photons. Taken along with quark number scaling suggested by the recombination model, it may help to estimate v2 of the parton distributions in terms of azimuthal asymmetry of the decay photons at large kT.