Beam Energy and Centrality Dependence of Direct-Photon Emission from Ultrarelativistic Heavy-Ion Collisions

The PHENIX collaboration presents first measurements of low-momentum (0.4 < p(T) < 3 GeV/c) direct-photon yields from Au + Au collisions at root s(NN) =39 and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au +horn Au collisions at root s(NN) =200. Analyzing the photon yield as a function of the experimental observable dN(ch)/d(eta) eveals that the low-momentum (> 1 GeV= c) direct-photon yield dN(gamma)(dir)/d(eta) is a smooth function f dN(ch)/d eta and can be well described as proportional to (dN(ch)/d(eta))(alpha) with a approximate to 1.25. This scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and the Large Hadron Collider, for centrality selected samples, as well as for different A thorn A collision systems. At a given beam energy, the scaling also holds for high p(T) (> 5 GeV= c), but when results from different collision energies are compared, an additional root s(NN) -dependent multiplicative factor is needed to describe the integrated-direct-photon yield.