Beam energy and centrality dependence of direct-photon emission from ultrarelativistic heavy-ion collisions

Adare, A.*; Hasegawa, Shoichi   ; Imai, Kenichi; Nagamiya, Shoji*; Sako, Hiroyuki   ; Sato, Susumu  ; Tanida, Kiyoshi   ; PHENIX Collaboration*; 666 of others*

The PHENIX collaboration presents first measurements of low-momentum (0.4 3 GeV/) direct-photon yields from AuAu collisions at = 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 AuAu collisions at = 200. Analyzing the photon yield as a function of the experimental observable reveals that the low-momentum (1 GeV/) direct-photon yield is a smooth function of and can be well described as proportional to with 1.25. This new scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and Large Hadron Collider, for centrality selected samples, as well as for different, collision systems. At a given beam energy the scaling also holds for high (5 GeV/) but when results from different collision energies are compared, an additional -dependent multiplicative factor is needed to describe the integrated-direct-photon yield.