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

超相対論的重イオン衝突における直接光子放出のビームエネルギーおよび中心衝突度依存性

Adare, A.*; 長谷川 勝一   ; 今井 憲一; 永宮 正治*; 佐甲 博之   ; 佐藤 進  ; 谷田 聖   ; PHENIX Collaboration*; 他666名*

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.