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 Au
Au 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 Au
Au 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.