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Wei R, Wessels J, White S N, Willis N, Winter D, Wolin S, Wong C P, Woody C L, Wright R M, Wysocki M, Xia B, Xie W, Xu C, Xu Q, Yamaguchi Y L, Yamaura K, Yang R, Yanovich A, Yasin Z, Ying J, Yokkaichi S, Yoo J H, You Z, Young G R, Younus I, Yu H, Yushmanov I E, Zajc W A, Zaudtke O, Zelenski A, Zhang C, Zharko S, Zhou S, Zimamyi J, Zolin L, Zou L
University of Colorado, Boulder, Colorado 80309, USA.
Joint Institute for Nuclear Research, Dubna 141980, Moscow Region, Russia.
Phys Rev Lett. 2019 Jul 12;123(2):022301. doi: 10.1103/PhysRevLett.123.022301.
The PHENIX collaboration presents first measurements of low-momentum (0.4<p_{T}<3 GeV/c) direct-photon yields from Au+Au collisions at sqrt[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+Au collisions at sqrt[s_{NN}]=200. Analyzing the photon yield as a function of the experimental observable dN_{ch}/dη reveals that the low-momentum (>1 GeV/c) direct-photon yield dN_{γ}^{dir}/dη is a smooth function of dN_{ch}/dη and can be well described as proportional to (dN_{ch}/dη)^{α} with α≈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+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 sqrt[s_{NN}]-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.
PHENIX合作组给出了在(\sqrt{s_{NN}} = 39)和(62.4)GeV下金离子与金离子碰撞中低动量（(0.4 < p_{T} < 3 GeV/c)）直接光子产额的首次测量结果。对于这两个束流能量，直接光子产额相对于瞬发过程的预期都有显著增强，类似于在(\sqrt{s_{NN}} = 200)的金离子与金离子碰撞中观察到的产额。将光子产额作为实验可观测量(dN_{ch}/dη)的函数进行分析表明，低动量（(> 1 GeV/c)）直接光子产额(dN_{γ}^{dir}/dη)是(dN_{ch}/dη)的平滑函数，并且可以很好地描述为与((dN_{ch}/dη)^{α})成正比，其中(α≈1.25)。这种标度行为在相对论重离子对撞机和大型强子对撞机的广泛束流能量范围内、对于中心度选定的样本以及不同的A + A碰撞系统都成立。在给定的束流能量下，该标度对于高(p_{T})（(> 5 GeV/c)）也成立，但是当比较不同碰撞能量的结果时，需要一个额外的与(\sqrt{s_{NN}})相关的乘性因子来描述积分直接光子产额。
