Direct real photons are arguably the most versatile tools to study relativistic heavy ion collisions. They are produced, by various mechanisms, during the entire space-time history of the strongly interacting system. Also, being colorless, most the time they escape without further interaction, \ie they are penetrating probes. This makes them rich in information, but hard to decypher and interpret. This review presents the experimental and theoretical developments related to direct real photons since the 1970s, with a special emphasis on the recently emerged ``direct photon puzzle'', the simultaneous presence of large yields and strong azimuthal asymmetries of photons in heavy ion collisions, an observation that so far eluded full and coherent explanation.

中文翻译：

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相对论重离子碰撞中的直接实光子

直接真实光子可以说是研究相对论重离子碰撞的最通用的工具。它们是通过各种机制在强相互作用系统的整个时空历史中产生的。此外，它们是无色的，大多数时候它们会在没有进一步相互作用的情况下逃逸，\即它们是穿透探针。这使得它们的信息丰富，但难以破译和解释。这篇综述介绍了自 1970 年代以来与直接真实光子相关的实验和理论发展，特别强调了最近出现的“直接光子难题”，即重离子碰撞中光子的高产率和强方位角不对称性的同时存在，到目前为止还没有得到完整和连贯的解释的观察结果。