Direct Photon Production in Heavy-Ion Collisions: Theory and Experiment

Direct photons provide a possibility to test properties of hot matter created in proton-proton (pp), proton-nucleus (p-A) or nucleus-nucleus (A-A) collisions. As they are created in charged particles' scatterings and freely escape the hot region, they provide a tool to test all stages of the collision: the scattering of the partons of incoming nucleons, pre-equilibrium evolution and collective expansion of hot quark-gluon matter created in nucleus-nucleus collisions. Comparing direct photon production in pp, p-A and A-A collisions, one can check the scaling with the number of binary collisions expected at a high transverse momentum range and obtain insight into the hot and cold hadronic matter properties with soft photons. The collective elliptic flow of direct photons is a unique possibility to trace the collective flow formation and space-time evolution of the fireball. We review the experimental results on direct photon production in pp, p-A and A-A collisions at the Super Proton Synchroton (SPS), the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) energies and discuss an agreement of theoretical predictions with measurements. Finally, we present predictions of direct photon spectra and collective flow for lower energy collisions expected at the Nuclotron-based Ion Collider fAcility (NICA) and the Facility for Antiproton and Ion Research (FAIR).