Direct photon production from a longitudinally expanding quark-gluon plasma (QGP) at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies is studied with a real-time kinetic description that is consistently incorporated with hydrodynamics. Within Bjorken's hydrodynamical model, energy nonconserving (anti)quark bremsstrahlung q(q̄) → q(q̄)γ and quark-antiquark annihilation qq̄ → γ are shown to be the dominant nonequilibrium effects during the transient lifetime of the QGP. For central collisions we find a significant excess of direct photons in the range of transverse momentum 1 - 2 ≲ pT≲ 5 GeV/c as compared to equilibrium results. The photon rapidity distribution exhibits a central plateau. The transverse momentum distribution at midrapidity falls off with a power law pT-vwith 2.5 ≲ ν v ≲ 3 as a consequence of these energy nonconserving processes, providing a distinct experimental nonequilibrium signature. The power law exponent ν increases with the initial temperature of the QGP and hence with the total multiplicity rapidity distribution dNπ/dy. © 2002 Elsevier Science B.V. All rights reserved.
Wang, S. Y., Boyanovsky, D., & Ng, K. W. (2002). Direct photons: A nonequilibrium signal of the expanding quark-gluon plasma. Nuclear Physics A, 699(3–4), 819–846. https://doi.org/10.1016/S0375-9474(01)01288-X