Direct photons: a nonequilibrium signal of the expanding quark–gluon plasma

Abstract

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( q ̄ )→q( q ̄ )γ and quark–antiquark annihilation q q ̄ →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≲ p T≲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 p − ν T with 2.5≲ ν≲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 d N π /d y.