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

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 Bjorkenʹs hydrodynamical model, energy nonconserving (anti)quark bremsstrahlung View the MathML source and quark–antiquark annihilation View the MathML source 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 lawp−νT 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 dNπ/dy.