High-energy photons from expanding quark-gluon plasma and hot hadronic matter

The emission of high-energy photons from an expanding quark-gluon plasma (QGP), which undergoes a first-order phase transition to a hot hadronic gas before freeze-out, is evaluated. Compton and annihilation processes in the quark-gluon plasma and an exhaustive array of hh→hγ reactions as well as decays in hot hadronic matter are considered. We find that, if the initial temperature of the QGP is more than twice the critical temperature, then, beyond a transverse momentum of about 2-3 GeV, the photons from the QGP outshine those having their origin in hadronic matter. We further note that the increase in degrees of freedom in the hot hadronic matter reduces the lifetime of the mixed phase.