Transverse flow effects on high-energy photons emitted by expanding quark-gluon plasma

High-energy photons emitted in ultrarelativistic nucleus-nucleus collisions at energies reached at the CERN SPS and LHC and the BNL RHIC are discussed as a source of information on the collision dynamics and as a signature of the deconfining phase transition. The energy density and velocity fields of the expanding matter have been computed assuming cylindrical symmetry along the transverse direction and boost invariance along the longitudinal direction. The emission of photons from an expanding quark-gluon plasma undergoing a first-order phase transition has been evaluated. When the transverse flow of the system is accounted for, it is found that the photons having their origin in the hadronic matter are comparable to those originating in the quark-gluon plasma for ${\mathit{p}}_{\mathit{T}}$ greater than 2 GeV, at energies reached at the SPS and RHIC. At energies reached at the LHC, photons having a ${\mathit{p}}_{\mathit{T}}$ larger than about 4 GeV are shown to have their origin predominantly in the quark-gluon plasma. The location of the ${\mathit{p}}_{\mathit{T}}$ window is shown to be very sensitive to the value of the freeze-out temperature chosen. The feasibility of evaluating the direct (QCD) photons has been studied. Photons fragmented off final state quarks in parton-parton scattering are shown to provide the largest background beyond ${\mathit{p}}_{\mathit{T}}$≊3 GeV at energies reached at the LHC. Accounting for hadronic degrees of freedom beyond the pion is found to be important.