NUCLEAR SHADOWING EFFECTS ON PROMPT PHOTONS AT ULTRARELATIVISTIC ENERGIES

The transverse momentum distribution of prompt photons coming from the very early phase of ultrarelativistic heavy-ion collisions for the ul trarelativistic energies is calculated by means of perturbative QCD. W e calculate the single photon cross section (A+B-gamma+X) by taking in to account the partonic subprocesses q + (q) over bar-->gamma+g and q + g-->gamma+q as well as the bremsstrahlung corrections to those proce sses. We choose a lower momentum cutoff k(0) = 2 GeV separating the so ft physics from perturbative QCD. We compare the results for those pri mary collisions with the photons produced in reactions of the thermali zed secondary particles, which are calculated within scaling hydrodyna mics. The QCD processes are taken in leading order. Nuclear shadowing corrections, which alter the involved nuclear structure functions are explicitly taken into account and compared to unshadowed results. Empl oying the GRV parton distribution parametrizations we find that at ult rarelativistic energies prompt QCD photons dominate over the thermal r adiation down to transverse momenta k(T) approximate to 2 GeV. At LHC, however, thermal radiation from the quark-gluon plasma dominates for photon transverse momenta k(T) less than or equal to 5 GeV, if nuclear shadowing effects on prompt photon production are taken into account.