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 ultrarelativistic energies is calculated by means of perturbative QCD. We calculate the single photon cross section $(A+\overrightarrow{B}\gamma +X)$ by taking into account the partonic subprocesses $q+\overline{q}\to \gamma +g$ and $q+\overrightarrow{g}\gamma +q$ as well as the bremsstrahlung corrections to those processes. We choose a lower momentum cutoff $k_0=2\mathrm{}$ GeV separating the soft physics from perturbative QCD. We compare the results for those primary collisions with the photons produced in reactions of the thermalized secondary particles, which are calculated within scaling hydrodynamics. 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. Employing the GRV parton distribution parametrizations we find that at ultrarelativistic energies prompt QCD photons dominate over the thermal radiation down to transverse momenta $k_T\approx 2$ GeV. At LHC, however, thermal radiation from the quark-gluon plasma dominates for photon transverse momenta $k_T<~5$ GeV, if nuclear shadowing effects on prompt photon production are taken into account.