Direct photons at large pT : from RHIC to LHC

Abstract

Using the color dipole formalism we study the production of direct photons in proton-nucleus and nucleus-nucleus collisions at energies corresponding to RHIC and LHC experiments. Prompt photons produced in a hard reaction are not accompanied with any final state interaction, either energy loss or absorption. Therefore, in the RHIC energy range besides small isotopic corrections one should not expect any nuclear effects at large pT . However, data from the PHENIX experiment indicates a significant large-pT suppression in d+Au and central Au+Au collisions that cannot be accompanied by coherent phenomena. We demonstrate that such an unexpected result is subject to the energy sharing problem universally induced by multiple initial state interactions (ISI) at large pT and/or at forward rapidities. In the LHC kinematic region ISI corrections are irrelevant at mid rapidities but cause rather strong suppression at forward rapidities. We present for the first time predictions for expected nuclear effects at large pT in p + Pb and Pb + Pb collisions at different rapidities. We include and analyze also a contribution of coherent effects associated with gluon shadowing modifying nuclear effects predominantly at small and medium-high pT.