Challenges of direct photon production at forward rapidities and large p_T

Direct photons produced in interactions with nuclear targets represent a cleaner probe for investigation of nuclear effects than hadrons, since photons have no final state interaction and no energy loss or absorption is expected in the produced hot medium. Therefore, besides the Cronin enhancement at medium-high transverse momenta p_T and isospin effects at larger p_T, one should not expect any nuclear effects. However, this fact is in contrast to the PHENIX data providing an evidence for a significant large-p_T suppression at mid rapidities in central d + Au and Au + Au collisions that cannot be induced by coherent phenomena (gluon shadowing, Color Glass Condensate). We demonstrate that such an unexpected results is subject to deficit of energy induced universally by multiple initial state interactions (ISI) towards the kinematic limits (large Feynman x_F and/or large ${x_T} = 2{p_T}/\sqrt s$). For this reason, in order to enhance the effects of coherence, one should be cautious going to forward rapidities and higher energies. In the LHC kinematic region ISI corrections are irrelevant at mid rapidities but cause rather strong suppression at forward rapidities and large p_T. Numerical calculations of invariant p_T spectra and the nuclear modification factor were performed within two different models, the color dipole formalism and the model based on k_T-factorization, which are successfully confronted with available data from the RHIC and LHC collider experiments. Finally, we perform also predictions for a strong onset of ISI corrections at forward rapidities and corresponding expected suppression can be verified by the future measurements at LHC.