Center of mass energy and system-size dependence of photon production at forward rapidity at RHIC

Abstract

We present the multiplicity and pseudorapidity distributions of photons produced in Au_s16Au and Cu_s16Cu collisions at sqrt{s_NN} = 62.4 and 200 GeV. The photons are measured in the region -3.7 < eta < -2.3 using the photon multiplicity detector in the STAR experiment at RHIC. The number of photons produced per average number of participating nucleon pairs increases with the beam energy and is independent of the collision centrality. For collisions with similar average numbers of participating nucleons the photon multiplicities are observed to be similar for Au_s16Au and Cu_s16Cu collisions at a given beam energy. The ratios of the number of charged particles to photons in the measured pseudorapidity range are found to be 1.4 _s16/- 0.1 and 1.2 _s16/- 0.1 for sqrt{s_NN} = 62.4 GeV and 200 GeV, respectively. The energy dependence of this ratio could reflect varying contributions from baryons to charged particles, while mesons are the dominant contributors to photon production in the given kinematic region. The photon pseudorapidity distributions normalized by average number of participating nucleon pairs, when plotted as a function of eta - ybeam, are found to follow a longitudinal scaling independent of centrality and colliding ion species at both beam energies.