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

TL;DR

This study measures photon production in gold-gold and copper-copper collisions at RHIC, revealing energy and system-size dependencies, and demonstrates longitudinal scaling of photon pseudorapidity distributions across different collision centralities and ion species.

Contribution

It provides the first detailed measurements of photon multiplicity and pseudorapidity distributions at forward rapidity in these collision systems, highlighting energy dependence and scaling behaviors.

Findings

Photon multiplicity per participant pair increases with beam energy.

Photon production is similar for Au+Au and Cu+Cu at the same energy and participant number.

Photon to charged particle ratio varies with energy, indicating different particle contributions.

Abstract

We present the multiplicity and pseudorapidity distributions of photons produced in Au+Au and Cu+Cu 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+Au and Cu+Cu 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 +/- 0.1 and 1.2 +/- 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.