Probing the Gluon Sivers Function through direct photon production at RHIC

TL;DR

This paper investigates the gluon Sivers function through direct photon production at RHIC, analyzing asymmetries and the impact of initial/final state interactions within a generalized parton model framework.

Contribution

It provides the first detailed analysis of the gluon Sivers function's effects on direct photon production at RHIC, including the impact of initial and final state interactions.

Findings

Asymmetries up to 10% are possible with a maximal gluon Sivers function.

Existing GSF fits predict negligible asymmetry in the studied kinematic region.

Fragmentation photons can dilute the asymmetry by 10-50%.

Abstract

We study the production of prompt-photons at RHIC in the context of a generalised parton model framework, with a view to obtain information on the gluon Sivers function (GSF). At RHIC energy ($\sqrt{s}=200$ GeV), the Compton process, $gq\to\gamma q$ contributes significantly to the production of direct-photons at midrapidity and dominates it in the negative (backward) rapdity region. We find that for direct photons, asymmetries of upto 10\% are allowed by a maximal gluon Sivers function. However, the asymmetry obtained using existing fits of the GSF available is literature is negligible. We also estimate the impact that photons produced via fragmentation can have on the signal and find that their inclusion can dilute the asymmetry by between 10-50\% of the direct-photon value. Finally, using the Colour-Gauge Invariant generalised parton model (CGI-GPM) approach, we consider the effects of initial state and final state interactions which can affect the universality of the Sivers functions in different processes. We find that the inclusion of these effects leads to the size of the gluon contributions being roughly halved. However, in the backward region which we are interested in, the sizes of the quark contributions are suppressed even further, leading to increased dominance of the gluon contributions.