Probing the gluon Sivers function through direct photon production at RHIC

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 ( √ s = 200 GeV), the Compton process, gq → γq 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 directphoton 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.

Unraveling the gluon Sivers function in hadronic collisions at RHIC

“…Note added: at the very last stage of this work we have become aware of a similar study on SSAs in p ↑ p → γ X within the CGI-GPM approach [45]. While the theoretical findings are in perfect agreement, see Eqs.…”

Probing gluon spin-momentum correlations in transversely polarized protons through midrapidity isolated direct photons in $p^\uparrow+p$ collisions at $\sqrt{s}=200$ GeV

“…The quark Sivers function has been extracted through polarized SIDIS measurements, but the gluon Sivers function has remained comparatively less constrained because SIDIS is not sensitive to gluons at leading order [9]. The direct photon TSSA in protonproton collisions has been shown to be sensitive to the gluon Sivers function [10], but the k T moment of TMD functions must be used to apply these functions to the single-scale inclusive TSSAs measured in proton-proton collisions.…”

Gluon Sivers function and transverse single spin asymmetries in $e + p^\uparrow \rightarrow γ+ X$

“…However, contribution from this FSI vanishes, when we sum the different cut diagrams which are shown in Fig. 3 P ↑ The modified hard parts for the processes under consideration have been calculated in [28,29] and are given below for the sake of completeness :…”

“…The hadronic cross-sections are then obtained by convoluting the universal Sivers function with the modified hard part, which now contains the process dependence and which is obtained by combining the partonic cross section with the initial/final state interactions [26,27]. CGI-GPM approach has been applied to J/ψ and D -meson production at RHIC [24] and has recently also been applied to the study of TSSA in prompt photon production at RHIC by us and also by D'Alesio et al independently [28,29]. Both of these studies indicate that prompt photon production in p ↑ p collisions can be used to discriminate between GPM and CGI-GPM approaches.…”