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.
Highlights
Transverse single-spin asymmetries (SSA) can provide information on the three-dimensional structure of hadrons
We find that for direct photons, asymmetries of up to 10% are allowed by a maximal gluon Sivers function
We show the asymmetry in the direct photons alone, as well as the asymmetry inclusive of direct and fragmentation photons
Summary
Transverse single-spin asymmetries (SSA) can provide information on the three-dimensional structure of hadrons. [26], we proposed the low-virtuality leptoproduction of open charm and studied it in the context of the GPM framework Unlike hadroproduction, this process does not involve any contributions from quarks and can probe the gluon Sivers function effectively. Since the photon is produced in the hard process and is colorless, the probe is unaffected by theoretical uncertainties related to hadronization or final-state interactions For this reason, SSA in the production of direct photons in the backward region was first proposed as a probe of the gluon Sivers function by Schmidt, Soffer and Yang (SSY) [35]. In this work, following our earlier studies of the GSF in open charm production [25,26], we consider estimates for the asymmetry obtained using saturated quark and gluon Sivers functions. V we present estimates of the asymmetry in both the GPM and CGIGPM frameworks
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