Abstract
We present estimates of transverse single-spin asymmetry in prompt photon production in the scattering of low virtuality photons off a polarized proton target and discuss the possibility of using this as a probe to get information about the gluon Sivers function (GSF). Using a generalized parton model (GPM) framework, we estimate the asymmetries at electron-ion collider energy ($\sqrt{s}$ =140 GeV) taking into account both direct and resolved photon processes and find that the dominant contribution, up to $10\%$, comes from quark Sivers function (QSF) while the contribution from GSF is found to be up to $2\%$. However, upon taking into account the effects of the process-dependent initial and final state interactions through the color-gauge invariant generalized parton model approach we find that the situation is significantly changed, with near zero contributions from the QSFs and up to a $1\%$ level contribution from the \textit{f}-type GSF. Our results indicate that this process may be useful for distinguishing between GPM and color-gauge invariant generalized parton models and can be used as a good probe of \textit{f}-type GSF.
Highlights
Transverse single-spin asymmetries (TSSAs) and the underlying physics is an area of hadron physics that has been explored with keen interest in the past years as these asymmetries provide useful tools for probing the three dimensional and spin structure of the nucleon
We have studied the Sivers asymmetry in prompt photon production in electron proton collisions
We have taken into account both direct as well as resolved contributions to the production of photons. We find that this probe can be useful for discriminating between generalized parton model (GPM) and color-gauge invariant generalized parton model (CGI-GPM) as the predictions of asymmetry given by these models are entirely different
Summary
Transverse single-spin asymmetries (TSSAs) and the underlying physics is an area of hadron physics that has been explored with keen interest in the past years as these asymmetries provide useful tools for probing the three dimensional and spin structure of the nucleon. The second approach is based on a generalization of collinear factorization of QCD wherein the collinear parton distribution functions (PDFs) and fragmentation functions (FFs) are replaced by corresponding transverse momentum dependent (TMD) distribution functions which, on being integrated upon the transverse momentum variables, yield the collinear PDFs and FFs [10]. The modified GPM approach, known as color-gauge invariant generalized parton model (CGI-GPM), amounts to absorbing the effects of ISIs and FSIs into the hard part of the process under consideration, restoring the universality of the Sivers function. CGI-GPM approach has been applied to J=ψ and Dmeson production at RHIC [25] and has recently been applied to the study of TSSA in prompt photon production at RHIC by us and by D’Alesio et al independently [29,30]. V, we present numerical estimates of TSSAs in both GPM and CGI-GPM frameworks
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