Transverse single-spin asymmetry in the low-virtuality leptoproduction of open charm as a probe of the gluon Sivers function

Abstract

We study the low-virtuality inclusive leptoproduction of open charm, $p^{\uparrow }l\to D^0+X$ as a probe of the gluon Sivers function. We perform the analysis in a generalised parton model framework. At leading order, this process is sensitive only to the gluon content of the proton. Hence any detection of a transverse single-spin asymmetry in this process would be clear indication of a non-zero gluon Sivers function (GSF). Considering COMPASS and a future Electron-Ion Collider (EIC), we present predictions for asymmetry using fits for the GSF available in literature. Predictions for peak asymmetry values lie in the range of 0.8\% to 13\%. We also present estimates of the upper bound on the asymmetry as obtained with a maximal gluon Sivers function. Further, for the case of the Electron-Ion Collider, we evaluate the asymmetry in the muons decaying from the $D$-meson and find that the asymmetry is well preserved in the kinematics of the muons. Peak values of the muon asymmetry are close to those obtained for the $D$-meson and lie in the range $0.75\%$ to 11\%.

Comments: 24 pages, 8 figures, v4: To appear in Physical Review D. Added results for SSA in quasi-real photoproduction obtained using the QED result for the TMD distribution of photons in a lepton (from Phys. Rev. D 93, 013005, arXiv:1508.06964). Also added details of how the SSA in terms of $D$-mesons is converted into SSA in open-charm decay muons, using the narrow width approximation