$J/\psi$ Production in Polarized and Unpolarized $ep$ Collision and Sivers and $\cos 2\varphi$ Asymmetries

Abstract

We calculate the Sivers and $\cos 2\varphi$ azimuthal asymmetries in $J/\psi$ production in polarized and unpolarized semi-inclusive $ep$ collision respectively, using the formalism based on transverse momentum dependent parton distributions (TMDs). Non-relativistic QCD based color octet model is employed for calculating the $J/\psi$ production rate. The Sivers asymmetry in this process directly probes the gluon Sivers function. The estimated Sivers asymmetry at $z=1$ is negative which is in good agreement with COMPASS data. We also investigate the effect of TMD evolution on the Sivers asymmetry. The $\cos 2\varphi$ asymmetry is sizable and probes the linearly polarized gluon distribution in an unpolarized proton.

Comments: 29 pages, 11 figures. Details of calculation added