Gluon TMDs and NRQCD matrix elements in J/ψ production at an EIC

Abstract

In this paper we analyze azimuthal asymmetries in the processes of unpolarized and polarized $J/\psi (\mathrm{{\rm Y}})$ production at an Electron-Ion Collider. Apart from giving access to various unknown gluon transverse momentum distributions, we suggest to use them as a new method to extract specific color-octet NRQCD long-distance matrix elements, i.e. $⟨0|{\mathcal{O}}_8^{J/\psi }{(}^1{S}_0)|0⟩$ and $⟨0|{\mathcal{O}}_8^{J/\psi }{(}^3{P}_0)|0⟩$, whose values are still quite uncertain and for which lattice calculations are unavailable. The new method is based on combining measurements of analogous asymmetries in open heavy-quark pair production which can be performed at the same energy. We also study for the first time the effects of transverse-momentum smearing in the quarkonium formation process. To enhance the gluon contribution one can consider smaller values of x and, in order to assess the impact of small-x evolution, we perform a numerical study using the MV model as a starting input and evolve it with the JIMWLK equations.