Searching for gluon saturation effects in the momentum transfer dependence of coherent charmonium electroproduction off nuclei

TL;DR

This paper investigates the momentum transfer dependence in coherent charmonium production off nuclei, revealing potential gluon saturation effects and the importance of shadowing corrections in high-energy electron-ion collisions.

Contribution

It introduces a comprehensive QCD dipole approach with Green function formalism to analyze $d\sigma/dt$, incorporating shadowing and color transparency effects for the first time.

Findings

Shadowing corrections significantly reduce $d\sigma/dt$ at high energies.

Predicted non-monotonic energy dependence of $d\sigma/dt$ suggests gluon saturation.

Color transparency influences the shape of nuclear modification factors.

Abstract

We study for the first time the transverse momentum transfer distributions $d\sigma/dt$ in coherent production of charmonia in nuclear ultra-peripheral and electron-ion collisions within the QCD color dipole approach based on a rigorous Green function formalism. This allows us to treat properly the color transparency effects, as well as the higher and leading-twist shadowing corrections associated with the $|Q\bar Q\rangle$ and $|Q\bar QnG\rangle$ Fock components of the photon. While the multi-gluon photon fluctuations represent the dominant source of nuclear shadowing at kinematic regions related to the recent LHC and its future upgrade to LHeC, the upcoming electron-ion collider at RHIC will additionally require the proper incorporation of reduced quark shadowing. The latter effect leads to a significant decrease in the differential cross sections $d\sigma/dt$ compared to standard calculations based on the eikonal form for the dipole-nucleus amplitude. The leading-twist shadowing corrections, corresponding to a non-linear QCD evolution of a partial dipole-nucleus amplitude, reduce substantially charmonium $t$-distributions in the LHeC energy range. We predict a non-monotonic energy dependence of $d\sigma/dt$ suggesting so possible gluon saturation effects with increased onset at larger $t$-values. In addition to shadowing corrections, we study how the color transparency effects affect the shape of $t$-dependent nuclear modification factor. We also briefly discuss several aspects that can modify the charmonium production rate and thus may have a large impact on the search for gluon saturation effects.