Probing proton structure with $c \bar c$ correlations in ultraperipheral $pA$ collisions

TL;DR

This paper investigates the production and correlations of charm quark pairs in ultraperipheral proton-nucleus collisions using advanced gluon distribution models, providing detailed predictions for various observable distributions.

Contribution

It introduces two formulations for calculating gluon GTMDs in the context of charm pair production, incorporating realistic phase space integration and azimuthal correlation analysis.

Findings

Azimuthal correlations are generally below 5%.

Different gluon models produce distinct azimuthal modulations.

Distributions in rapidity, transverse momentum, and momentum transfer are provided.

Abstract

We study the exclusive diffractive $c \bar c$ photoproduction in ultraperipheral $pA$ collisions. The formalism makes use of off-diagonal generalizations of the unintegrated gluon distribution, the so-called generalized transverse momentum dependent distributions (GTMDs). We present two different formulations. The first one is based directly on gluon GTMD parametrizations in momentum space. Another option is the calculation of the GTMD as a Fourier transform of the dipole-nucleon scattering amplitude $N(Y,\vec{r}_{\perp},\vec{b}_{\perp})$. The latter approach requires some extra regularization discussed in the paper. Different dipole amplitudes from the literature are used. Compared to previous calculations in the literature, we integrate over the full phase space and therefore cross sections for realistic conditions are obtained. We present distributions in rapidity of $c$ or $\bar c$, transverse momentum of the $c \bar c$ pair, four-momentum transfer squared as well as the azimuthal correlation between a sum and a difference of the $c$ and $\bar c$ transverse momenta. The azimuthal correlations are partially due to the so-called elliptic gluon Wigner distribution. Different models lead to different modulations in the azimuthal angle. The modulations are generally smaller than 5%. They depend on the range of transverse momentum selected for the calculation.