Photoproduction of $J/\psi$ and $\Upsilon$ states in exclusive and proton-dissociative diffractive events

TL;DR

This paper models the photoproduction of $J/eta$ and $$ states in proton-proton collisions using pQCD, compares predictions with LHCb data, and explores saturation effects and dissociative backgrounds.

Contribution

It introduces a comprehensive pQCD $k_{T}$-factorization approach including electromagnetic form factors and light-cone wave functions, improving predictions for exclusive and dissociative diffractive events.

Findings

Predicted cross sections align with LHCb data.

Identified potential constraints on saturation effects.

Quantified electromagnetic dissociation contributions.

Abstract

The amplitude for $\gamma p \to V p$, where $V$ is a $J/\psi$ or $\Upsilon$ ground state or excited vector meson, is calculated in a pQCD $k_{T}$-factorization approach. We use this amplitude to predict the cross section for exclusive photoproduction of $J/\psi, \psi', \Upsilon$ mesons in proton-proton collisions. Calculations are performed for a variety of unintegrated gluon distributions, and we compare our results to LHCb data. Here we especially focus on the possibilty of constraining saturation effects. Compared to earlier calculations we include both Dirac and Pauli electromagnetic form factors. We discuss the role of the $Q\bar Q$ light-cone wave functions for differential distributions for ratios such as $\sigma(\psi')/\sigma(J/\psi)$. Absorption effects are always taken into account. We also discuss the related diffractive production in proton dissociative events. Here we concentrate on electromagnetic dissociation , which is calculable without additional free parameters. Besides being of interest in their own right, dissociative events constitute an important experimental background to exclusive production.