First measurement of the ${\mathbf{|\textit t|}}$-dependence of coherent $\mathbf{\rm{J/\psi}}$ photonuclear production

TL;DR

This paper reports the first measurement of the $|t|$-dependence of coherent J/ψ photonuclear production in ultra-peripheral Pb-Pb collisions, providing new insights into gluonic structure at very low Bjorken-$x$.

Contribution

It presents the first measurement of the $|t|$-dependence of coherent J/ψ photoproduction and compares it with models including shadowing and gluon saturation effects.

Findings

$|t|$-dependence not explained by nuclear form factor alone

Models with shadowing or gluon saturation better reproduce data

Results constrain gluonic structure at low Bjorken-$x$

Abstract

The first measurement of the cross section for coherent J/$\psi$ photoproduction as a function of $|t|$, the square of the momentum transferred between the incoming and outgoing target nucleus, is presented. The data were measured with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV with the J/$\psi$ produced in the central rapidity region $|y|<0.8$, which corresponds to the small Bjorken-$x$ range $(0.3-1.4) \times 10^{-3}$. The measured $|t|$-dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky-Kovchegov equation. These new results are therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken-$x$.