First measurement of the $|t|$-dependence of coherent J/$\psi$ photonuclear production

Abstract The first measurement of the cross section for coherent J / ψ 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 s NN = 5.02 TeV with the J / ψ produced in the central rapidity region | y | 0.8 , which corresponds to the small Bjorken-x range ( 0.3 − 1.4 ) × 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.