First measurement of the |t|-dependence of coherent J/ψ photonuclear production

The first measurement of the cross section for coherent $\text{J}/\psi$ photoproduction as a function of $\left|t\right|$, 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 $\sqrt{s_{\mathrm{NN}}}=5.02\text{TeV}$ with the $\text{J}/\psi$ produced in the central rapidity region $\left|y\right|<0.8$, which corresponds to the small Bjorken-x range $(0.3-1.4)\times {10}^{-3}$. The measured $\left|t\right|$-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.