  The first measurement of the dependence on $|t|$, the square of the momentum
transferred between the incoming and outgoing target nucleus, of coherent
J/$\psi$ photoproduction 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 $(0.3-1.4)\times10^{-3}$.
  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. This new observable is therefore a valid tool to
constrain the relevant model parameters and to investigate the transverse
gluonic structure at very low Bjorken-$x$.
