First Measurement of the $\left|t\right|$ Dependence of Incoherent $J/\psi$ Photonuclear Production

The first measurement of the cross section for incoherent photonuclear production of $J/\psi$ vector mesons as a function of the Mandelstam $\left|t\right|$ variable is presented. The measurement was carried out with the ALICE detector at midrapidity, $\left|y\right|<0.8$, using ultraperipheral collisions of Pb nuclei at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. This rapidity interval corresponds to a Bjorken-$x$ range $\left(0.3–1.4\right)\times {10}^{-3}$. Cross sections are given in five $\left|t\right|$ intervals in the range $0.04<\left|t\right|<1{\mathrm{GeV}}^2$ and compared to the predictions by different models. Models that ignore quantum fluctuations of the gluon density in the colliding hadron predict a $\left|t\right|$ dependence of the cross section much steeper than in data. The inclusion of such fluctuations in the same models provides a better description of the data.