First Measurement of the $|t|$ 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 $|t|$ variable is presented. The measurement was carried out with the ALICE detector at midrapidity, $|y|<0.8$, 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 $(0.3–1.4)\times {10}^{-3}$. Cross sections are given in five $|t|$ intervals in the range $0.04<|t|<1{\mathrm{GeV}}^2$ and compared to the predictions by different models. Models that ignore quantum fluctuations of the gluon density in the colliding hadron predict a $|t|$ 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.

Figure 1

Left: Invariant mass distribution of muon pairs (full symbols) and fit to a model (solid blue line, see text). Right: transverse momentum distribution of muon pairs with $3.0<m_{\mu \mu }<3.2\mathrm{GeV}/c^2$ (full symbols) and fit to a model (solid blue line) along with the different contributions to the fit (other lines, see text).

Figure 2

Cross section for the incoherent photoproduction of $J/\psi$ vector mesons in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ measured at midrapidity. The uncorrelated uncertainty (statistical and systematic added in quadrature) is indicated with the vertical bar, while the correlated uncertainty by the gray band. The width of each $|t|$ range is given by the horizontal bars. The lines show the predictions of the different models described in the text. The bottom panel presents the ratio of the integral of the predicted to that of the measured cross section in each $|t|$ range. The relative uncertainties on the ratios calculated from GSZ are 45%.