PHYSICAL REVIEW LETTERS 132, 162302 (2024)

The first experimental measurement of the incoherent photonuclear production of J/ψ in ultraperipheral heavy-ion collisions is better explained by the presence of subnuclear quantum fluctuations of the gluon field.

The first measurement of the cross-section for incoherent photonuclear production of 𝐽/𝜓 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_NN)=5.02  TeV. This rapidity interval corresponds to a Bjorken-𝑥 range (0.3–1.4)×10⁻³. Cross sections are given in five |𝑡| intervals in the range 0.04<|𝑡|<1  GeV² 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.