According to quantum chromodynamics, at sufficiently high energy, the structure of hadrons reveals a dynamic equilibrium between gluon splitting and gluon recombination -- a phenomenon known as saturation. The process of diffractive photonuclear production of a J/$\psi$ vector meson provides a direct insight into the gluon composition of hadrons. The J/$\psi$ production as a function of momentum transferred in the interaction, quantified by the Mandelstam-$t$ variable, serves as an excellent probe for studying the structure of hadrons within the impact-parameter plane, because different ranges in $t$ are sensitive to the dynamics of the gluon field at varying spatial size scales. The ALICE collaboration has measured the energy dependence of incoherent photonuclear production of J/$\psi$ mesons off lead ions, at $\sqrt{s_{\rm NN}} = 5.02$ TeV, for three Mandelstam-$t$ intervals. The energy dependence of the photonuclear cross section at the highest $|t|$ range measured, $(0.81<~ |t| <~1.44)$ GeV$^2$, is sensitive to subnucleonic structures of the Pb target. The increase of the cross section with energy at large $|t|$ shows evidence of suppression with respect to the increase seen at low $|t|$. The observed pattern of the energy evolution in data is similar to that of gluon saturation models.
Submitted to: PRL
e-Print: arXiv:2503.18708 | PDF | inSPIRE
Figure group
Figure 1
The energy dependence of the cross section of incoherent \Jpsi photonuclear production off lead nuclei is shown for three different ranges of the Mandelstam-$t$ variable (left). The symbols denote the measured cross section, the vertical lines across them represent the uncorrelated uncertainty, the open boxes the correlated uncertainty, and the horizontal shadow box the uncertainty from the computation of the photon flux. Ratio of the cross sections at different Mandelstam-$t$ ranges (right) normalised such that the lower energy ratio is one. The vertical line denotes the statistical and \mant-uncorrelated systematic uncertainties added inquadrature. In the left panel the data symbols have been slightly displaced horizontally to improve their visibility. |   |
Figure 2
The energy dependence of the cross section of incoherent \Jpsi photonuclear production off lead nuclei is shown for three ranges of the Mandelstam-$t$ variable and compared with model predictions. The shadowing based model of Guzey et al. is shown with a dot-dashed line. Two saturation-based predictions are also shown: from M{\"a}ntysaari et al. with a dashed line and from Cepila et al. with a solid line. |  |