First measurement of the energy and Mandelstam-$t$ dependence of both coherent and incoherent $J/ψ$ photonuclear production

TL;DR

This work addresses how gluon saturation and nuclear shadowing shape the gluon density in heavy nuclei at small $x$. It employs diffractive $J/\psi$ photoproduction in ultra-peripheral Pb-Pb collisions to access both the energy dependence and the spatial-scale dependence via the momentum transfer $|t|$, resolving photon directions with ZDC tagging. The analysis finds a suppression of coherent production at high photon-nucleus energy and reveals that incoherent production grows more slowly at larger $|t|$, consistent with subnucleonic gluon fluctuations and saturation effects. The first multi-differential measurement of incoherent production as a function of both energy and $|t|$ provides strong constraints on models of the nuclear gluon density, guiding future explorations at lower $x$ and finer spatial scales.

Abstract

A new phenomenon, gluon saturation, is expected to emerge in quantum chromodynamics (QCD) at high energies, when gluon splitting and recombination processes reach a dynamic equilibrium. In heavy nuclei, this balance is expected to be achieved at lower energies than in protons, making lead-lead collisions at the LHC an ideal environment to probe the onset of saturation. The diffractive photoproduction of the $J/ψ$ vector meson provides an excellent tool to study this regime since it directly probes the gluon distribution in the target. ALICE offers unique kinematic coverage of the photon-nucleon centre-of-mass energy, spanning from 20 to 800 GeV, corresponding to three orders of magnitude in Bjorken-$x$ from about $10^{-2}$ down to $10^{-5}$. This contribution presents the latest ALICE results on the energy dependence of coherent $J/ψ$ production, which is sensitive to the average gluon density, and on the energy and Mandelstam-$t$ dependence of incoherent production, which probes fluctuations of the gluon field at different spatial scales. These measurements provide unprecedented constraints on models of QCD in the high-energy limit and mark a milestone in studying the gluonic structure of nuclei.

Figures (3)

• Figure 1: The dependence of the cross section for coherent $J/\psi$ photoproduction on photon-nucleus energy, $W_{\gamma \mathrm{Pb}}$, or Bjorken-$x$ as measured with ALICE ALICE:2023jgu and CMS Tumasyan:2023PRL (left). Mandelstam-$|t|$ dependence of the cross section for coherent ALICE:2021tyx ($t\lesssim0.01$ GeV$^2$) and incoherent ALICE:2023gcs ($t\gtrsim 0.1$ GeV$^2$) $J/\psi$ photoproduction.
• Figure 2: The energy dependence of the cross section of incoherent $J/\psi$ photonuclear production off lead nuclei is shown for three different ranges of the Mandelstam-$|t|$ variable ALICE:2025cuw (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. Ratios of the cross sections at different Mandelstam-$|t|$ ranges ALICE:2025cuw normalized such that the lower energy ratio is one. The vertical line denotes the statistical and $|t|$-uncorrelated systematic uncertainties added in quadrature.
• Figure 3: The energy dependence of the cross section of incoherent J/$\psi$ photonuclear production at three ranges of the Mandelstam-$|t|$ compared with model predictions ALICE:2025cuw. The shadowing-based model of Guzey et al. Guzey:2018tlk is shown with a dot-dashed line. Two saturation-based predictions are also shown: from Mäntysaari et al. Mantysaari:2022suxMantysaari:2023xcu with a dashed line and from Cepila et al. Cepila:2023dxn with a solid line.