Investigating subnucleonic structures via new measurements of incoherent J/$ψ$ photoproduction in ultra-peripheral Pb--Pb collisions with ALICE

TL;DR

This study uses ultra-peripheral Pb-Pb collisions to probe the fluctuating gluon structure of nuclei through incoherent J/$\psi$ photoproduction, mapping the transverse gluon density via $|t|$ and the photon-nucleus energy $W_{\gamma \mathrm{Pb},n}$. Utilizing ALICE data at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV and forward neutron tagging, the analysis achieves a multi-differential measurement of the incoherent cross section, extracting the signal with unbinned fits and constraining feed-down, across three $|t|$ intervals and several $W_{\gamma \mathrm{Pb},n}$ bins. The results show a cross-section rise with energy at low $|t|$ but a suppressed growth at high $|t|$, with a measured ratio $R(W=633\text{ GeV})=0.52 \pm 0.13$ between high and low $|t|$ regions, signaling sensitivity to subnucleonic gluon configurations. Model comparisons favor saturation-based descriptions with subnucleonic fluctuations over purely shadowing-based approaches, with Mäntysaari's model offering the best quantitative agreement. The findings support gluon saturation at small spatial scales and establish incoherent photoproduction as a powerful probe of the transverse gluon structure, with future Run 3/4 data and FoCal extending access to smaller Bjorken-$x$ values.

Abstract

Ultra-peripheral collisions of heavy ions provide a unique environment to study the gluon structure of nuclei through photon-induced reactions. In particular, the incoherent photoproduction of J/$ψ$ vector meson is sensitive to event-by-event fluctuations of the gluon field at nucleon and subnucleonic scales. We report new ALICE measurement of incoherent J/$ψ$ production in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV, differential in both photon-nucleus energy and Mandelstam-$|t|$. At low $|t|$, the cross section rises with increasing photon-nucleus energy, while at larger $|t|$ the growth is suppressed with respect to the trend at low $|t|$. This measurement represents the first study of incoherent photoproduction at the LHC performed as a function of both $W_{γ\mathrm{Pb},n}$ and $|t|$.

Figures (2)

• Figure 1: Ratios of the cross sections at different Mandelstam-$t$ ranges normalized such that the lower energy ratio is one. The vertical line denotes the statistical and $|t|$-uncorrelated systematic uncertainties added in quadrature.
• Figure 2: The energy dependence of the cross section of incoherent J/$\psi$ photonuclear production at three ranges of the Mandelstam-$t$ compared with model predictions. The shadowing-based model of Guzey et al. Guzey is shown with a dot-dashed line. Two saturation-based predictions are also shown: from Mäntysaari et al. Mantysaari with a dashed line and from Cepila et al. Cepila with a solid line.