Exclusive heavy vector meson photoproduction in $pp$ and $pPb$ collisions within the GPD approach: A phenomenological analysis

TL;DR

The paper investigates exclusive heavy vector meson photoproduction in ultraperipheral $pp$ and $pPb$ collisions at LHC energies to constrain the small‑$x$ gluon content of the proton. It uses the Goloskokov–Kroll GPD framework to relate the gluon GPD $H_g(x,\xi,t,\mu_F)$ to standard gluon PDFs and computes $J/\psi$ and $\Upsilon$ photoproduction, comparing linear DGLAP evolution with a nonlinear gluon‑recombination parameterization (DGHP), including factorization‑scale dependence. Cross sections and rapidity distributions are confronted with HERA/LHC data, revealing that linear PDFs describe $J/\psi$ data well in several cases while nonlinear DGHP tends to reduce small‑$x$ gluons and can better accommodate $\Upsilon$ data, though current measurements do not decisively favor one scenario. The study highlights the significant role of the factorization scale and calls for more precise $\Upsilon$ measurements to disentangle the small‑$x$ gluon dynamics relevant for high‑energy QCD.

Abstract

The exclusive $J/ψ$ and $Υ$ photoproduction in proton - proton ($pp$) and proton - lead ($pPb$) collisions at the LHC energies is investigated using the Generalized Parton Distribution (GDP) approach. Assuming the Goloskokov - Kroll (GK) model, we estimate the corresponding total cross - sections and rapidity distributions considering different parametrizations for the unpolarized gluon distribution of the proton. In particular, we compare the linear predictions, associated with DGLAP evolution, with those derived taking into account of the leading nonlinear corrections resulting from gluon recombination. The dependence of our predictions on the choice for the factorization scale is discussed.

Figures (9)

• Figure 1: (a) Exclusive heavy vector meson photoproduction in $pp$ and $pPb$ collisions. (b) Scattering amplitude for the $\gamma p$ interaction within the GPD approach.
• Figure 2: Predictions of the different PDF sets considered in our study for the $x$ - dependence of the gluon distribution, $xg(x,\mu^2)$, for two different values of the hard scale: $\mu^2 = 10$ GeV$^2$ (left panel) and $\mu^2 = 100$ GeV$^2$ (right panel).
• Figure 3: Energy dependence of the exclusive $J/\psi$ (left panel) and $\Upsilon$ (right panel) production cross - section in photon-proton interactions. The measured cross sections for the $J/\psi$ and $\Upsilon$ photoproduction are taken from Refs.Camerini:1975cyBinkley:1981kvFrabetti:1993uxAdloff:2000vmChekanov:2002xiLHCb:2018rcmALICE:2014eofALICE:2018oyo and ZEUS:1998cdrH1:2000kisChekanov:2009zzCMS:2018bbk, respectively.
• Figure 4: Rapidity distributions for the exclusive $J/\psi$ photoproduction in $pp$ (upper panels) and $pPb$ (lower panels) collisions. Experimental data of pp are taken from Refs.LHCb:2014acgLHCb:2016oceLHCb:2018rcm while pPb are from ALICE:2014eofALICE:2018oyoALICE:2023mfc.
• Figure 5: Rapidity distributions for the exclusive $\Upsilon$ photoproduction in $pp$ (left panel) and $pPb$ (right panel) collisions. Experimental data are taken from Refs LHCb:2015wlxCMS:2018bbk.