Global Bayesian Analysis of $\mathrm{J}/ψ$ Photoproduction on Proton and Lead Targets

TL;DR

The paper addresses whether a CGC-based description can simultaneously describe diffractive $J/\psi$ photoproduction in $\gamma+p$ and $\gamma+\mathrm{Pb}$ collisions. A global Bayesian analysis with GP emulators and an extended MCMV initial state (including a flexible proton shape and substructure) tests this possibility, and introduces a global normalization factor $K$ to absorb nonperturbative/wave-function and higher-order uncertainties. The results show that, without $K$, the $\gamma+p$ and $\gamma+\mathrm{Pb}$ data cannot be reconciled within the standard CGC setup; allowing $K$ to vary yields a good description only for $K\sim 0.3$, which in turn implies larger saturation scales and stronger nuclear suppression, effectively slowing the $W$-evolution in nuclei. The findings highlight the necessity of non-linear CGC dynamics and possibly missing higher-order corrections for a unified description, guiding future work toward NLO improvements and more flexible initial-state modeling beyond the MV framework.

Abstract

We perform a global Bayesian analysis of diffractive $\mathrm{J}/ψ$ production in $γ+p$ and $γ+\mathrm{Pb}$ collisions using a color glass condensate (CGC) based calculation framework. As past calculations have shown that CGC-based models typically overpredict the $\mathrm{J}/ψ$ production in $γ+\mathrm{Pb}$ collisions at high center of mass energy, we address the question of whether it is possible to describe coherent and incoherent diffractive $\mathrm{J}/ψ$ data from $γ+p$ collisions at HERA and the LHC, and from $γ+\mathrm{Pb}$ collisions at the LHC simultaneously. Our results indicate that a simultaneous description of $γ+p$ and $γ+\mathrm{Pb}$ data is challenging, with results improving when an overall $K$-factor -- scaling $γ+p$ and $γ+\mathrm{Pb}$ cross sections to absorb model uncertainties -- is introduced.

Figures (4)

• Figure 1: Posterior distributions obtained by fitting the $\gamma+p$ (dashed, lower corner) and $\gamma+\mathrm{Pb}$ (dotted, upper corner) data separately. The numbers presented at the top of the figure are the median values along with their corresponding 90% credible intervals.
• Figure 2: Integrated $W$ dependent and $|t|$-differential cross sections from two separate fits containing only $\gamma+p$ (full), or only $\gamma+\mathrm{Pb}$ (dashed) data using the standard parameter setup. The uncertainty bands indicate the 68% credible intervals from 25 posterior sample runs of the model.
• Figure 3: Posterior distribution obtained in the global analysis result with (full, upper corner) and without (dash-dotted, lower corner) the model extension by the normalization factor $K$. The numbers presented at the top of the figure are the median values along with their corresponding 90% credible intervals.
• Figure 4: Integrated $W$ dependent and $|t|$-differential cross sections from the fit containing $\gamma+p$ and $\gamma+\mathrm{Pb}$ data in the standard parameter setup (dash-dotted) and including an additional $K$ factor (full) the uncertainty bands indicate the 68% credible intervals from 25 posterior sample runs of the model.