Exclusive J/Psi and Psi(2s) photo-production as a probe of QCD low x evolution equations

TL;DR

This study compares linear and non-linear QCD evolution equations using vector meson photo-production data, extending previous work to excited states and analyzing uncertainties, with implications for understanding low-x gluon dynamics.

Contribution

It introduces an improved charmonium wave function set, extends analysis to Psi(2s), and estimates uncertainties in gluon distribution fits based on HERA and LHC data.

Findings

Difference between linear and non-linear evolution diminishes with current data.

The ratio of Psi(2s) to J/Psi cross-sections enhances differences between gluon models.

Current data cannot clearly distinguish between the two evolution approaches.

Abstract

We investigate photo-production of vector mesons J/Psi and Psi(2s), based on both HERA and LHC data, using 2 fits of unintegrated gluon distributions. The latter are subject to non-linear Balitsky-Kovchegov evolution (Kutak-Sapeta gluon; KS) and linear next-to-leading order Balitsky-Kuraev-Fadin-Lipatov evolution (Hentschinski-Sabio Vera-Salas; HSS gluon) respectively. Apart from extending previous studies to the case of radially excited charmonium Psi(2s), we further use an improved set of charmonium wave functions and provide an estimate of the uncertainties associated with the HSS gluon. While we observe that the difference between linear and non-linear evolution somehow diminishes and a clear distinction between both HSS and KS gluon is no longer possible using the currently available data-set, we find that the differences between both gluon distributions are enhanced for the ratio of the photo-production cross-sections of Psi(2s) and J/Psi vector mesons.