# QCD evolution based evidence for the onset of gluon saturation in exclusive photo-production of vector mesons

## TL;DR

This paper provides evidence for gluon saturation in protons by analyzing vector meson photo-production data from HERA and LHC, comparing non-linear and linear gluon evolution models.

## Contribution

It demonstrates that non-linear gluon evolution models better describe experimental data, indicating the onset of gluon saturation in high-energy proton interactions.

## Key findings

- Linear NLO evolution fails unless unnaturally large corrections are used
- Non-linear BK evolution accurately fits energy dependence of data
- Evidence supports high gluon densities and gluon saturation in protons

## Abstract

We investigate photo-production of vector mesons J/Psi and Upsilon measured both at HERA and LHC, using 2 particular fits of inclusive unintegrated gluon distributions, based on non-linear Balitsky-Kovchegov evolution (Kutak-Sapeta gluon; KS) and next-to-leading order Balitsky-Fadin-Kuraev-Lipatov evolution (Hentschinski-Sabio Vera-Salas gluon; HSS). We find that linear next-to-leading order evolution can only describe production at highest energies, if perturbative corrections are increased to unnaturally large values; rendering this corrections to a perturbative size, the growth with energy is too strong and the description fails. At the same time, the KS gluon, which we explore both with and without non-linear corrections, requires the latter to achieve an accurate description of the energy dependence of data. We interpret this observation as a clear signal for the presence of high gluon densities in the proton, characteristic for the onset of gluon saturation.