The ratio of Ψ(2S) and J/Ψ exclusive photoproduction cross-sections as a tool to detect non-linear QCD evolution

Abstract

We study the proposal that the rise with energy of the ratio of Ψ(2s) and J/Ψ exclusive photoproduction cross-sections might serve as an indicator of the presence of non-linear QCD evolution, related to the presence of high and potentially saturated gluon densities in both the proton and a lead nucleus. Our study employs recent fits of the GBW and BGK dipole models and provides predictions for both exclusive photoproduction on a proton and on a lead nucleus. While the cross-sections for photoproduction on a proton depend only weakly on non-linear low x corrections, we find an increased sensitivity for the cross-section ratio, which is directly related to the node in the Ψ(2s)  wave function. We further give a description of recent ALICE data for exclusive J/Ψ photoproduction on a lead nucleus and provide predictions for Ψ(2s)  photoproduction on a lead nucleus as well as for the corresponding cross-section ratio.