Coherent \(\psi(2S)\) photo-production in ultra-peripheral Pb-Pb collisions at \(\sqrt{s_{\rm NN}}\)= 2.76 TeV

Abstract

The ALICE Collaboration has performed the first measurement of the coherent \(\psi(2S)\) photo-production cross section in ultra-peripheral Pb-Pb collisions at the LHC. This charmonium excited state is reconstructed via the \(\psi(2S)\rightarrow l^+l^-\) and \(\psi(2S)\rightarrow{\rm J}/\psi \pi^+\pi^-\) decays, where the J/\(\psi\) decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 \(\mu\)b\)^{-1}\), The cross section for coherent \(\psi(2S)\) production in the rapidity interval \(-0.9 < y < 0.9\) is \({\rm d}\sigma^{\rm coh}_{\psi(2S)}/{\rm d}y = 0.83\pm0.19 {\rm (stat+syst)}\) mb. The \(\psi(2S)/{\rm J}/\psi\) coherent cross section ratio is 0.34\)^{+0.08}_{-0.07}{\rm (stat+syst)}\), The obtained cross section and the \(\psi(2S)/{\rm J}/\psi\) coherent cross section ratio are compared to those from theoretical models.