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

Abstract

We have 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) $\rightarrow$ 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 $\mu\rm{b}^{-1}$. The cross section for coherent $\psi$(2S) production in the rapidity interval $-0.9<y<0.9$ is $\mathrm{d}\sigma_{\psi{\rm(2S)}}^{\rm coh}/\mathrm{d}y =0.83\pm 0.19\big(\mathrm{\rm{stat}+{\rm syst}}\big)$ mb. The $\psi$(2S) to J/$\psi$ coherent cross section ratio is $0.34^{+0.08}_{-0.07}(\rm{stat}+{\rm syst})$. The obtained results are compared to predictions from theoretical models.