Coherent $ψ$(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 $ψ$(2S) photo-production cross section in ultra-peripheral Pb-Pb collisions at the LHC. This charmonium excited state is reconstructed via the $ψ$(2S) $\rightarrow l^{+}l^{-}$ and $ψ$(2S) $\rightarrow$ J/$ψπ^{+}π^{-}$ decays, where the J/$ψ$ decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 $μ\rm{b}^{-1}$. The cross section for coherent $ψ$(2S) production in the rapidity interval $-0.9<y<0.9$ is $\mathrm{d}σ_{ψ{\rm(2S)}}^{\rm coh}/\mathrm{d}y =0.83\pm 0.19\big(\mathrm{\rm{stat}+{\rm syst}}\big)$ mb. The $ψ$(2S) to J/$ψ$ 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.