Coherent ψ(2S) photo-production in ultra-peripheral PbPb collisions at √s_NN = 2.76 TeV

We have 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) → l⁺l⁻ and ψ(2S) → 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 μb⁻¹. The cross section for coherent ψ(2S) production in the rapidity interval −0.9 < y < 0.9 is dσ_ψ(2S)^coh/dy = 0.83±0.19(stat + syst ) mb. The ψ(2S) to J/ψ coherent cross section ratio is 0.34^+0.08_−0.07 (stat + syst). The obtained results are compared to predictions from theoretical models.