Coherent ψ(2S) photo-production in ultra-peripheral Pb–Pb 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_−0.07 ^+0.08(stat+syst). The obtained results are compared to predictions from theoretical models.