Coherent ψ (2S) photo-production in ultra-peripheral Pb Pb collisions at sNN=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 dσψ(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.