Photo-production of ψ(2S) mesons at HERA

Quasi-elastic (z>0.95) photo-production of ψ(2S) mesons has been observed at HERA for photon-proton centre-of-mass energies in the range 40 to 160 GeV. The ψ(2S) mesons were identified through their decays to ℓ+ℓ−, and to J/ψπ+π−, where the J/ψ subsequently decays to ℓ+ℓ−, the lepton ℓ being either a muon or an electron. The cross-section for quasi-elastic photoproduction was measured to be (18.0±2.8(stat)±3.0(syst)) nb at a photon-proton centre-of-mass energy of 80 GeV. The ratio of the ψ(2S) to J/ψ quasi-elastic cross-sections is (0.150±0.027(stat)±0.022(syst)).