  Quasi-elastic (z >0.95) photo-production of psi' mesons has been observed at
HERA for photon-proton centre-of-mass energies in the range 40 to 160 GeV. The
psi' mesons were identified through their decays to l+l- and to J/psi pi+ pi-,
where the J/psi 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 psi' to J/psi quasi-elastic
cross-sections is 0.150 +- 0.027 (stat) +- 0.022 (syst).

High Energy Physics - Experiment