Measurement of elastic ρ0 photoproduction at HERA

Elastic ρ0 photoproduction has been measured using the ZEUS detector at HERA. Untagged photoproduction events from ep interactions were used to measure the reaction γp→ρ0p (ρ0→π+π-) at photon-proton centre-of-mass energies between 60 and 80 GeV and vertical stroke tvertical stroke <0.5 GeV2, where t is the square of the four-momentum transferred at the proton vertex. The differential cross section dσ/dMππ, where Mππ is the invariant mass of the two pions, and the integrated cross section, σsub(γp→ρ0p), are presented; the latter was measured to be 14.7±0.4(stat.)±2.4(syst.) μb. The differential cross section dσ/dt has an approximately exponential shape; a fit of the type A'texp(-b'tvertical stroke tvertical stroke +c'tt2) yields a t-slope b't=9.9±1.2(stat.)±1.4(syst.) μb. The results, when compared to low energy data, show a weak energy dependence of both σsub(γp→ρ0p) and of the t-slope. The ρ0 is produced predominantly with transverse polarisation, demonstrating that s-channel helicity conservation holds at these energies. (orig.)