Measurement Of Elastic P(0) Photoproduction At Hera

Elastic rho(0) photoproduction has been measured using the ZEUS detector at HERA. Untagged photoproduction events from ep interactions were used to measure the reaction gamma p --> rho(0)P (rho(0) --> pi(+)pi(-)) at photon-proton centre-of-mass energies between 60 and 80 GeV and \t\ < 0.5 GeV2, where t is the square of the four-momentum transferred at the proton vertex. The differential cross section d sigma/dM(pi pi), where M(pi pi) is the invariant mass of the two pions, and the integrated cross section, sigma(gamma p-->rho)o(p), are presented; the latter was measured to be 14.7 +/- 0.4 (stat.) +/- 2.4 (syst.) mu b. The differential cross section d sigma/dt has an approximately exponential shape; a fit of the type A'(t) exp (-b'(t)\t\ + c'(t)t(2)) yields a t-slope b'(t) = 9.9 +/- 1.2 (stat.) +/- 1.4 (syst.) GeV-2. The results, when compared to low energy data, show a weak energy dependence of both sigma(gamma p-->rho)o(P) and of the t-slope. The p(0) is produced predominantly with transverse polarisation, demostrating that s-channel helicity conservation holds at these energies.