MEASUREMENT OF ELASTIC P(0) PHOTOPRODUCTION AT HERA RID B-9165-2008 RID C-5889-2009 RID A-4818-2008 RID C-1693-2008

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.