MEASUREMENT OF ELASTIC OMEGA-PHOTOPRODUCTION AT HERA

Elastic photoproduction of vector mesons, gamma p --> Vp, has been ext ensively studied in fixed target experiments at centre-of-mass energie s up to W approximate to 20 GeV. The production of rho(0), omega and p hi is usually described in the framework of the vector meson dominance model (VDM) [1] and Regge theory [2]. The W dependence of the cross s ection can be parametrised in Regge theory by the sum of two terms, on e due to Pomeron exchange and the other to Reggeon exchange, While the latter falls with W, the former is almost flat, Whereas rho(0) and ph i production is predominantly due to Pomeron exchange at all energies, the energy behaviour of omega production investigated before HERA sug gests a non-negligible contribution from Reggeon exchange. It is there fore of interest to analyse omega photoproduction at HERA, where Pomer on exchange should dominate. More specifically, it is important to est ablish if the features typical of elastic rho(0) and phi vector meson production are also observed in omega photoproduction at high energy. Among these features are the weak dependence of the elastic cross sect ion on W, the exponential shape of the differential cross section in t , where t is the squared four momentum transferred at the proton verte x, and the observation that the vector meson retains the helicity of t he photon (s-channel helicity conservation, SCHC), In addition, a comp arison of the photoproduction cross sections of the light vector meson s rho(0) [3, 4], omega and phi [5] at HERA energies allows another che ck of their diffractive production mechanism. This paper reports a mea surement of the photoproduction of omega mesons using the reaction ep --> e omega p with the ZEUS detector at HERA. The omega meson is obser ved via its decay into pi(+)pi(-)pi(0)(pi(0) --> gamma gamma) in the k inematic range 70 < W < 90 GeV and p(T)(2) < 0.6 GeV2, where p(T) is t he transverse momentum of the omega with respect to the beam axis. For these events the scattered positron was not observed in the detector, thereby restricting the photon virtuality Q(2) to values smaller than 4 GeV2, with a median Q(2) of about 10(-4) GeV2.