51ÁÔÆæÈë¿Ú

Abstract:

The production of φ mesons in the reaction e⁺p → e⁺φp (φ → K⁺K^-), for 7 < Q² < 25 GeV² and for virtual photon-proton centre of mass energies (W) in the range 42-134 GeV, has been studied with the ZEUS detector at HERA. When compared to lower energy data at similar Q², the results show that the γp → p⁰p cross section rises strongly with W. This behaviour is similar to that previously found for the γ*p → p⁰p cross section. This strong dependence cannot be explained by production through soft pomeron exchange. It is, however, consistent with perturbative QCD expectations, where it reflects the rise of the gluon momentum density in the proton at small x. The ratio of σ(φ)/σ(p⁰), which has previously been determined by ZEUS to be 0.065 ± 0.013 (stat.) in photoproduction at a mean W of 70 GeV, is measured to be 0.18 ± 0.05 (stat.) ± 0.03 (syst.) at a mean Q² of 12.3 GeV2 and mean W of ≈ 100 GeV and is thus approaching at large Q² the value of 2/9 predicted from the quark charges of the vector mesons and a flavour independent production mechanism.

Abstract:

The production of φ mesons in the reaction e⁺p → e⁺φp (φ → K⁺K^-) at a median Q² of 10^-4 GeV² has been studied with the ZEUS detector at HERA. The differential φ photoproduction cross section dσ/dt has an exponential shape and has been determined in the kinematic range 0.1 < |t| < 0.5 GeV² and 60 < W < 80 GeV. An integrated cross section of σγ±è→φp = 0.96 ± 0.19^+0.21-.18 μb has been obtained by extrapolating to t = 0. When compared to lower energy data, the results show a weak energy dependence of both σγ±è→φp and the slope of the t distribution. The φ decay angular distributions are consistent with s-channel helicity conservation. From lower energies to HERA energies, the features of φ photoproduction are compatible with those of a soft diffractive process.