Measurement of diffractive photoproduction of vector mesons at large momentum transfer at HERA

Elastic and proton-dissociative photoproduction of rho /sup 0/, phi and J/ psi vector mesons ( gamma p to Vp, gamma p to VN, respectively) have been measured in e/sup +/p interactions at HERA up to -t=3 GeV/sup 2/, where t is the four-momentum transfer squared at the photon-vector-meson vertex. The analysis is based on a data sample in which photoproduction reactions were tagged by detection of the scattered positron in a special-purpose calorimeter. This limits the photon virtuality, Q/sup 2/, to values less than 0.01 GeV/sup 2/, and selects a gamma p average center-of-mass energy of [W]=94 GeV. Results for the differential cross sections, d sigma /dt, for rho /sup 0/, phi and J/ psi mesons are presented and compared to the results of recent QCD calculations. Results are also presented for the t-dependence of the pion-pair invariant-mass distribution in the rho /sup 0/ mass region and of the spin-density matrix elements determined from the decay-angle distributions. The Pomeron trajectory has been derived from measurements of the W dependence of the elastic differential cross sections d sigma /dt for both rho /sup 0/ and phi mesons

Search for contact interactions in deep inelastic e(+)p → e(+)X scattering at HERA

In a search for signatures of physics processes beyond the Standard Model, various eeqq vector contact-interaction hypotheses have been tested using the high-Q(2) deep inelastic neutral-current e(+)p scattering data collected with the ZEUS detector at HERA. The data correspond to an integrated luminosity of 47.7pb(-1) of e(+)p interactions at 300 GeV center-of-mass energy. No significant evidence of a contact- interaction signal has been found. Limits at the 95% confidence level are set on the contact-interaction amplitudes. The effective mass scales Lambda corresponding to these limits range from 1.7 TeV to 5 TeV for the contact-interaction scenarios considere

Measurement of deeply virtual Compton scattering at HERA

The cross section for deeply virtual Compton scattering in the reaction ep -> e gamma p has been measured with the ZEUS detector at HERA using integrated luminosities of 95.0 pb-1 of e+p and 16.7 pb-1 of e-p collisions. Differential cross sections are presented as a function of the exchanged-photon virtuality, Q2, and the centre-of-mass energy, W, of the gamma*p system in the region 5 < Q2 < 100 GeV2 and 40 < W < 140 GeV. The measured cross sections rise steeply with increasing W. The measurements are compared to QCD-based calculations.Comment: 23 pages, 6 figures, 3 tables. Minor wording and changes to reference

Measurement of subject multiplicities in neutral current deep inelastic scattering at HERA and determination of alpha(s)

The subjet multiplicity has been measured in neutral current e(+) p interactions at Q(2) > 125 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb(-1). Jets were identified in the laboratory frame using the longitudinally invariant k(T) cluster algorithm. The number of jet-like substructures within jets, known as the subjet multiplicity, is defined as the number of clusters resolved in a jet by reapplying the jet algorithm at a smaller resolution scale y(cut). Measurements of the mean subjet multiplicity, , for jets with transverse energies E-T,E-jet > 15 GeV are presented. Next-to-leading-order perturbative QCD calculations describe the measurements well. The value of alpha(s) (M-Z), determined from at y(cut) = 10(-2) for jets with 25 < E-T,E-jet < 71 GeV, is alpha(s) (M-Z) = 0.1187 +/- 0.0017(stat.)(-0.0009)(+0.0024)(syst.)(-0.0076)(+0.0093)(th.). (C) 2003 Published by Elsevier Science B.V

Observation of the strange sea in the proton via inclusive phi-meson production in neutral current deep inelastic scattering at HERA

Inclusive phi (1020)-meson production in neutral current deep inelastic e(+) p scattering has been Measured with the ZEUS detector at HERA using an integrated luminosity of 45 pb(-1) The phi mesons were studied in the range 10 < Q(2) < 100 GeV2, where Q(2) is the virtuality of the exchanged photon and in restricted kinematic regions in the transverse momentum, p(T) pseudorapidity, eta and the scaled momentum in the Breit frame, x(p). Monte Carlo models with the strangeness-suppression factor as determined by analyses of e(+)e(+) annihilation events overestimate the cross sections. A smaller value of the strangeness-suppression factor reduces the predicted cross sections but fails to reproduce the shapes of the Measured differential cross sections. High-momentum phi mesons in the current region of the Breit frame give the first direct evidence for the strange sea in the proton at low x. (C) 2002 Published by Elsevier Science B.V

Measurement of dijet production in neutral current deep inelastic scattering at high Q(2) and determination of alpha(s)

Dijet production has been studied in neutral current deep inelastic e(+)p scattering for 470 lt Q(2) lt 20000 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb(-1). Dijet differential cross sections are presented in a kinematic region where both theoretical and experimental uncertainties are small. Next-to-leading-order (NLO) QCD calculations describe the measured differential cross sections well. A QCD analysis of the measured dijet fraction as a function of Q(2) allows both a precise determination of alpha (s)(M-z) and a test of the energy-scale dependence of the strong coupling constant. A detailed analysis provides an improved estimate of the uncertainties of the NLO QCD cross sections arising from the parton distribution functions of the proton. The value of cu,(Mz), as determined from the QCD fit, is alpha (s)(M-z) = 0.1166 +/- 0.0019(stat.)(- 0.0033)(+0.0024)(exp.)(-0.0044)(+0.0057)(th.)