Study of the azimuthal asymmetry of jets in neutral current deep inelastic scattering at HERA

The azimuthal distribution of jets produced in the Breit frame in high-Q(2) deep inelastic e(+)p scattering has been studied with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb(-1). The measured azimuthal distribution shows a structure that is well described by next-to-leading-order QCD predictions over the Q(2) range considered, Q(2) > 125 GeV2. (C) 2002 Elsevier Science B.V. All rights reserved

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 the E**2(T,jet) / Q**2 dependence of forward jet production at HERA

The forward-jet cross section in deep inelastic ep scattering has been measured using the ZEUS detector at HERA with an integrated luminosity of 6.36 pb^-1. The jet cross section is presented as a function of jet transverse energy squared, E(T,jet)^2, and Q^2 in the kinematic ranges 10^-2<E(T,jet)^2/Q^2<10^2 and 2.5 10^-4<x<8.0 10^-2. Since the perturbative QCD predictions for this cross section are sensitive to the treatment of the log(E_T/Q)^2 terms, this measurement provides an important test. The measured cross section is compared to the predictions of a next-to-leading order pQCD calculation as well as to various leading-order Monte Carlo models. Whereas the predictions of all models agree with the measured cross section in the region of small E(T,Jet)^2/Q^2, only one model, which includes a resolved photon component, describes the data over the whole kinematic range.Comment: 8 pages including 3 figure

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.)

Measurement of diffractive production of D*+-(2010) mesons in deep inelastic scattering at HERA

Diffractive production of D*+-(2010) mesons in deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 44.3 pb-1. Diffractive charm production is identified by the presence of a large rapidity gap in the final state of events in which a D*+-(2010) meson is reconstructed in the decay channel D*+ -> (D0 -> K-pi+) pi+ (+ charge conjugate). Differential cross sections when compared with theoretical predictions indicate the importance of gluons in such diffractive interactions.Comment: 19 pages, 4 figures; Submitted to Physics Letters. Comments from Journal referee incorporated, including one additional referenc

Measurement of inclusive D+-(s) photoproduction at HERA

The first measurement of inclusive Ds+- photoproduction at HERA has been performed with the ZEUS detector for photon-proton centre-of-mass energies 130 < W < 280 GeV. The measured cross section for 3 < pt(Ds) < 12 GeV and |eta(Ds)| Ds X) = 3.79 +- 0.59 (stat.) +0.26-0.46 (syst.) +- 0.94 (br.) nb, where the last error arises from the uncertainty in the Ds decay branching ratio. The measurements are compared with inclusive D*+- photoproduction cross sections in the same kinematic region and with QCD calculations. The Ds cross sections lie above a fixed-order next-to-leading order calculation and agree better with a tree-level O(alpha,alpha_s^3) calculation that was tuned to describe the ZEUS D* cross sections. The ratio of Ds+- to D*+- cross sections is 0.41 +- 0.07 (stat.) +0.03-0.05 (syst.) +- 0.10 (br.). From this ratio, the strangeness-suppression factor in charm photoproduction, within the LUND string fragmentation model, has been calculated to be gamma_s = 0.27 +- 0.05 +- 0.07 (br.). The cross-section ratio and gamma_s are in good agreement with those obtained in charm production in e+e- annihilation.Comment: 20 pages, 3 figures, submitted to Phys. Lett. B; 1 footnote adde

Inclusive jet cross-sections in the Breit frame in neutral current deep inelastic scattering at HERA and determination of alpha(s)

Inclusive jet differential cross sections have been measured in neutral current deep inelastic e+p scattering for boson virtualities Q**2>125 GeV**2. The data were taken using the ZEUS detector at HERA and correspond to an integrated luminosity of 38.6 pb-1. Jets were identified in the Breit frame using the longitudinally invariant K_T cluster algorithm. Measurements of differential inclusive jet cross sections are presented as functions of jet transverse energy (E_T,jet), jet pseudorapidity and Q**2, for jets with E_T,jet>8 GeV. Next-to-leading-order QCD calculations agree well with the measurements both at high Q**2 and high E_T,jet. The value of alpha_s(M_Z), determined from an analysis of dsigma/dQ**2 for Q**2>500 GeV**2, is alpha_s(M_Z) = 0.1212 +/- 0.0017 (stat.) +0.0023 / -0.0031 (syst.) +0.0028 / -0.0027 (th.).Comment: 34 pages, 6 figures, 8 tables; submitted to Phys. Let

Measurement of azimuthal asymmetries in deep inelastic scattering

The distribution of the azimuthal angle for the charged hadrons has been studied in the hadronic centre-of-mass system for neutral current deep inelastic positron-proton scattering with the ZEUS detector at HERA. Measurements of the dependence of the moments of this distribution on the transverse momenta of the charged hadrons are presented. Asymmetries that can be unambiguously attributed to perturbative QCD processes have been observed for the first time.Comment: 19 pages - 4 figures. Included referee's comment