Measurement of event shapes in deep inelastic scattering at HERA

Inclusive event-shape variables have been measured in the current region of the Breit frame for neutral current deep inelastic ep scattering using an integrated luminosity of 45.0 pb^-1 collected with the ZEUS detector at HERA. The variables studied included thrust, jet broadening and invariant jet mass. The kinematic range covered was 10 < Q^2 < 20,480 GeV^2 and 6.10^-4 < x < 0.6, where Q^2 is the virtuality of the exchanged boson and x is the Bjorken variable. The Q dependence of the shape variables has been used in conjunction with NLO perturbative calculations and the Dokshitzer-Webber non-perturbative corrections (`power corrections') to investigate the validity of this approach.Comment: 7+25 pages, 6 figure

Inclusive jet cross sections in the Breit frame in neutral current deep inelastic scattering at HERA and determination of αs\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.)

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

Measurements of inelastic J / psi and psi-prime photoproduction at HERA

The cross sections for inelastic photoproduction of J/psi and psi' mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 38.0 pb-1. The events were required to have 0.1 < z < 0.9 and 50 < W < 180 GeV, where z is the fraction of the incident photon energy carried by the J/psi in the proton rest frame and W is the photon-proton centre-of-mass energy. The psi' to J/psi cross-section ratio was measured in the range 0.55 < z < 0.9. The J/psi data, for various ranges of transverse momentum, are compared to theoretical models incorporating colour-singlet and colour-octet matrix elements. Predictions of a next-to-leading-order colour-singlet model give a good description of the data, although there is a large normalisation uncertainty. The J/psi helicity distribution for z > 0.4 is compared to leading-order QCD predictions.Comment: 31 pages, 10 figure

Sociologies de l'enfance

Differential cross sections for jet photoproduction in the reaction ep → e jet X have been measured with the ZEUS detector at HERA using 82.2 pb-1 of integrated luminosity. Inclusive jet cross sections are presented as a function of the jet transverse energy, ET jet, for jets with ET jet > 17 GeV and pseudorapidity -1 <ηjet <2.5, in the γp centre-of-mass-energy range 142 < Wγp < 293 GeV. Scaled jet invariant cross sections are presented as a function of the dimensionless variable xT ≡ 2ET jet/Wγp for (Wγp) = 180 and 255 GeV. Next-to-leading-order QCD calculations give a good description of the measured differential cross sections in both magnitude and shape. The ratio of scaled jet invariant cross sections at the two (Wγp) values shows clear non-scaling behaviour. A value for the strong coupling constant of αs(MZ) = 0.1224 ± 0.0001 (stat.)-0.0019 +0.0022(exp.)-0.0042 +0.0054(th.) has been extracted from a QCD analysis of the measured dσ/dET jet. The variation of αs with ET jet is in good agreement with the running of αs as predicted by QCD

Measurement of high Q**2 e- p neutral current cross-sections at HERA and the extraction of xF(3)

Cross sections for e(-)p neutral current deep inelastic scattering have been measured at a centre-of-mass energy of 318 GeV using an integrated luminosity of 15.9 pb(-1) collected with the ZEUS detector at HERA. Results on the double- differential cross-section d(2)sigma/dx dQ(2) in the range 185 lt Q(2) lt 50 000 GeV2 and 0.0037 lt x lt 0.75, as well as the single-differential cross-sections dsigma/dQ(2), dsigma/dx and dsigma/dy for Q(2) gt 200 GeV2, are presented. To study the effect of Z-boson exchange, dsigma/dx has also been measured for Q(2) gt 10 000 GeV2. The structure function xF(3) has been extracted by combining the e(-)p results presented here with the recent ZEUS measurements of e(+)p neutral current deep inelastic scattering. All results agree well with the predictions of the Standard Model

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