Professor Claire Gwenlan

Abstract:

Events with a final-state proton carrying a large fraction of the proton beam momentum, x L >0.6, and the square of the transverse momentum p T ² <0.5 GeV ² , have been studied in e ⁺ p collisions with the ZEUS detector at HERA. Data with different photon virtualities were used: Q ² <0.02 GeV ² , 0.12 <0.74 GeV ² and 32 <254 GeV ² , corresponding to integrated luminosities of 0.9, 1.85 and 3.38 pb ^-1 . The cross sections are given as a function of x L , p T ² , Q ² and the Bjorken scaling variable, x. The ratio of the cross section for leading proton production to the inclusive e ⁺ p cross section shows only a mild dependence on Q ² and on x. In the region 0.6L <0.97, the relative yield of protons is only a weak function of x L . © 2003 Elsevier Science B.V. All rights reserved.

Abstract:

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 <η^jet <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. © 2003 Published by Elsevier Science B.V.

Abstract:

The subjet multiplicity has been measured in neutral current e⁺p interactions at Q² > 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 kT 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 ycut. Measurements of the mean subjet multiplicity, (nsbj), for jets with transverse energies ET,jet > 15 GeV are presented. Next-to-leading-order perturbative QCD calculations describe the measurements well. The value of αs(MZ), determined from (nsbj) at ycut = 10^-2 for jets with 25 < ET,jet < 71 GeV, is αs(MZ) = 0.1187 ± 0.0017(stat.)-0.0009^+0.0024(syst.) -0.0076^+0.0093(th.). © 2003 Published by Elsevier Science B.V.

Abstract:

Inclusive φ(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 φ mesons were studied in the range 10 < Q2 < 100 GeV2, where Q2 is the virtuality of the exchanged photon, and in restricted kinematic regions in the transverse momentum, pT, pseudorapidity, η, and the scaled momentum in the Breit frame, xp. 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 φ mesons in the current region of the Breit frame give the first direct evidence for the strange sea in the proton at low x. © 2002 Published by Elsevier Science B.V.