Prof Roman Walczak

Abstract:

Production of D^*±(2010) mesons in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb^-1. Diffractive events were identified by the presence of a large rapidity gap in the final state. Differential cross sections have been measured in the kinematic region 1.52<200 GeV², 0.02P<0.035, β<0.8, pT(D^*±)>1.5 GeV and η(D^*±) <1.5. The measured cross sections are compared to theoretical predictions. The results are presented in terms of the open-charm contribution to the diffractive proton structure function. The data demonstrate a strong sensitivity to the diffractive parton densities. © 2003 Published by Elsevier B.V.

Abstract:

The production rates and substructure of jets have been studied in charged current deep inelastic e⁺p scattering for Q² > 200 GeV² with the ZEUS detector at HERA using an integrated luminosity of 110.5 pb^-1. Inclusive jet cross sections are presented for jets with transverse energies ET^jet > 14 GeV and pseudorapidities in the range -1 < η^jet < 2. Dijet cross sections are presented for events with a jet having ET^jet > 14 GeV and a second jet having ET^jet > 5 GeV. Measurements of the mean subjet multiplicity, 〈ηsbj〉, of the inclusive jet sample are presented. Predictions based on parton-shower Monte Carlo models and next-to-leading-order QCD calculations are compared to the measurements. The value of αs (MZ), determined from 〈ηsbj〉 at ycut = 10^-2 for jets with 25 < ET^jet < 119 GeV, is α s (MZ) = 0.1202 ± 0.0052 (stat.) -0.0019^+0.0060 (syst.)-0.0053 ^+0.0065 (th.). The mean subjet multiplicity as a function of Q ² is found to be consistent with that measured in NC DIS.

Abstract:

Dijet angular distributions of photoproduction events in which a D*^± meson is produced in association with one of two energetic jets have been measured with the ZEUS detector at HERA, using an integrated luminosity of 120 pb^-1. Differential cross sections as a function of the angle between the charm-jet and the proton-beam direction in the dijet rest frame have been measured for samples enriched in direct or resolved photon events. The results are compared with predictions from leading-order parton-shower Monte Carlo models and with next-to-leading-order QCD calculations. The angular distributions show clear evidence for the existence of charm originating from the photon. © 2003 Published by Elsevier B.V.

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.