Plasma-Wakefield Accelerator Group

Abstract:

The production of the neutral strange hadrons K⁰ S, Λ and Λ has been measured in ep collisions at HERA using the ZEUS detector. Cross sections, baryon-to-meson ratios, relative yields of strange and charged light hadrons, Λ (Λ) asymmetry and polarization have been measured in three kinematic regions: Q² > 25 GeV²; 5 < Q² < 25 GeV²; and in photoproduction (Q ² 0). In photoproduction the presence of two hadronic jets, each with at least 5 GeV transverse energy, was required. The measurements agree in general with Monte Carlo models and are consistent with measurements made at e⁺e^- colliders, except for an enhancement of baryon relative to meson production in photoproduction. © Springer-Verlag / Società Italiana di Fisica 2007.

Abstract:

The production of D^{* ±} (2010) mesons in e^± p scattering in the range of exchanged photon virtuality 0.05 < Q² < 0.7   GeV² has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb^-1. The decay channels D^{* +} → D⁰ π⁺ with D⁰ → K^- π⁺ and corresponding antiparticle decay were used to identify D^* mesons and the ZEUS beampipe calorimeter was used to identify the scattered electron. Differential D^* cross sections as functions of Q², inelasticity, y, transverse momentum of the D^* meson, pT (D^*), and pseudorapidity of the D^* meson, η (D^*), have been measured in the kinematic region 0.02 < y < 0.85, 1.5 < pT (D^*) < 9.0   GeV and | η (D^*) | < 1.5. The measured differential cross sections are in agreement with two different NLO QCD calculations. The cross sections are also compared to previous ZEUS measurements in the photoproduction and DIS regimes. © 2007 Elsevier B.V. All rights reserved.

Abstract:

Differential inclusive-jet cross sections have been measured for different jet radii in neutral current deep inelastic ep scattering for boson virtualities Q2 > 125   GeV2 with the ZEUS detector at HERA using an integrated luminosity of 81.7 pb-1. Jets were identified in the Breit frame using the kT cluster algorithm in the longitudinally inclusive mode for different values of the jet radius R. Differential cross sections are presented as functions of Q2 and the jet transverse energy, ET, Bjet. The dependence on R of the inclusive-jet cross section has been measured for Q2 > 125 and 500   GeV2 and found to be linear with R in the range studied. Next-to-leading-order QCD calculations give a good description of the measurements for 0.5 ≤ R ≤ 1. A value of αs (MZ) has been extracted from the measurements of the inclusive-jet cross section d σ / d Q2 with R = 1 for Q2 > 500   GeV2: αs (MZ) = 0.1207 ± 0.0014 (stat .)-0.0033+0.0035 (exp .)-0.0023+0.0022 (th .). The variation of αs with ET, Bjet is in a good agreement with the running of αs as predicted by QCD. © 2007 Elsevier B.V. All rights reserved.

Abstract:

Mean values and differential distributions of event-shape variables have been studied in neutral current deep inelastic scattering using an integrated luminosity of 82.2 pb^-1 collected with the ZEUS detector at HERA. The kinematic range is 80 < Q² < 20 480 GeV² and 0.0024 < x < 0.6, where Q² is the virtuality of the exchanged boson and x is the Bjorken variable. The data are compared with a model based on a combination of next-to-leading-order QCD calculations with next-to-leading-logarithm corrections and the Dokshitzer-Webber non-perturbative power corrections. The power-correction method provides a reasonable description of the data for all event-shape variables studied. Nevertheless, the lack of consistency of the determination of αs and of the non-perturbative parameter of the model, over(α0, -), suggests the importance of higher-order processes that are not yet included in the model. © 2006 Elsevier B.V. All rights reserved.

Abstract:

The production of beauty quarks with a D*± and a muon in the final state has been measured with the ZEUS detector at HERA using an integrated luminosity of 114 pb^-1. Low transverse-momentum thresholds for the muon and D^* meson allow for a measurement of beauty production closer to the production threshold than previous measurements. The beauty signal was extracted using the charge correlations and angular distributions of the muon with respect to the D* meson. Cross sections for photoproduction and deep inelastic scattering are somewhat higher than, but compatible with, next-to-leading-order QCD predictions, and compatible with other measurements. © Springer-Verlag Berlin Heidelberg 2007.