Plasma-Wakefield Accelerator Group

Abstract:

The first measurement of inclusive D(s)(±) photoproduction at HERA has been performed with the ZEUS detector for photon-proton centre-of-mass energies 130 < W < 280GeV. The measured cross section for 3 < p (is perpendicular to)/(D(s)) < 12GeV and |η(D(s))| < 1.5 is σ(ep → D(s)X) = 3.79 ± 0.59(stat.) -0.46/+0.26 (syst.) ± 0.94(br.)nb, where the last error arises from the uncertainty in the D(s)(±) decay branching ratio. The measurements are compared with inclusive D*(±) photoproduction cross sections in the same kinematic region and with QCD calculations. The D(s)(±) cross sections lie above a fixed-order next-to-leading order calculation and agree better with a tree-level O(αα(s)/3) calculation that was tuned to describe the ZEUS D*(±) cross sections. The ratio of D(s)(±) to D*(±) cross sections is 0.41 ± 0.07 (stat.) -0.05/+0.03 (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 γ(s) = 0.27 ± 0.05 ± 0.07(br.). The cross-section ratio and γ(s) are in good agreement with those obtained in charm production in e+e- annihilation. (C) 2000 Elsevier Science B.V.

Abstract:

Correlations between charged particles in deep inelastic e+ p scattering have been studied in the Breit frame with the ZEUS detector at HERA using an integrated luminosity of 6.4pb-1. Short-range correlations are analysed in terms of the angular separation between current-region particles within a cone centred around the virtual photon axis. Long-range correlations between the current and target regions have also been measured. The data support predictions for the scaling behaviour of the angular correlations at high Q2 and for anti-correlations between the current and target regions over a large range in Q2 and in the Bjorken scaling variable x. Analytic QCD calculations and Monte Carlo models correctly describe the trends of the data at high Q2, but show quantitative discrepancies. The data show differences between the correlations in deep inelastic scattering and e+e- annihilation.

Abstract:

The production of D*^± (2010) mesons in deep inelastic scattering has been measured in the ZEUS detector at HERA using an integrated luminosity of 37 pb^-1. The decay channels D*⁺ → D⁰π⁺ (+c.c.), with D⁰ → K^-π⁺ or D⁰ → K^-π^-π⁺π⁺, have been used to identify the D mesons. The e⁺p cross section for inclusive D*^± production with 1 < Q² < 600 GeV² and 0.02 < y < 0.7 is 8.31±0.31(stat.)^+0.30 -0.50 (syst.) nb in the kinematic region 1.5 < pT(D*^±) < 15 GeV and \η(D*^±)\ < 1.5. Differential cross sections are consistent with a next-to-leading-order perturbative-QCD calculation when using charm-fragmentation models which take into account the interaction of the charm quark with the proton remnant. The observed cross section is extrapolated to the full kinematic region in pT(D*^±) and η(D*^±) in order to determine the charm contribution, F^cc̄ 2 (x, Q²), to the proton structure function. The ratio F^cc̄ 2/F2 rises from ≃10% at Q² ≃ 1.8GeV² to ≃30% at Q² ≃130GeV² for x values in the range 10^-4 to 10^-3.

Abstract:

The e+p charged-current deep inelastic scattering cross sections, dσ/dQ2 for Q2 between 200 and 60000 GeV2, and dσ/dx and dσ/dy for Q2 > 200 GeV2, have been measured with the ZEUS detector at HERA. A data sample of 47.7 pb-1, collected at a center-of-mass energy of 300 GeV, has been used. The double-differential cross-section dσ/dQ2 falls by a factor of about 50000 as Q2 increases from 280 to 30000 GeV2. The double differential cross section d2σ/dxdQ2 has also been measured. A comparison between the data and Standard Model (SM) predictions shows that contributions from antiquarks (ū and c̄) and quarks (d and s) are both required by the data. The predictions of the SM give a good description of the full body of the data presented here. A comparison of the charged-current cross-section dσ/dQ2 with the recent ZEUS results for neutral-current scattering shows that the weak and electromagnetic forces have similar strengths for Q2 above M2W, M2Z. A fit to the data for dσ/dQ2 with the Fermi constant GF and MW as free parameters yields GF = (1.171±0.034 (stat.)+0.026-0.032 (syst.)+0.016-0.015 (PDF)) × 10-5 Gev-2 and MW = 80.8+4.9-4.5 (stat.)+5.0-4.3(syst.)+1.4-1.3 (PDF) GeV. Results For MW, where the propagator effect alone or the SM constraint between GF and MW have been considered, are also presented.