Prof Roman Walczak

Abstract:

The photoproduction of prompt photons, together with an accompanying jet, has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb-1. A study of the effective transverse momentum, (kT), of partons in the proton, as modelled within the framework of the PYTHIA Monte Carlo, gives a value of (kT) = 1.69 ± 0.18 -0.20+0.18 GeV for the γp centre-of-mass energy range 134 < W < 251 GeV. This result is in agreement with the previously observed trend in hadron-hadron scattering for (kT) to rise with interaction energy. © 2001 Elsevier Science B.V.

Abstract:

Multiplicity moments of charged particles in deep inelastic e+p scattering have been measured with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb-1. The moments for Q2 > 1000 GeV2 were studied in the current region of the Breit frame. The evolution of the moments was investigated as a function of restricted regions in polar angle and, for the first time, both in the transverse momentum and in absolute momentum of final-state particles. Analytic perturbative QCD predictions in conjunction with the hypothesis of Local Parton-Hadron Duality (LPHD) reproduce the trends of the moments in polar-angle regions, although some discrepancies are observed. For the moments restricted either in transverse or absolute momentum, the analytic results combined with the LPHD hypothesis show considerable deviations from the measurements. The study indicates a large influence of the hadronisation stage on the multiplicity distributions in the restricted phase-space regions studied here, which is inconsistent with the expectations of the LPHD hypothesis. © 2001 Published by Elsevier Science B.V.

Abstract:

Dijet production has been studied in neutral current deep inelastic e⁺p scattering for 470 < Q² < 20000 GeV² with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb^-1. Dijet differential cross sections are presented in a kinematic region where both theoretical and experimental uncertainties are small. Next-to-leading-order (NLO) QCD calculations describe the measured differential cross sections well. A QCD analysis of the measured dijet fraction as a function of Q² allows both a precise determination of αs(Mz) and a test of the energy-scale dependence of the strong coupling constant. A detailed analysis provides an improved estimate of the uncertainties of the NLO QCD cross sections arising from the paxton distribution functions of the proton. The value of αs(Mz), as determined from the QCD fit, is αs (MZ) = 0.1166 ± 0.0019(stat.)^+0.0024 -0.0044 (exp.) ^+0.0057 0.0044(th.). © 2001 Elsevier Science B.V.

Abstract:

Differential cross sections for dijet photoproduction in association with a leading neutron using the reaction e+ + p → e+ + n + jet + jet + Xr have been measured with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb-1. The fraction of dijet events with a leading neutron in the final state was studied as a function of the jet kinematic variables. The cross sections were measured for jet transverse energies EjetT > 6 GeV, neutron energy En > 400 GeV, and neutron production angle θn < 0.8 mrad. The data are broadly consistent with factorization of the lepton and hadron vertices and with a simple one-pion-exchange model. © 2001 Elsevier Science B.V. All rights reserved.

Abstract:

The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process e⁺p → e⁺ + dijet + e^- + X with the ZEUS detector at HERA using an integrated luminosity of 38.5 pb^-1. Events with photon-proton centre-of-mass energies, Wγp, between 134 and 269 GeV and a photon virtuality, Q², less than 1 GeV² were selected requiring at least two jets of transverse energy E^jet1(2) T > 7(6) GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement. The contribution of beauty quarks was determined using the transverse momentum of the electron relative to the axis of the closest jet, p^rel T. The data, after background subtraction, were fit with a Monte Carlo simulation including beauty and charm decays. The measured beauty cross section was extrapolated to the parton level with the b quark restricted to the region of transverse momentum p^b T > = 5 GeV and pseudorapidity |η^b| < 2. The extrapolated cross section is 1.6 ± 0.4 (stat.)^+0.3 -0.5(syst.)^+0.2 -0.4(ext.) nb. The result is compared to a perturbative QCD calculation performed to next-to-leading order.