Prof Roman Walczak

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:

Cross sections for e- p neutral current deep inelastic scattering have been measured at a centre-of-mass energy of 318 GeV using an integrated luminosity of 15.9 pb-1 collected with the ZEUS detector at HERA. Results on the double-differential cross-section d2σ/dx dQ2 in the range 185 < Q2 < 50 000 GeV2 and 0.0037 < x < 0.75, as well as the single-differential cross-sections dσ/dQ2, dσ/dx and dσ/dy for Q2 > 200GeV2, are presented. To study the effect of Z-boson exchange, dσ/dx has also been measured for Q2 > 10 000 GeV2. The structure function xF3 has been extracted by combining the e-p results presented here with the recent ZEUS measurements of e+ p neutral current deep inelastic scattering. All results agree well with the predictions of the Standard Model.

Abstract:

A search for single-top production, ep → et X, has been made with the ZEUS detector at HERA using an integrated luminosity of 130.1 pb^-1. Events from both the leptonic and hadronic decay channels of the W boson resulting from the decay of the top quark were sought. For the leptonic mode, the search was made for events with isolated high-energy leptons and significant missing transverse momentum. For the hadronic decay mode, three-jet events in which two of the jets had an invariant mass consistent with that of the W were selected. No evidence for top production was found. The results are used to constrain single-top production via flavour-changing neutral current (FCNC) transitions. The ZEUS limit excludes a substantial region in the FCNC tuγ coupling not ruled out by other experiments. © 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.