Prof Roman Walczak

Abstract:

Jet cross sections were measured in charged-current deep inelastic e±p scattering at high boson virtualities Q2 with the ZEUS detector at HERA II using an integrated luminosity of 0.36fb-1. Differential cross sections are presented for inclusive-jet production as functions of Q2, Bjorken x and the jet transverse energy and pseudorapidity. The dijet invariant mass cross section is also presented. Observation of three- and four-jet events in charged-current e±p processes is reported for the first time. The predictions of next-to-leading-order (NLO) QCD calculations are compared to the measurements. The measured inclusive-jet cross sections are well described in shape and normalization by the NLO predictions. The data have the potential to constrain the u and d valence-quark distributions in the proton if included as input to global fits. © 2008 The American Physical Society.

Abstract:

The charged multiplicity distributions and the mean charged multiplicity have been investigated in inclusive neutral current deep inelastic ep scattering with the ZEUS detector at HERA, using an integrated luminosity of 38.6 pb-1. The measurements were performed in the current region of the Breit frame, as well as in the current fragmentation region of the hadronic centre-of-mass frame. The KNO-scaling properties of the data were investigated and the energy dependence was studied using different energy scales. The data are compared to results obtained in e+e- collisions and to previous DIS measurements as well as to leading-logarithm parton-shower Monte Carlo predictions. © SISSA 2007.

Abstract:

Diffractive photoproduction of dijets was measured with the ZEUS detector at the ep collider HERA using an integrated luminosity of 77.2 pb-1. The measurements were made in the kinematic range Q2 < 1 GeV 2, 0.207.5 and 6.5 GeV, respectively, and to lie in the pseudorapidity range -1.5<ηjet<1.5. Differential cross sections were compared to perturbative QCD calculations using available parameterisations of diffractive parton distributions of the proton. © 2008 Springer-Verlag / Società Italiana di Fisica.

Abstract:

Three- and four-jet final states have been measured in photoproduction at HERA using the ZEUS detector with an integrated luminosity of 121   pb -1. The results are presented for jets with transverse energy E Tjet > 6   GeV and pseudorapidity | η jet | < 2.4, in the kinematic region given by the virtuality of the photon Q 2 < 1   GeV 2 and the inelasticity 0.2 ≤ y ≤ 0.85 and in two mass regions defined as 25 ≤ M n j < 50   GeV and M n j ≥ 50   GeV, where M n j is the invariant mass of the n-jet system. The four-jet photoproduction cross section has been measured for the first time and represents the highest-order process studied at HERA. Both the three- and four-jet cross sections have been compared with leading-logarithmic parton-shower Monte Carlo models, with and without multi-parton interactions. The three-jet cross sections have been compared to an O (α α s2) perturbative QCD calculation. © 2007 Elsevier B.V. All rights reserved.

Abstract:

The ATF extraction line laser-wire (LW) aims to achieve a micron-scale laser spot size and to verify that micron scale beam profile measurements can be performed at the International Linear Collider beam delivery system. Recent upgrades to the LW system are presented together with recent results including the first use of the LW as a beam diagnostic tool.