Prof Roman Walczak

Abstract:

Events with high energy isolated electrons, muons or tau leptons and missing transverse momentum are studied using the full e ± p data sample collected by the H1 experiment at HERA, corresponding to an integrated luminosity of 474 pb -1. Within the Standard Model, events with isolated leptons and missing transverse momentum mainly originate from the production of single W bosons. The total single W boson production cross section is measured as 1.14±0.25 (stat.)±0.14 (sys.) pb, in agreement with the Standard Model expectation. The data are also used to establish limits on the WW γ gauge couplings and for a measurement of the W boson polarisation. © 2009 Springer-Verlag / Società Italiana di Fisica.

Abstract:

The scaled momentum distributions of charged particles in jets have been measured for dijet photoproduction with the ZEUS detector at HERA using an integrated luminosity of 359 pb-1. The distributions are compared to predictions based on perturbative QCD carried out in the framework of the modified leading-logarithmic approximation (MLLA) and assuming local parton-hadron duality (LPHD). The universal MLLA scale, Λeff, and the LPHD parameter, κch, are extracted. © SISSA 2009.

Abstract:

Subjet distributions were measured in neutral current deep inelastic ep scattering with the ZEUS detector at HERA using an integrated luminosity of 81.7 pb-1. Jets were identified using the k T cluster algorithm in the laboratory frame. Subjets were defined as jet-like substructures identified by a reapplication of the cluster algorithm at a smaller value of the resolution parameter ycut. Measurements of subjet distributions for jets with exactly two subjets for ycut=0.05 are presented as functions of observables sensitive to the pattern of parton radiation and to the colour coherence between the initial and final states. Perturbative QCD predictions give an adequate description of the data. © 2009 Springer-Verlag / Società Italiana di Fisica.

Abstract:

The production of D-+/-- and D-0-mesons has been measured with the ZEUS detector at HERA using an integrated luminosity of 133.6 pb(-1). The measurements cover the kinematic range 5 < Q(2) < 1000 GeV2, 0.02 < y < 0.7, 1.5 < p(T)(D) < 15 GeV and |eta(D)| < 1.6. Combinatorial background to the D-meson signals is reduced by using the ZEUS microvertex detector to reconstruct displaced secondary vertices. Production cross sections are compared with the predictions of next-to-leading-order QCD, which is found to describe the data well. Measurements are extrapolated to the full kinematic phase space in order to obtain the open-charm contribution, F-2(c (c) over bar), to the proton structure function, F-2.

Abstract:

The exclusive photoproduction reaction γ p → Υ{hooked} p has been studied with the ZEUS experiment in ep collisions at HERA using an integrated luminosity of 468 pb-1. The measurement covers the kinematic range 60 < W < 220 GeV and Q2 < 1   GeV2, where W is the photon-proton centre-of-mass energy and Q2 is the photon virtuality. These results, which represent the analysis of the full ZEUS data sample for this channel, are compared to predictions based on perturbative QCD. © 2009 Elsevier B.V.