Prof Roman Walczak

Abstract:

Diffractive dissociation of virtual photons, γ*p → Xp, has been studied in ep interactions with the ZEUS detector at HERA. The data cover photon virtualities 0.17 < Q2 < 0.70 GeV2 and 3 < Q2 < 80 GeV2 with 3 < MX < 38 GeV, where MX is the mass of the hadronic final state. Diffractive events were selected by two methods: the first required the detection of the scattered proton in the ZEUS leading proton spectrometer (LPS); the second was based on the distribution of MX. The integrated luminosities of the low- and high-Q2 samples used in the LPS-based analysis are ≃ 0.9 pb-1 and ≃ 3.3 pb-1, respectively. The sample used for the MX-based analysis corresponds to an integrated luminosity of ≃ 6.2 pb-1. The dependence of the diffractive cross section on W, the virtual photon-proton centre-of-mass energy, and on Q2 is studied. In the low-Q2 range, the energy dependence is compatible with Regge theory and is used to determine the intercept of the Pomeron trajectory. The W dependence of the diffractive cross section exhibits no significant change from the low-Q2 to the high-Q2 region. In the low-Q2 range, little Q2 dependence is found, a significantly different behaviour from the rapidly falling cross section measured for Q2 > 3 GeV2. The ratio of the diffractive to the virtual photon-proton total cross section is studied as a function of W and Q2. Comparisons are made with a model based on perturbative QCD.

Abstract:

Cross sections for e- p charged current deep inelastic scattering have been measured at a centre-of-mass energy of 318 GeV with an integrated luminosity of 16.4 pb-1 using the ZEUS detector at HERA. Differential cross sections dσ/dQ2, dσ/dx and dσ/dy are presented for Q2 > 200 GeV2. In addition, d2σ/(dxdQ2) was measured in the kinematic range 280 GeV2 < Q2 < 30 000 GeV2 and 0.015 < × < 0.42. The predictions of the Standard Model agree well with the measured cross sections. The mass of the W boson, determined from a fit to dσ/dQ2, is MW = 80.3±2.1(stat.) ± 1.2(syst.) ± 1.0(PDF) GeV. © 2002 Published by Elsevier Science B.V.

Abstract:

The exclusive photoproduction of J/ψ mesons, γp → J/ψp, has been studied in ep collisions with the ZEUS detector at HERA, in the kinematic range 20 < W < 290 GeV, where W is the photon-proton centre-of-mass energy. The J/ψ mesons were reconstructed in the ninon and the electron decay channels using integrated luminosities of 38 pb-1 and 55 pb-1, respectively. The helicity structure of J/ψ production shows that the hypothesis of s-channel helicity conservation is satisfied within two standard deviations. The total cross section and the differential cross-section dσ/dt, where t is the squared four-momentum transfer at the proton vertex, are presented as a function of W, for |t| < 18 Ge2. The t distribution exhibits an exponential shape with a slope parameter increasing logarithmically with W with a value b = 4.15 ± 0.05(stat.)-0.18+0.30(syst.) GeV-2 at W = 90 GeV. The effective parameters of the Pomeron trajectory are αℙ(0) = 1.200 ± 0.009(stat.) -0.010+0.004(Syst.) and α′ℙ = 0.115 ± 0.018(stat.)-0.015+0.008(syst.) GeV-2.

Abstract:

A search has been made for lepton-flavor-violating interactions of the type e+p→lX, where l denotes a μ or τ with high transverse momentum, at a center-of-mass energy √s of 300 GeV with an integrated luminosity of 47.7 pb-1 using the ZEUS detector at HERA. No evidence was found for lepton-flavor violation and constraints were derived on leptoquarks (LQs) that could mediate such interactions. For LQ masses below √s, limits are set on λeq1βlq, where λeq1 is the coupling of the LQ to an electron and a first-generation quark q1 and βlq is the branching ration of the LQ to l and a quark. For LQ masses exceeding √s, limits are set on the four-fermion contact-interaction term λeqα λlqβ/M2LQ for leptoquarks that couple to an electron and a quark qα and also to l and a quark qβ. Some of the limits are also applicable to lepton-flavor-violating processes mediated by squarks in R-parity-violating supersymmetric models. In some cases involving heavy quarks and especially for l=τ, the ZEUS limits are the most stringent published to date.