Prof Roman Walczak

Abstract:

A search for QCD-instanton-induced events in deep inelastic ep scattering has been performed with the ZEUS detector at the HERA collider, using data corresponding to an integrated luminosity of 38 pb-1. A kinematic range defined by cuts on the photon virtuality, Q2 > 120 GeV 2, and on the Bjorken scaling variable, x > 10-3, has been investigated. The QCD-instanton induced events were modelled by the Monte Carlo generator QCDINS. A background-independent, conservative 95% confidence level upper limit for the instanton cross section of 26 pb is obtained, to be compared with the theoretically expected value of 8.9pb.

Abstract:

Bose-Einstein correlations in one and two dimensions have been studied in deep inelastic ep scattering events measured with the ZEUS detector at HERA using an integrated luminosity of 121 pb-1. The correlations are independent of the virtuality of the exchanged photon, Q2, in the range 0.1 < Q2 < 8000 GeV2. There is no significant difference between the correlations in the current and target regions of the Breit frame for Q2 > 100 GeV2. The two-dimensional shape of the particle-production source was investigated, and a significant difference between the transverse and the longitudinal dimensions of the source is observed. This difference also shows no Q2 dependence. The results demonstrate that Bose-Einstein interference, and hence the size of the particle-production source, is insensitive to the hard subprocess. © 2004 Published by Elsevier B.V.

Abstract:

A search for events containing isolated tau leptons and large missing transverse momentum, not originating from the tau decay, has been performed with the ZEUS detector at the electron-proton collider HERA, using 130 pb -1 of integrated luminosity. A search was made for isolated tracks coming from hadronic tau decays. Observables based on the internal jet structure were exploited to discriminate between tau decays and quark- or gluon-induced jets. Three tau candidates were found, while 0.40 -0.13+0.12 were expected from Standard Model processes, such as charged current deep inelastic scattering and single W ±-boson production. To search for heavy-particle decays, a more restrictive selection was applied to isolate tau leptons produced together with a hadronic final state with high transverse momentum. Two candidate events survive, while 0.20±0.05 events are expected from Standard Model processes. © 2004 Published by Elsevier B.V.

Abstract:

Cross sections for e+ p neutral current deep inelastic scattering have been measured at a center-of-mass energy of √s = 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 63.2 pb -1. The double-differential cross section, d2σ/ dxdQ2, is presented for 200 GeV2 < Q2 < 30 000 GeV2 and for 0.005 < x < 0.65. The single-differential cross sections dσ/dQ2, dσ/dx and dσ/dy are presented for Q2 > 200 GeV2. The effect of Z-boson-exchange is seen in dσ/dx measured for Q2 > 10 000 GeV2. The data presented here were combined with ZEUS e+ p neutral current data taken at √s = 300 GeV and the structure function F2em was extracted. All results agree well with the predictions of the Standard Model.