Professor Brian Foster OBE, FRS

Abstract:

Dijet production has been studied in neutral current deep inelastic e⁺p scattering for 470 < Q² < 20000 GeV² with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb^-1. Dijet differential cross sections are presented in a kinematic region where both theoretical and experimental uncertainties are small. Next-to-leading-order (NLO) QCD calculations describe the measured differential cross sections well. A QCD analysis of the measured dijet fraction as a function of Q² allows both a precise determination of αs(Mz) and a test of the energy-scale dependence of the strong coupling constant. A detailed analysis provides an improved estimate of the uncertainties of the NLO QCD cross sections arising from the paxton distribution functions of the proton. The value of αs(Mz), as determined from the QCD fit, is αs (MZ) = 0.1166 ± 0.0019(stat.)^+0.0024 -0.0044 (exp.) ^+0.0057 0.0044(th.). © 2001 Elsevier Science B.V.

Abstract:

Using a data sample of 23 × 106 BB̄ pairs recorded at the Y(4S) resonance, a sample BCP of neutral B mesons decaying to the CP eigenstates was reconstructed. Each of the events in this sample was examined for evidence that the other neutral B meson decayed as B0 or a B̄0. The obtained value is consistent with the range implied by measurements and theoretical estimates of the magnitudes of CKM matrix elements.

Abstract:

Differential cross sections for dijet photoproduction in association with a leading neutron using the reaction e⁺ + p → e⁺ + n + jet + jet + Xr have been measured with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb^-1. The fraction of dijet events with a leading neutron in the final state was studied as a function of the jet kinematic variables. The cross sections were measured for jet transverse energies E^jet T > 6 GeV, neutron energy En > 400 GeV, and neutron production angle θn < 0.8 mrad. The data are broadly consistent with factorization of the lepton and hadron vertices and with a simple one-pion-exchange model. © 2001 Elsevier Science B.V. All rights reserved.

Abstract:

Low-χ physics is reviewed, with particular emphasis on searches for deviations from Gribov-Lipatov-Altarelli-Parisi (GLAP) evolution of the parton densities. Although there are several intriguing indications, both in HERA and in Tevatron data, as yet there is no unambiguous evidence for other than standard next-to-leading-order GLAP evolution. The framework of dipole models and saturation of parton densities is examined and confronted with the data. Although such models give a good qualitative description of the data, so do other, more conventional, explanations.

Abstract:

The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process e⁺p → e⁺ + dijet + e^- + X with the ZEUS detector at HERA using an integrated luminosity of 38.5 pb^-1. Events with photon-proton centre-of-mass energies, Wγp, between 134 and 269 GeV and a photon virtuality, Q², less than 1 GeV² were selected requiring at least two jets of transverse energy E^jet1(2) T > 7(6) GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement. The contribution of beauty quarks was determined using the transverse momentum of the electron relative to the axis of the closest jet, p^rel T. The data, after background subtraction, were fit with a Monte Carlo simulation including beauty and charm decays. The measured beauty cross section was extrapolated to the parton level with the b quark restricted to the region of transverse momentum p^b T > = 5 GeV and pseudorapidity |η^b| < 2. The extrapolated cross section is 1.6 ± 0.4 (stat.)^+0.3 -0.5(syst.)^+0.2 -0.4(ext.) nb. The result is compared to a perturbative QCD calculation performed to next-to-leading order.