Professor Brian Foster OBE, FRS

Abstract:

Inclusive-jet and dijet differential cross sections have been measured in neutral current deep inelastic ep scattering for exchanged boson virtualities Q² > 125   GeV² with the ZEUS detector at HERA using an integrated luminosity of 82 pb^-1. Jets were identified in the Breit frame using the kT cluster algorithm. Jet cross sections are presented as functions of several kinematic and jet variables. The results are also presented in different regions of Q². Next-to-leading-order QCD calculations describe the measurements well. Regions of phase space where the theoretical uncertainties are small have been identified. Measurements in these regions have the potential to constrain the gluon density in the proton when used as inputs to global fits of the proton parton distribution functions. © 2006.

Abstract:

A new method is employed to measure the neutral current cross section up to Bjorken-x values of one with the ZEUS detector at HERA using an integrated luminosity of 65.1 pb^-1 for e⁺p collisions and 16.7 pb^-1 for e^-p collisions at √s = 318 GeV and 38.6 pb^-1 for e⁺p collisions at √s = 300 GeV. Cross sections have been extracted for Q² ≥648 GeV² and are compared to predictions using different parton density functions. For the highest x bins, the data have a tendency to lie above the expectations using recent parton density function parametrizations. © Springer-Verlag Berlin Heidelberg 2007.

Abstract:

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential γ+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and xγobs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the kT-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the kT- factorisation calculations are in good agreement with the data. © Springer-Verlag Berlin Heidelberg 2007.

Abstract:

A search for stop production in R-parity-violating supersymmetry has been performed in e+p interactions with the ZEUS detector at HERA, using an integrated luminosity of 65 pb-1. At HERA, the R-parity-violating coupling λ' allows resonant squark production, e+d→̃ q. Since the lowest-mass squark state in most supersymmetry models is the light stop, ̃t, this search concentrated on production of ̃t, followed either by a direct R-parity-violating decay, or by the gauge decay to b̃x1+. No evidence for stop production was found and limits were set on λ'131 as a function of the stop mass in the framework of the minimal supersymmetric standard model. The results have also been interpreted in terms of constraints on the parameters of the minimal supergravity model. © Springer-Verlag Berlin Heidelberg 2007.