Plasma-Wakefield Accelerator Group

Abstract:

A global event shape analysis of the multihadronic final states observed in neutral current deep inelastic scattering events with a large rapidity gap with respect to the proton direction is presented. The analysis is performed in the range 5 ≤ Q2 ≤ 185 GeV2 and 160 ≤ W ≤ 250 GeV, where Q2 is the virtuality of the photon and W is the virtual-photon proton centre of mass energy. Particular emphasis is placed on the dependence of the shape variables, measured in the γ * -pomeron rest frame, on the mass of the hadronic final state, MX. With increasing MX the multihadronic final state becomes more collimated and planar. The experimental results are compared with several models which attempt to describe diffractive events. The broadening effects exhibited by the data require in these models a significant gluon component of the pomeron. © 1998 Elsevier Science B.V.

Abstract:

Charged particles (h±) and K0 mesons have been studied in photoproduced events containing at least one jet of ET > 8 GeV n a pseudorapidity interval (-0.5, 0.5) in the ZEUS laboratory frame. Distributions are presented in terms of transverse momentum, pseudorapidity and distance of the particle from the axis of a jet. The properties of h± within the jet are described well using the standard settings of PYTHIA, but the use of the multiparton interaction option improves the description outside the jets.

Abstract:

Results from the HI and ZEUS experiments at HERA on deep inelastic scattering are reviewed. The data lead to a consistent picture of a steep rise in the F2 structure function and in the gluon density within the proton. Important new information on the partonic structure of diffraction is emerging from H1 and ZEUS. The spacelike region in which the weak and electromagnetic interactions become of equal strength is being explored for the first time. A possible excess of events at high x and Q2 compared to the expectations of the Standard Model has been observed in both experiments. © World Scientific Publishing Company.

Abstract:

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (xγOBS) and pomeron (βOBS) momentum participating in the production of the dijet system. The observed xγOBS dependence shows evidence for the presence of a resolved-as well as a direct-photon component. The measured cross section dσ/dβOBS increases as βOBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.

Abstract:

Dijet cross sections are presented using photoproduction data obtained with the ZEUS detector during 1994. These measurements represent an extension of previous results, as the higher statistics allow cross sections to be measured at higher jet transverse energy (E ). Jets are identified in the hadronic final state using three different algorithms, and the cross sections compared to complete next-to-leading order QCD calculations. Agreement with these calculations is seen for the pseudorapidity dependence of the direct photon events with E > 6 GeV and of the resolved photon events with E > 11 GeV. Calculated cross sections for resolved photon processes with 6 GeV < E < 11 GeV lie below the data. T T T T jet jet jet jet