Laser-plasma accelerator group

Abstract:

The exclusive photoproduction reaction γ p → Υ{hooked} p has been studied with the ZEUS experiment in ep collisions at HERA using an integrated luminosity of 468 pb-1. The measurement covers the kinematic range 60 < W < 220 GeV and Q2 < 1   GeV2, where W is the photon-proton centre-of-mass energy and Q2 is the photon virtuality. These results, which represent the analysis of the full ZEUS data sample for this channel, are compared to predictions based on perturbative QCD. © 2009 Elsevier B.V.

Abstract:

A search for events containing two or more high-transverse-momentum isolated leptons has been performed in ep collisions with the ZEUS detector at HERA using the full collected data sample, corresponding to an integrated luminosity of 480 pb ^-1. The number of observed events has been compared with the prediction from the Standard Model, searching for possible deviations, especially for multi-lepton events with invariant mass larger than 100 GeV. Good agreement with the Standard Model has been observed. Total and differential cross sections for di-lepton production have been measured in a restricted phase space dominated by photon-photon collisions. © 2009 Elsevier B.V.

Abstract:

The production of D ^±- and D ⁰-mesons has been measured with the ZEUS detector at HERA using an integrated luminosity of 133.6 pb^-1. The measurements cover the kinematic range 52<1000 GeV², 0.02T ^D <15 GeV and |η ^D |<1.6. Combinatorial background to the D-meson signals is reduced by using the ZEUS microvertex detector to reconstruct displaced secondary vertices. Production cross sections are compared with the predictions of next-to-leading-order QCD, which is found to describe the data well. Measurements are extrapolated to the full kinematic phase space in order to obtain the open-charm contribution, F^cc 2 to the proton structure function, F 2. © 2009 Springer-Verlag / Società Italiana di Fisica.

Abstract:

Measurements of the neutral current cross sections for deep inelastic scattering in e ^- p collisions at HERA with a longitudinally polarised electron beam are presented. The single-differential cross-sections d σ/dQ ², d σ/dx and d σ/dy and the double-differential cross sections in Q ² and x are measured in the kinematic region y<0.9 and Q ²>185∈GeV ² for both positively and negatively polarised electron beams and for each polarisation state separately. The measurements are based on an integrated luminosity of 169.9∈pb ^-1 taken with the ZEUS detector in 2005 and 2006 at a centre-of-mass energy of 318∈GeV. The structure functions F3 and xF 3 ^{γ Z} are determined by combining the e ^- p results presented in this paper with previously measured e ⁺ p neutral current data. The asymmetry parameter A ^- is used to demonstrate the parity violating effects of electroweak interactions at large spacelike photon virtuality. The measurements agree well with the predictions of the Standard Model. © 2009 Springer-Verlag / Società Italiana di Fisica.

Abstract:

A search for excited quarks is performed using the full e ^± p data sample collected by the H1 experiment at HERA, corresponding to a total integrated luminosity of 475   pb ^{- 1}. The electroweak decays of excited quarks q ^* → q γ, q ^* → q Z and q ^* → q W with subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for first generation excited quark production is found. Mass dependent exclusion limits on q ^* production cross sections and on the ratio f / Λ of the coupling to the compositeness scale are derived within gauge mediated models. These limits extend the excluded region compared to previous excited quark searches. © 2009 Elsevier B.V.