Laser-plasma accelerator group

Abstract:

Quasi-monoenergetic, laser-driven electron beams of up to ∼ 200 MeV in energy have been generated from steady-state-flow gas cells [1], These beams are emitted within a low-divergence cone of 2.1 ± 0.5 mrad FWHM and feature unparalleled shot-to-shot stability in energy (2.5% rms), pointing direction (1.4 mrad rms) and charge (16% rms) owing to a highly reproducible plasma-density profile within the laser-plasma-interaction volume. Laser-wakefield acceleration (LWFA) in gas cells of this type constitutes a simple and reliable source of relativistic electrons with well defined properties, which should allow for applications such as the production of extreme-ultraviolet undulator radiation in the near future. © 2009 American Institute of Physics.

Abstract:

The production of excited charm, D 1(2420)⁰ and D 2 ^* (2460)⁰, and charm-strange, D s1(2536)^±, mesons in ep collisions was measured with the ZEUS detector at HERA using an integrated luminosity of 126 pb ^-1. Masses, widths and helicity parameters were determined. The measured yields were converted to the rates of c quarks hadronising as a given excited charm meson and to the ratios of the dominant D 2 ^* (2460)⁰ and D s1(2536) ^± branching fractions. A search for the radially excited charm meson, D ^*(2640)^±, was also performed. The results are compared with those measured previously and with theoretical expectations. © 2009 Springer-Verlag / Società Italiana di Fisica.

Abstract:

A search for events with an isolated high-energy lepton and large missing transverse momentum has been performed with the ZEUS detector at HERA using a total integrated luminosity of 504 pb^-1. The results agree well with Standard Model predictions. The cross section for production of single W bosons in electron-proton collisions with unpolarised electrons is measured to be 0.89-0.22^+0.25 (stat.) ± 0.10 (syst.) pb. © 2009 Elsevier B.V.

Abstract:

Deeply virtual Compton scattering, γ*p→γp has been measured in e⁺p collisions at HERA with the ZEUS detector using an integrated luminosity of 61.1 pb^-1. Cross sections are presented as a function of the photon virtuality, Q², and photon-proton centre-of-mass energy, W, for a wide region of the phase space, Q² > 1.5GeV² and 40 < W < 170GeV. A subsample of events in which the scattered proton is measured in the leading proton spectrometer, corresponding to an integrated luminosity of 31.3 pb^-1, is used for the first direct measurement of the differential cross section as a function of t, where t is the square of the four-momentum transfer at the proton vertex. © SISSA 2009.