Laser-plasma accelerator group

Abstract:

Exclusive electroproduction of ρ0 mesons has been measured using the ZEUS detector at HERA in two Q2 ranges, 0.25 < Q2 < 0.85 GeV2 and 3 < Q2 < 30 GeV2. The low-Q2 data span the range 20 < W < 90 GeV; the high-Q2 data cover the 40 < W < 120 GeV interval. Both samples extend up to four-momentum transfers of |t| = 0.6 GeV2. The distribution in the azimuthal angle between the positron scattering plane and the ρ0 production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) ρ0 mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed.

Abstract:

In a search for signatures of physics processes beyond the Standard Model, various eeqq vector contact-interaction hypotheses have been tested using the high-Q2 deep inelastic neutral-current e+p scattering data collected with the ZEUS detector at HERA. The data correspond to an integrated luminosity of 47.7pb-1 of e+p interactions at 300 GeV center-of-mass energy. No significant evidence of a contact-interaction signal has been found. Limits at the 95% confidence level are set on the contact-interaction amplitudes. The effective mass scales Λ corresponding to these limits range from 1.7 TeV to 5 TeV for the contact-interaction scenarios considered.

Abstract:

The e⁺-jet invariant mass spectrum produced in the reaction e⁺p → e⁺ X has been studied at a center-of-mass energy of 300 GeV. The data were collected using the ZEUS detector operating at the HERA collider, and correspond to an integrated luminosity of 47.7 pb^-1. The observed mass spectrum is in good agreement with Standard Model expectations up to an e⁺-jet mass of 210 GeV. Above this mass, some excess is seen. The angular distribution of these events is typical of high-Q² neutral current events and does not give convincing evidence for the presence of a narrow scalar or vector state. Limits are presented on the product of cross section and branching ratio for such a state and are interpreted as limits on leptoquark or R-parity-violating squark production. Specific leptoquark types are ruled out at 95% confidence level for coupling strength λ = 0.3 for masses between 150 and 280 GeV.

Abstract:

We present the results of simulations of the propagation of high-intensity laser pulses in discharge-ablated capillary waveguides. The limitations of this type of waveguide for pulse intensities of the order of 1016 W cm-2 are discussed. However, for higher intensities we describe a new regime of quasi-matched guiding, that results in significant improvements in guiding performance. The effect on quasi-matched guiding of changing the atomic numbers of the atoms that compose the plasma waveguide is discussed. Calculations are presented for boron capillaries that show quasi-matched guiding over lengths of 32 mm for an input intensity of 5 × 10 17 Wcm-2. © 2000 Optical Society of America.

Abstract:

An overview is given on the results of numerical simulations of the propagation of high-intensity pules through discharge-ablated capillary waveguides. The numerical code solves the paraxial wave equation for a plasma undergoing optical field ionization, and so accounts for refractive defocusing due to further ionization by the propagating pulse.