Laser-plasma accelerator group

Abstract:

Inclusive φ(1020)-meson production in neutral current deep inelastic e+p scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 45 pb-1. The φ mesons were studied in the range 10 < Q2 < 100 GeV2, where Q2 is the virtuality of the exchanged photon, and in restricted kinematic regions in the transverse momentum, pT, pseudorapidity, η, and the scaled momentum in the Breit frame, xp. Monte Carlo models with the strangeness-suppression factor as determined by analyses of e+e- annihilation events overestimate the cross sections. A smaller value of the strangeness-suppression factor reduces the predicted cross sections, but fails to reproduce the shapes of the measured differential cross sections. High-momentum φ mesons in the current region of the Breit frame give the first direct evidence for the strange sea in the proton at low x. © 2002 Published by Elsevier Science B.V.

Abstract:

The azimuthal distribution of jets produced in the Breit frame in high-Q² deep inelastic e⁺p scattering has been studied with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb-¹. The measured azimuthal distribution shows a structure that is well described by next-to-leading-order QCD predictions over the Q² range considered, Q² > 125 GeV². © 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We describe in detail the operation of the gas-filled capillary discharge waveguide for high-intensity laser pulses and discuss measurements and magnetohydrodynamic simulations that show that the plasma channel produced is parabolic and essentially fully ionized. We present the results of experiments in which laser pulses with a peak input intensity of 1.2 × 1017 W cm-2 were guided through hydrogen-filled capillary discharges with lengths of 30 and 50 mm. The pulse energy coupling and transmission losses were determined to be <4% and (7 ± 1) m-1, respectively. We discuss the application of waveguides of this type to driving short-wavelength lasers and laser wakefield accelerators. © 2003 Optical Society of America.

Abstract:

Diffractive photoproduction of vector mesons, γp → VY, where Y is a proton-dissociative system, has been measured in e⁺p interactions with the ZEUS detector at HERA using an integrated luminosity of 25 pb^-1. The differential cross section, dσ/dt, is presented for 1.2 < -t < 12 GeV², where t is the square of the four-momentum transferred to the vector meson. The data span the range in photon-proton centre-of-mass energy, W, from 80 GeV to 120 GeV. The t distributions are well fit by a power law, dσ/dt ∝ (-t)^-n. The slope of the effective Pomeron trajectory, measured from the W dependence of the ρ⁰ and φ cross sections in bins of t, is consistent with zero. The ratios dσγp→φY/dt to dσγp→ρ0Y/dt and dσγp→J/ψY/dt to dσγp→ρ0Y/dt increase with increasing -t. Decay-angle analyses for ρ⁰, φ and J/ψ mesons have been carried out. For the ρ⁰ and φ mesons, contributions from single and double helicity flip are observed. The results are compared to expectations of theoretical models.

Abstract:

A search for narrow-width resonances that decay into electron+jet or neutrino+jet has been performed with the ZEUS detector at the DESY ep collider HERA operating at center-of-mass energies of 300 and 318 GeV. An integrated e⁺p luminosity of 114.8 pb^-1 and e^-p luminosity of 16.7 pb^-1 were used. No evidence for any resonance was found. Limits were derived on the Yukawa coupling λ as a function of the mass of a hypothetical resonance that has arbitrary decay branching ratios into eq or vq. These limits also apply to squarks predicted by R-parity-violating supersymmetry. Limits for the production of leptoquarks described by the Buchmüller-Rückl-Wyler model were also derived for masses up to 400 GeV. For λ = 0.1, leptoquark masses up to 290 GeV are excluded.