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Abstract:

Inclusive event-shape variables have been measured in the current region of the Breit frame for neutral current deep inelastic ep scattering using an integrated luminosity of 45.0 pb^-1 collected with the ZEUS detector at HERA. The variables studied included thrust, jet broadening and invariant jet mass. The kinematic range covered was 10 < Q² < 20480 GeV² and 6 × 10^-4 < x < 0.6, where Q² is the virtuality of the exchanged boson and x is the Bjorken variable. The Q dependence of the shape variables has been used in conjunction with NLO perturbative calculations and the Dokshitzer-Webber non-perturbative corrections ('power corrections') to investigate the validity of this approach.

Abstract:

The cross sections for inelastic photoproduction of J/ψ and ψ′ mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 38.0 pb^-1. The events were required to have 0.1 < z < 0.9 and 50 < W < 180 GeV, where z is the fraction of the incident photon energy carried by the J/ψ in the proton rest frame and W is the photon-proton centre-of-mass energy. The ψ′ to J/ψ cross-section ratio was measured in the range 0.55 < z < 0.9. The J/ψ data, for various ranges of transverse momentum, are compared to theoretical models incorporating colour-singlet and colour-octet matrix elements. Predictions of a next-to-leading-order colour-singlet model give a good description of the data, although there is a large normalisation uncertainty. The J/ψ helicity distribution for z > 0.4 is compared to leading-order QCD predictions.

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 dσγ��→φY/dt to dσγ��→ρ0��/dt and dσγ��→J/ψ��/dt to dσγ��→ρ0��/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.