Measurement of the spin-density matrix elements in exclusive electroproduction of ρ0 mesons at HERA
European Physical Journal C 12:3 (2000) 393-410
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.Search for contact interactions in deep inelastic e+p → e+X scattering at HERA
European Physical Journal C 14:2 (2000) 239-254
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.Search for resonances decaying to e+-jet in e+p interactions at HERA
European Physical Journal C 16:2 (2000) 253-267
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.W production and the search for events with an isolated high-energy lepton and missing transverse momentum at HERA
PHYSICS LETTERS B 471:4 (2000) 411-428
Angular and current-target correlations in deep inelastic scattering at HERA
EUROPEAN PHYSICAL JOURNAL C 12:1 (2000) 53-68