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Abstract:

Using the ZEUS detector at the HERA electron-proton collider, we have searched for lepton flavor violation in ep collisions at a center-of-mass energy (√s) of 300 GeV. Events of the type e + p → ℓ + X with a final-state lepton of high transverse momentum, ℓ = μ or τ, were sought. No evidence was found for lepton flavor violation in the combined 1993 and 1994 data samples, for which the integrated luminosities were 0.84 pb-1 for e-p collisions and 2.94 pb-1 for e+p collisions. Limits on coupling vs. mass are provided for leptoquarks and R-parity violating squarks. For flavor violating couplings of electromagnetic strength, we set 95% confidence level lower limits on leptoquark masses between 207 GeV and 272 GeV, depending on the leptoquark species and final-state lepton. For leptoquark masses larger than 300 GeV, limits on flavor-changing couplings are determined, many of which supersede prior limits from rare decay processes. © Springer-Verlag 1997.

Abstract:

The reaction γp →ωp(ω → π+π−π0 and π0 → γ γ) has been studied in ep interactions using the ZEUS detector at photon-proton centre-of-mass energies between 70 and 90GeV and ∣t∣ < 0.6GeV2, where t is the squared four momentum transferred at the proton vertex. The elastic ω photoproduction cross section has been measured to be σγp → ωp = 1.21 ±0.12 ±0.23 μb. The differential cross section dσγp→ωp/d¦t¦ has an exponential shape e−b∣t∣ with a slope b =10.0 ± 1.2 ± 1.3 GeV−2. The angular distributions of the decay pions are consistent with s-channel helicity conservation. When compared to low energy data, the features of ω photoproduction as measured at HERA energies are in agreement with those of a soft diffractive process. Previous measurements of the ρ0 and ϕ photoproduction cross sections at HERA show a similar behaviour.

Abstract:

The differential cross section dσ/dt for elastic ρ⁰ photoproduction, γp → ρ⁰ρ (ρ⁰ → π⁺π^-), has been measured in ep interactions at HERA. The squared four-momentum exchanged at the proton vertex, t, has been determined directly by measuring the momentum of the scattered proton using the ZEUS Leading Proton Spectrometer (LPS), a large scale system of silicon micro-strip detectors operating close to the HERA proton beam. The LPS allows the measurement of the momentum of high energy protons scattered at small angles with accuracies of 0.4% for the longitudinal momentum and 5 MeV for the transverse momentum. Photoproduction of ρ⁰ mesons has been investigated in the interval 0.073 < |t| < 0.40 GeV², for photon virtualities Q² < 1 GeV² and photon-proton centre-of-mass energies W between 50 and 100 GeV. In the measured range, the t distribution exhibits an exponential shape with a slope parameter b = 9.8 ± 0.8 (stat.) ± 1.1 (syst.) GeV^-2. The use of the LPS eliminates the contamination from events with diffractive dissociation of the proton into low mass states. © Springler-Verlag 1997.