51ÁÔÆæÈë¿Ú

Abstract:

Differential cross sections for dijet photoproduction in association with a leading neutron using the reaction e⁺ + p → e⁺ + n + jet + jet + Xr have been measured with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb^-1. The fraction of dijet events with a leading neutron in the final state was studied as a function of the jet kinematic variables. The cross sections were measured for jet transverse energies E^jet T > 6 GeV, neutron energy En > 400 GeV, and neutron production angle θn < 0.8 mrad. The data are broadly consistent with factorization of the lepton and hadron vertices and with a simple one-pion-exchange model. © 2001 Elsevier Science B.V. All rights reserved.

Abstract:

Low-χ physics is reviewed, with particular emphasis on searches for deviations from Gribov-Lipatov-Altarelli-Parisi (GLAP) evolution of the parton densities. Although there are several intriguing indications, both in HERA and in Tevatron data, as yet there is no unambiguous evidence for other than standard next-to-leading-order GLAP evolution. The framework of dipole models and saturation of parton densities is examined and confronted with the data. Although such models give a good qualitative description of the data, so do other, more conventional, explanations.

Abstract:

The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process e⁺p → e⁺ + dijet + e^- + X with the ZEUS detector at HERA using an integrated luminosity of 38.5 pb^-1. Events with photon-proton centre-of-mass energies, Wγ±è, between 134 and 269 GeV and a photon virtuality, Q², less than 1 GeV² were selected requiring at least two jets of transverse energy E^jet1(2) T > 7(6) GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement. The contribution of beauty quarks was determined using the transverse momentum of the electron relative to the axis of the closest jet, p^rel T. The data, after background subtraction, were fit with a Monte Carlo simulation including beauty and charm decays. The measured beauty cross section was extrapolated to the parton level with the b quark restricted to the region of transverse momentum p^b T > = 5 GeV and pseudorapidity |η^b| < 2. The extrapolated cross section is 1.6 ± 0.4 (stat.)^+0.3 -0.5(syst.)^+0.2 -0.4(ext.) nb. The result is compared to a perturbative QCD calculation performed to next-to-leading order.

Abstract:

A study of the vÌ„-jet mass spectrum in e⁺p→vÌ„X events at a center-of-mass energy 300 GeV has been performed with the ZEUS detector at the HERA collider at DESY using an integrated luminosity of 47.7 pb^-1. The mass spectrum is in good agreement with that expected from standard model processes over the vÌ„-jet mass range studied. No significant excess attributable to the decay of a narrow resonance is observed. By using both e⁺±è→>±ð⁺X and e⁺p-vX data, mass-dependent limits are set on the s-channel production of scalar and vector resonant states. Couplings to first-generation quarks are considered and limits are presented as a function of the e⁺q and vÌ„q branching ratios. These limits are used to constrain the production of leptoquarks and A-parity violating squarks. © 2001 The American Physical Society.