Dissociation of virtual photons in events with a leading proton at HERA

The ZEUS detector has been used to study dissociation of virtual photons in events with a leading proton, gamma^* p -> X p, in e^+p collisions at HERA. The data cover photon virtualities in two ranges, 0.03<Q^2<0.60 GeV^2 and 2<Q^2<100 GeV^2, with M_X>1.5 GeV, where M_X is the mass of the hadronic final state, X. Events were required to have a leading proton, detected in the ZEUS leading proton spectrometer, carrying at least 90% of the incoming proton energy. The cross section is presented as a function of t, the squared four-momentum transfer at the proton vertex, Phi, the azimuthal angle between the positron scattering plane and the proton scattering plane, and Q^2. The data are presented in terms of the diffractive structure function, F_2^D(3). A next-to-leading-order QCD fit to the higher-Q^2 data set and to previously published diffractive charm production data is presented

Search for resonance decays to an v‾\overline v-neutrino plus jet in e+p\mathrm{e^+ p} scattering at DESY HERA

A study of the antineutrino-jet mass spectrum in e^+p -> antineutrino+X events at center-of-mass energy 300 GeV has been performed with the ZEUS detector at HERA 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 antineutrino -jet mass range studied. No significant excess attributable to the decay of a narrow resonance is observed. By using both e^+p -> e^+ X and e^+p -> antineutrino X 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 antineutrino-q branching ratios. These limits are used to constrain the production of leptoquarks and R-parity violating squarks

Three jet production in diffractive deep inelastic scattering at HERA

Three-jet production in the reaction ep --> eXp has been studied with the ZEUS detector at HERA using an integrated luminosity of 42.74 pb^-1. The data were measured in the kinematic region 5 < Q^2 < 100 GeV^2, 200 < W < 250 GeV and 23 < M_X < 40 GeV. The diffractive signal was selected by requiring a large rapidity gap in the outgoing proton direction. Jets were reconstructed in the centre-of-mass system of X using the exclusive k_T-algorithm. A sample of three-jet events in diffraction has been identified. Differential cross sections were measured as a function of the jet pseudorapidity and jet transverse momentum with respect to the virtual photon-Pomeron axis. The jets going in the Pomeron direction are broader than those going in the virtual-photon direction. This is consistent with models predicting that gluons are predominantly produced in the Pomeron direction and quarks in the virtual-photon direction.Comment: 14 pages, 5 figures and 7 tables. Submitted to Physics Letters

Measurement of dijet cross-sections for events with a leading neutron in photoproduction at HERA

Differential cross sections for dijet photoproduction in association with a leading neutron using the reaction e^+ + p --> e^+ + n + jet + jet + X_r 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 E_n > 400 GeV, and neutron production angle theta_n < 0.8 mrad. The data are broadly consistent with factorization of the lepton and hadron vertices and with a simple one-pion-exchange model.Comment: 28 pages, 9 figures, accepted by Nuclear Physics B, correction to neutron yiel