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Abstract: Three- and four-jet final states have been measured in photoproduction at HERA using the ZEUS detector with an integrated luminosity of . The results are presented for jets with transverse energy and pseudorapidity , in the kinematic region given by the virtuality of the photon and the inelasticity and in two mass regions defined as and , where is the invariant mass of the n-jet system. The four-jet photoproduction cross section has been measured for the first time and represents the highest-order process studied at HERA. Both the three- and four-jet cross sections have been compared with leading-logarithmic parton-shower Monte Carlo models, with and without multi-parton interactions. The three-jet cross sections have been compared to an perturbative QCD calculation. [Copyright &y& Elsevier]

The production of dijets in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 61 pb-1. The dijet cross section has been measured for virtualities of the exchanged virtual photon, 5 < Q2 < 100 GeV2, and γ*p centre-of-mass energies, 100 < W < 250 GeV. The jets, identified using the inclusive kT algorithm in the γ*p frame, were required to have a transverse energy E* T,jet > 4 GeV and the jet with the highest transverse energy was required to have E* T,jet > 5 GeV. All jets were required to be in the pseudorapidity range -3.5<η* jet<0. The differential cross sections are compared to leading-order predictions and next-to-leading-order QCD calculations based on recent diffractive parton densities extracted from inclusive diffractive deep inelastic scattering data. [ABSTRACT FROM AUTHOR]

Abstract: The first observation of (anti)deuterons in deep inelastic scattering at HERA has been made with the ZEUS detector at a centre-of-mass energy of 300–318 GeV using an integrated luminosity of 120 pb−1. The measurement was performed in the central rapidity region for transverse momentum per unit of mass in the range . The particle rates have been extracted and interpreted in terms of the coalescence model. The (anti)deuteron production yield is smaller than the (anti)proton yield by approximately three orders of magnitude, consistent with the world measurements. [Copyright &y& Elsevier]

Abstract: Inclusive dijet and trijet production in deep inelastic ep scattering has been measured for and low Bjorken x, . The data were taken at the HERA ep collider with centre-of-mass energy using the ZEUS detector and correspond to an integrated luminosity of . Jets were identified in the hadronic centre-of-mass (HCM) frame using the cluster algorithm in the longitudinally invariant inclusive mode. Measurements of dijet and trijet differential cross sections are presented as functions of , , jet transverse energy, and jet pseudorapidity. As a further examination of low- dynamics, multi-differential cross sections as functions of the jet correlations in transverse momenta, azimuthal angles, and pseudorapidity are also presented. Calculations at generally describe the trijet data well and improve the description of the dijet data compared to the calculation at . [Copyright &y& Elsevier]

Abstract: Bose–Einstein correlations of charged and neutral kaons have been measured in deep inelastic scattering with an integrated luminosity of using the ZEUS detector at HERA. The two-particle correlation function was studied as a function of the four-momentum difference of the kaon pairs, , assuming a Gaussian shape for the particle source. The values of the radius of the production volume, r, and of the correlation strength, λ, were obtained for both neutral and charged kaons. The radii for charged and neutral kaons are similar and are consistent with those obtained at LEP. [Copyright &y& Elsevier]

Abstract: The production of energetic neutrons in ep collisions has been studied with the ZEUS detector at HERA. The neutron energy and distributions were measured with a forward neutron calorimeter and tracker in a sample of inclusive deep inelastic scattering (DIS) data and a sample of photoproduction data. The neutron yield in photoproduction is suppressed relative to DIS for the lower neutron energies and the neutrons have a steeper distribution, consistent with the expectation from absorption models. The distributions are compared to HERA measurements of leading protons. The neutron energy and transverse-momentum distributions in DIS are compared to Monte Carlo simulations and to the predictions of particle exchange models. Models of pion exchange incorporating absorption and additional secondary meson exchanges give a good description of the data. [Copyright &y& Elsevier]

The distribution of the azimuthal angle of charged and neutral hadrons relative to the lepton plane has been studied for neutral current deep inelastic ep scattering using an integrated luminosity of 45 pb-1 taken with the ZEUS detector. The kinematic range is 100

Diffractive photoproduction of D*±(2010) mesons was measured with the ZEUS detector at the ep collider HERA, using an integrated luminosity of 78.6 pb-1. The D* mesons were reconstructed in the kinematic range: transverse momentum pT(D*) > 1.9 GeV and pseudorapidity |η(D*)|<1.6, using the decay D*+→D0π+ s followed by D0→K-π+(+c.c.). Diffractive events were identified by a large gap in pseudorapidity between the produced hadronic state and the outgoing proton. Cross sections are reported for photon–proton centre-of-mass energies in the range 130 < W < 300 GeV and for photon virtualities Q2 < 1 GeV2, in two ranges of the Pomeron fractional momentum xIP<0.035 and xIP<0.01. The relative contribution of diffractive events to the inclusive D*±(2010) photoproduction cross section is about 6%. The data are in agreement with perturbative QCD calculations based on various parameterisations of diffractive parton distribution functions. The results are consistent with diffractive QCD factorisation. [ABSTRACT FROM AUTHOR]

The production of the neutral strange hadrons K0 S, Λ and Λ̄ has been measured in ep collisions at HERA using the ZEUS detector. Cross sections, baryon-to-meson ratios, relative yields of strange and charged light hadrons, Λ (Λ̄) asymmetry and polarization have been measured in three kinematic regions: Q2 > 25 GeV2; 5 < Q2 < 25 GeV2; and in photoproduction (Q2≃0). In photoproduction the presence of two hadronic jets, each with at least 5 GeV transverse energy, was required. The measurements agree in general with Monte Carlo models and are consistent with measurements made at e+e- colliders, except for an enhancement of baryon relative to meson production in photoproduction. [ABSTRACT FROM AUTHOR]

Abstract: The production of mesons in scattering in the range of exchanged photon virtuality has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb−1. The decay channels with and corresponding antiparticle decay were used to identify mesons and the ZEUS beampipe calorimeter was used to identify the scattered electron. Differential cross sections as functions of , inelasticity, y, transverse momentum of the meson, , and pseudorapidity of the meson, , have been measured in the kinematic region , and . The measured differential cross sections are in agreement with two different NLO QCD calculations. The cross sections are also compared to previous ZEUS measurements in the photoproduction and DIS regimes. [Copyright &y& Elsevier]

Abstract: Differential inclusive-jet cross sections have been measured for different jet radii in neutral current deep inelastic ep scattering for boson virtualities with the ZEUS detector at HERA using an integrated luminosity of 81.7 pb−1. Jets were identified in the Breit frame using the cluster algorithm in the longitudinally inclusive mode for different values of the jet radius R. Differential cross sections are presented as functions of and the jet transverse energy, . The dependence on R of the inclusive-jet cross section has been measured for and and found to be linear with R in the range studied. Next-to-leading-order QCD calculations give a good description of the measurements for . A value of has been extracted from the measurements of the inclusive-jet cross section with for : . The variation of with is in a good agreement with the running of as predicted by QCD. [Copyright &y& Elsevier]

Abstract: Mean values and differential distributions of event-shape variables have been studied in neutral current deep inelastic scattering using an integrated luminosity of 82.2 pb−1 collected with the ZEUS detector at HERA. The kinematic range is and , where is the virtuality of the exchanged boson and x is the Bjorken variable. The data are compared with a model based on a combination of next-to-leading-order QCD calculations with next-to-leading-logarithm corrections and the Dokshitzer–Webber non-perturbative power corrections. The power-correction method provides a reasonable description of the data for all event-shape variables studied. Nevertheless, the lack of consistency of the determination of and of the non-perturbative parameter of the model, , suggests the importance of higher-order processes that are not yet included in the model. [Copyright &y& Elsevier]

Abstract: Inclusive-jet and dijet differential cross sections have been measured in neutral current deep inelastic ep scattering for exchanged boson virtualities with the ZEUS detector at HERA using an integrated luminosity of 82 pb−1. Jets were identified in the Breit frame using the cluster algorithm. Jet cross sections are presented as functions of several kinematic and jet variables. The results are also presented in different regions of . Next-to-leading-order QCD calculations describe the measurements well. Regions of phase space where the theoretical uncertainties are small have been identified. Measurements in these regions have the potential to constrain the gluon density in the proton when used as inputs to global fits of the proton parton distribution functions. [Copyright &y& Elsevier]

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential γ+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and xγ obs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the kT-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the kT-factorisation calculations are in good agreement with the data. [ABSTRACT FROM AUTHOR]

Abstract: The cross sections for charged and neutral current deep inelastic scattering in collisions with a longitudinally polarised positron beam have been measured using the ZEUS detector at HERA. The results, based on data corresponding to an integrated luminosity of 23.8 pb−1 at , are given for both charged current and neutral current deep inelastic scattering for both positive and negative values of the longitudinal polarisation of the positron beam. Single differential cross sections are presented for the kinematic region . The measured cross sections are compared to the predictions of the Standard Model. A fit to the data yields , which is consistent within two standard deviations with the absence of right-handed charged currents in the Standard Model. [Copyright &y& Elsevier]

Abstract: Differential inclusive jet cross sections in neutral current deep inelastic ep scattering have been measured with the ZEUS detector using an integrated luminosity of 38.7 pb−1. The jets have been identified using the cluster algorithm in the longitudinally invariant inclusive mode in the laboratory frame; they have been selected with jet transverse energy, above 6 GeV and jet pseudorapidity, , between −1 and 3. Measurements of cross sections as functions of , Björken x and the photon virtuality, , are presented. Three phase-space regions have been selected in order to study parton dynamics from the most global to the most restrictive region of forward-going (close to the proton-beam direction) jets at low x, where the effects of BFKL evolution might be present. The measurements have been compared to the predictions of leading-logarithm parton-shower Monte Carlo models and fixed-order perturbative QCD calculations. In the forward region, QCD calculations underestimate the data up to an order of magnitude at low x. An improved description of the data in this region is obtained by including QCD corrections, which account for the lowest-order -channel gluon-exchange diagrams, highlighting the importance of such terms in the parton dynamics at low x. [Copyright &y& Elsevier]

Abstract: Inclusive jet cross sections in photoproduction for events containing a meson have been measured with the ZEUS detector at HERA using an integrated luminosity of . The events were required to have a virtuality of the incoming photon, , of less than 1 GeV2, and a photon–proton centre-of-mass energy in the range . The measurements are compared with next-to-leading-order (NLO) QCD calculations. Good agreement is found with the NLO calculations over most of the measured kinematic region. Requiring a second jet in the event allowed a more detailed comparison with QCD calculations. The measured dijet cross sections are also compared to Monte Carlo (MC) models which incorporate leading-order matrix elements followed by parton showers and hadronisation. The NLO QCD predictions are in general agreement with the data although differences have been isolated to regions where contributions from higher orders are expected to be significant. The MC models give a better description than the NLO predictions of the shape of the measured cross sections. [Copyright &y& Elsevier]

Abstract: Exclusive electroproduction of ϕ mesons has been studied in collisions at with the ZEUS detector at HERA using an integrated luminosity of . The cross section is presented in the kinematic range , and . The cross sections as functions of , W, t and helicity angle are compared to cross sections for other vector mesons. The ratios R of the cross sections for longitudinally and transversely polarized virtual photons are presented as functions of and W. The data are also compared to predictions from theoretical models. [Copyright &y& Elsevier]

Abstract: Deep inelastic scattering and its diffractive component, , have been studied at HERA with the ZEUS detector using an integrated luminosity of . The measurement covers a wide range in the c.m. energy W (37–245 GeV), photon virtuality Q 2 (2.2–80 GeV2) and mass (0.28–35 GeV). The diffractive cross section for rises strongly with W; the rise is steeper with increasing Q 2. The latter observation excludes the description of diffractive deep inelastic scattering in terms of the exchange of a single pomeron. The ratio of diffractive to total cross section is constant as a function of W, in contradiction to the expectation of Regge phenomenology combined with a naive extension of the optical theorem to scattering. Above of 8 GeV, the ratio is flat with Q 2, indicating a leading-twist behaviour of the diffractive cross section. The data are also presented in terms of the diffractive structure function, , of the proton. For fixed β, the Q 2 dependence of changes with in violation of Regge factorisation. For fixed , rises as , the rise accelerating with increasing Q 2. These positive scaling violations suggest substantial contributions of perturbative effects in the diffractive DIS cross section. [Copyright &y& Elsevier]

Abstract: A search for pentaquarks decaying to () and corresponding antiparticles has been performed with the ZEUS detector at HERA. The data sample consists of deep inelastic ep scattering events at centre-of-mass energies of 300 and 318 GeV, and corresponds to 121 pb−1 of integrated luminosity. A clear signal for was observed. However, no signal for any new baryonic state was observed at higher masses in either the or channels. The searches in the antiparticle channels were also negative. Upper limits on the ratio of a possible () signal to the signal were set in the mass range 1650–2350 MeV. [Copyright &y& Elsevier]