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Abstract: ZEUS inclusive diffractive-cross-section measurements have been used in a DGLAP next-to-leading-order QCD analysis to extract the diffractive parton distribution functions. Data on diffractive dijet production in deep inelastic scattering have also been included to constrain the gluon density. Predictions based on the extracted parton densities are compared to diffractive charm and dijet photoproduction data. [Copyright &y& Elsevier]

Abstract: Differential cross sections for dijet photoproduction and this process in association with a leading neutron, , have been measured with the ZEUS detector at HERA using an integrated luminosity of 40 pb−1. The fraction of dijet events with a leading neutron was studied as a function of different jet and event variables. Single- and double-differential cross sections are presented as a function of the longitudinal fraction of the proton momentum carried by the leading neutron, , and of its transverse momentum squared, . The dijet data are compared to inclusive DIS and photoproduction results; they are all consistent with a simple pion-exchange model. The neutron yield as a function of was found to depend only on the fraction of the proton beam energy going into the forward region, independent of the hard process. No firm conclusion can be drawn on the presence of rescattering effects. [Copyright &y& Elsevier]

Abstract: The dissociation of virtual photons, , in events with a large rapidity gap between X and the outgoing proton, as well as in events in which the leading proton was directly measured, has been studied with the ZEUS detector at HERA. The data cover photon virtualities and centre-of-mass energies , with , where is the mass of the hadronic final state, X. Leading protons were detected in the ZEUS leading proton spectrometer. The cross section is presented as a function of t, the squared four-momentum transfer at the proton vertex and Φ, the azimuthal angle between the positron scattering plane and the proton scattering plane. It is also shown as a function of and , the fraction of the proton''s momentum carried by the diffractive exchange, as well as β, the Bjorken variable defined with respect to the diffractive exchange. [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 52.4 pb−1. The method has been used to extract the diffractive contribution. A wide range in the centre-of-mass energy W (37–245 GeV), photon virtuality (20–450 GeV2) and mass (0.28–35 GeV) is covered. The diffractive cross section for rises strongly with W, the rise becoming steeper as increases. The data are also presented in terms of the diffractive structure function, , of the proton. For fixed and fixed , shows a strong rise as , where is the fraction of the proton momentum carried by the pomeron. For Bjorken-, shows positive scaling violations, while for negative scaling violations are observed. The diffractive structure function is compatible with being leading twist. The data show that Regge factorisation is broken. [Copyright &y& Elsevier]

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]

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: 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]

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]

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]

The production of neutrons carrying at least 20% of the proton beam energy (xL> 0.2) in e+p collisions has been studied with the ZEUS detector at HERA for a wide range of Q2, the photon virtuality, from photoproduction to deep inelastic scattering. The neutron-tagged cross section, ep→e′Xn, is measured relative to the inclusive cross section, ep→e′X, thereby reducing the systematic uncertainties. For xL> 0.3, the rate of neutrons in photoproduction is about half of that measured in hadroproduction, which constitutes a clear breaking of factorisation. There is about a 20% rise in the neutron rate between photoproduction and deep inelastic scattering, which may be attributed to absorptive rescattering in the γp system. For 0.64, the rate of neutrons is almost independent of the Bjorken scaling variable x and Q2. However, at lower and higher xL values, there is a clear but weak dependence on these variables, thus demonstrating the breaking of limiting fragmentation. The neutron-tagged structure function, FLN(3)2(x,Q2,xL), rises at low values of x in a way similar to that of the inclusive F2(x,Q2) of the proton. The total γπ cross section and the structure function of the pion, Fπ2(xπ,Q2) where xπ=x/(1−xL), have been determined using a one-pion-exchange model, up to uncertainties in the normalisation due to the poorly understood pion flux. At fixed Q2, Fπ2 has approximately the same x dependence as F2 of the proton. [Copyright &y& Elsevier]

Abstract: Jet substructure and differential cross sections for jets produced in the photoproduction and deep inelastic ep scattering regimes have been measured with the ZEUS detector at HERA using an integrated luminosity of . The substructure of jets has been studied in terms of the jet shape and subjet multiplicity for jets with transverse energies . The data are well described by the QCD calculations. The jet shape and subjet multiplicity are used to tag gluon- and quark-initiated jets. Jet cross sections as functions of , jet pseudorapidity, the jet–jet scattering angle, dijet invariant mass and the fraction of the photon energy carried by the dijet system are presented for gluon- and quark-tagged jets. The data exhibit the behaviour expected from the underlying parton dynamics. A value of of was extracted from the measurements of jet shapes in deep inelastic scattering. [Copyright &y& Elsevier]

The exclusive electroproduction of J/ψ mesons, ep→epJ/ψ, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15 and 2 using integrated luminosities of 69 and 83 pb-1, respectively. The photon–proton centre-of-mass energy was in the range 30 and the squared four-momentum transfer at the proton vertex &z.sfnc;t&z.sfnc;<1 GeV2. The cross sections and decay angular distributions are presented as functions of Q2, W and t. The effective parameters of the pomeron trajectory are in agreement with those found in J/ψ photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, σL/σT, grows with Q2, whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton. [Copyright &y& Elsevier]