Measurement of high-Q^{2} neutral-current e^+p deep inelastic scattering cross-sections at HERA

The neutral-current deep inelastic scattering differential cross-sections , for GeV, and , for , 2500 and 10000 GeV, have been measured with the ZEUS detector at HERA. The data sample of 47.7 pb was collected at a center-of-mass energy of 300 GeV. The cross-section, , falls by six orders of magnitude between and 40000 GeV. The predictions of the Standard Model are in very good agreement with the data. Complementing the observations of time-like contributions to fermion-antifermion annihilation, the data provide direct evidence for the presence of exchange in the space-like region explored by deep inelastic scattering. Received: 18 May 1999 / Published online: 3 August 1999     

Measurement of jet shapes in high-Q^2 deep inelastic scattering at HERA

The shapes of jets with transverse energies, , up to 45 GeV produced in neutral- and charged-current deep inelastic scattering (DIS) at GeV have been measured with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the - plane with a cone radius of one unit. The jets become narrower as increases. The jet shapes in neutral- and charged-current DIS are found to be very similar. The jets in neutral-current DIS are narrower than those in resolved processes in photoproduction and closer to those in direct-photon processes for the same ranges in and jet pseudorapidity. The jet shapes in DIS are observed to be similar to those in interactions and narrower than those in collisions for comparable . Since the jets in interactions and DIS are predominantly quark initiated in both cases, the similarity in the jet shapes indicates that the pattern of QCD radiation within a quark jet is to a large extent independent of the hard scattering process in these reactions. Received: 2 April 1998 / Published online: 7 April 1999     

Search for contact interactions in deep inelastic e^+p\to e^+X scattering at HERA

In a search for signatures of physics processes beyond the Standard Model, various eeqq vector contact–interaction hypotheses have been tested using the high– deep inelastic neutral–current scattering data collected with the ZEUS detector at HERA. The data correspond to an integrated luminosity of of interactions at 300 GeV center–of–mass energy. No significant evidence of a contact–interaction signal has been found. Limits at the 95% confidence level are set on the contact–interaction amplitudes. The effective mass scales corresponding to these limits range from 1.7 TeV to 5 TeV for the contact–interaction scenarios considered. Received: 18 May 1999 / Revised version: 14 January 2000 / Published online: 25 February 2000     

Measurement of charged current deep inelastic scattering cross sections with a longitudinally polarised electron beam at HERA

Measurements of the cross sections for charged current deep inelastic scattering in e ⁻ p collisions with longitudinally polarised electron beams are presented. The measurements are based on a data sample with an integrated luminosity of 175 pb⁻¹ collected with the ZEUS detector at HERA at a centre-of-mass energy of 318 GeV. The total cross section is given for positively and negatively polarised electron beams. The differential cross-sections d σ/dQ ², d σ/dx and d σ/dy are presented for Q ²>200 GeV². The double-differential cross-section d ² σ/dxdQ ² is presented in the kinematic range 280<Q ²<30 000 GeV² and 0.015<x<0.65. The measured cross sections are compared with the predictions of the Standard Model. Deceased. Also affiliated with University College London, United Kingdom. Now at University of Liverpool, UK. On leave of absence at CERN, Geneva, Switzerland. Now at CERN, Geneva, Switzerland. Now at Bologna University, Bologna, Italy. Also at Institut of Theoretical and Experimental Physics, Moscow, Russia. Also at INP, Cracow, Poland. Also at FPACS, AGH-UST, Cracow, Poland. Partially supported by Warsaw University, Poland. Partly supported by Moscow State University, Russia. Royal Society of Edinburgh, Scottish Executive Support Research Fellow. Now at University of Salerno, Italy. Also affiliated with DESY, Germany. Also at University of Tokyo, Japan. Now at Kobe University, Japan. V. Aushev: Supported by DESY, Germany. Partly supported by Russian Foundation for Basic Research grant no. 05-02-39028-NSFC-a. STFC Advanced Fellow. Nee Korcsak-Gorzo. This material was based on work supported by the National Science Foundation, while working at the Foundation. Now at University of Kansas, Lawrence, USA. Also at Max Planck Institute, Munich, Germany, Alexander von Humboldt Research Award. Also working at Max Planck Institute, Munich, Germany. Now at KEK, Tsukuba, Japan. Now at Nagoya University, Japan. Member of Department of Radiological Science, Tokyo Metropolitan University, Japan. Now at SunMelx Co. Ltd., Tokyo, Japan. Also at Hamburg University, Inst. of Exp. Physics, Alexander von Humboldt Research Award and partially supported by DESY, Hamburg, Germany. Also at Łódź University, Poland. Member of Łódź University, Poland. Also at University of Podlasie, Siedlce, Poland. Now at Lund Universtiy, Lund, Sweden. Now at Institute of Aviation, Warsaw, Poland. Supported by the research grant no. 1 P03B 04529 (2005–2008). This work was supported in part by the Marie Curie Actions Transfer of Knowledge project COCOS (contract MTKD-CT-2004-517186). Now at University of Bonn, Germany. Now at DESY group FEB, Hamburg, Germany. Also at Moscow State University, Russia. Supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).     

Measurement of event shapes in deep inelastic scattering at HERA

Inclusive event-shape variables have been measured in the current region of the Breit frame for neutral current deep inelastic ep scattering using an integrated luminosity of collected with the ZEUS detector at HERA. The variables studied included thrust, jet broadening and invariant jet mass. The kinematic range covered was 10 &lt; Q ² &lt; 20 480 GeV ² and , where Q² is the virtuality of the exchanged boson and x is the Bjorken variable. The Q dependence of the shape variables has been used in conjunction with NLO perturbative calculations and the Dokshitzer-Webber non-perturbative corrections (`power corrections') to investigate the validity of this approach. Received: 13 November 2002 / Published online: 7 March 2003     

Multijet cross sections in deep inelastic scattering at next-to-leading order

We present the perturbative prediction for three-jet production cross section in deep inelastic scattering at the next-to-leading order accuracy. We study the dependence on the renormalization and factorization scales of exclusive three-jet cross section. The perturbative prediction for the three-jet differential distribution as a function of the momentum transfer is compared to the corresponding data obtained by the H1 Collaboration at HERA.     

Measurement of the diffractive structure function in deep inelastic scattering at HERA

This paper presents an analysis of the inclusive properties of diffractive deep inelastic scattering events produced inep interactions at HERA. The events are characterised by a rapidity gap between the outgoing proton system and the remaining hadronic system. Inclusive distributions are presented and compared with Monte Carlo models for diffractive processes. The data are consistent with models where the pomeron structure function has a hard and a soft contribution. The diffractive structure function is measured as a function ofx , the momentum fraction lost by the proton, of , the momentum fraction of the struck quark with respect tox , and ofQ ² in the range 6.3·10^–4x <>^–2, 0.1<0.8 and=">Q ²<100>². The dependence is consistent with the formx wherea=1.30±0.08(stat) _–0.14 ^+0.08 (sys) in all bins of andQ ². In the measuredQ ² range, the diffractive structure function approximately scales withQ ² at fixed . In an Ingelman-Schlein type model, where commonly used pomeron flux factor normalisations are assumed, it is found that the quarks within the pomeron do not saturate the momentum sum rule.supported by Worldlab, Lausanne, Switzerland     

Measurement of azimuthal asymmetries in neutral current deep inelastic scattering at HERA

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<Q²<8000 GeV², 0.2<y<0.8 and 0.01<x<0.1 where Q² is the virtuality of the exchanged boson, y is the inelasticity and x is the Bjorken variable. The measurements were made in the hadronic centre-of-mass system. The analysis exploits the energy-flow method, which allows the measurement to be made over a larger range of pseudorapidity compared to previous results. The dependence of the moments of the azimuthal distributions on the pseudorapidity and minimum transverse energy of the final-state hadrons are presented. Although the predictions from next-to-leading-order QCD describe the data better than do the Monte Carlo models incorporating leading-logarithm parton showers, they still fail to describe the magnitude of the asymmetries. This suggests that higher-order calculations may be necessary to describe these data.     

Charmonium production in deep inelastic scattering at HERA

The electroproduction of and mesons is studied in elastic, quasi-elastic and inclusive reactions for four momentum transfers and photon-proton centre of mass energies GeV. The data were taken with the H1 detector at the electron proton collider HERA in the years 1995 to 1997. The total virtual photon-proton cross section for elastic production is measured as a function of and W. The dependence of the production rates on the square of the momentum transfer from the proton (t) is extracted. Decay angular distributions are analysed and the ratio of the longitudinal and transverse cross sections is derived. The ratio of the cross sections for quasi-elastic and meson production is measured as a function of . The results are discussed in terms of theoretical models based upon perturbative QCD. Differential cross sections for inclusive and inelastic production of mesons are determined and predictions within two theoretical frameworks are compared with the data, the non-relativistic QCD factorization approach including colour octet and colour singlet contributions, and the model of Soft Colour Interactions. Received: 10 March 1999 / Published online: 8 September 1999     

Electromagnetic corrections to deep inelastic scattering at HERA

The dominant electromagnetic radiative corrections to neutral current deep inelastic scattering at HERA energies have been calculated analytically in collinear approximation for unpolarized beams. The results may be used for the construction of an event generator and are applied to discuss the influence of cuts on the magnitude of the correction. For final state collinear photon emission the measurement of the resulting electromagnetic jet (electron plus collinear photon) reducesx-andy-dependence of the correction factor. Electromagnetic corrections to charged current interactions are discussed in leading logarithmic approximation.     

Multiplicity moments in deep inelastic scattering at HERA

Multiplicity moments of charged particles in deep inelastic e⁺p scattering have been measured with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb⁻¹. The moments for Q²>1000 GeV² were studied in the current region of the Breit frame. The evolution of the moments was investigated as a function of restricted regions in polar angle and, for the first time, both in the transverse momentum and in absolute momentum of final-state particles. Analytic perturbative QCD predictions in conjunction with the hypothesis of Local Parton–Hadron Duality (LPHD) reproduce the trends of the moments in polar-angle regions, although some discrepancies are observed. For the moments restricted either in transverse or absolute momentum, the analytic results combined with the LPHD hypothesis show considerable deviations from the measurements. The study indicates a large influence of the hadronisation stage on the multiplicity distributions in the restricted phase-space regions studied here, which is inconsistent with the expectations of the LPHD hypothesis.     

Dijet production in diffractive deep inelastic scattering at HERA

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 k_T 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.     

Forward jet production in deep inelastic scattering at HERA

The inclusive forward jet cross section in deep inelastic e⁺pe^+p scattering has been measured in the region of x-Bjorken, 4.5 ·10^-44.5 \cdot 10^{-4} to 4.5 ·10^-24.5 \cdot 10^{-2}. This measurement is motivated by the search for effects of BFKL-like parton shower evolution. The cross section at hadron level as a function of x is compared to cross sections predicted by various Monte Carlo models. An excess of forward jet production at small x is observed, which is not reproduced by models based on DGLAP parton shower evolution. The Colour Dipole model describes the data reasonably well. Predictions of perturbative QCD calculations at the parton level based on BFKL and DGLAP parton evolution are discussed in the context of this measurement.     

Electroweak radiative corrections to deep inelastic scattering at HERA

Analytic results are presented on electroweak radiative corrections for neutral-current deepinelastic electron proton scattering at HERA energies. Single photon bremsstrahlung is included without cuts.     

Forward-jet production in deep inelastic ep scattering at HERA

Forward jet cross sections have been measured in neutral current deep inelastic scattering at low Bjorken-x with the ZEUS detector at HERA using an integrated luminosity of 81.8 pb^-1. Measurements are presented for inclusive forward jets as well as for forward jets accompanied by a dijet system. The explored phase space, with jet pseudorapidity up to 4.3 is expected to be particularly sensitive to the dynamics of QCD parton evolution at low x. The measurements are compared to fixed-order QCD calculations and to leading-order parton-shower Monte Carlo models.     

Forward jet production in deep inelastic scattering at HERA

The production of forward jets has been measured in deep inelastic ep collisions at HERA. The results are presented in terms of single differential cross sections as a function of the Bjorken scaling variable (x_Bj) and as triple differential cross sections d³σ/dx_BjdQ² , where Q² is the four momentum transfer squared and is the squared transverse momentum of the forward jet. Also cross sections for events with a di-jet system in addition to the forward jet are measured as a function of the rapidity separation between the forward jet and the two additional jets. The measurements are compared with next-to-leading order QCD calculations and with the predictions of various QCD-based models.     

Measurement of dijet cross sections in photoproduction at HERA

Dijet cross sections as functions of several jet observables are measured in photoproduction using the H1 detector at HERA. The data sample comprises data with an integrated luminosity of 34.9 pb. Jets are selected using the inclusive algorithm with a minimum transverse energy of 25 GeV for the leading jet. The phase space covers longitudinal proton momentum fraction and photon longitudinal momentum fraction in the ranges and . The predictions of next-to-leading order perturbative QCD, including recent photon and proton parton densities, are found to be compatible with the data in a wide kinematical range. Received: 4 January 2002 / Published online: 12 July 2002