Dijet production in neutral current deep inelastic scattering at HERA

Dijet cross sections in neutral current deep inelastic ep scattering have been measured in the range with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb. The cross sections, measured in the Breit frame using the jet algorithm, are compared with next-to-leading-order perturbative QCD calculations using proton parton distribution functions. The uncertainties of the QCD calculations have been studied. The predictions are in reasonable agreement with the measured cross sections over the entire kinematic range. Received: 11 September 2001 / Published online: 20 December 2001     

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     

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.     

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 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     

Neutral strange particle production in deep inelastic scattering at HERA

This paper presents measurements ofK ⁰ and production in neutral current, deep inelastic scattering of 26.7 GeV electrons and 820 GeV protons in the kinematic range 10Q ²<640>², 0.0003x<0.01,>y>0.04. Average multiplicities forK ⁰ and production are determined for transverse momentap _T>0.5 GeV and pseudorapidities ||<1.3. the=" multiplicities=" favour=" a=" stronger=" strange=" to=" light=" quark=" suppression=" in=" the=" fragmentation=" chain=" than=" found=">e ⁺ e ^– experiments. The production properties ofK ^0' s in events with and without a large rapidity gap with respect to the proton direction are compared. The ratio of neutralK ^0' s to charged particles per event in the measured kinematic range is, within the present statistics, the same in both samples.Supported by Worldlab, Lausanne, Switzerland     

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.     

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.     

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.     

Angular and current-target correlations in deep inelastic scattering at HERA

Correlations between charged particles in deep inelastic scattering have been studied in the Breit frame with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb. Short-range correlations are analysed in terms of the angular separation between current-region particles within a cone centred around the virtual photon axis. Long-range correlations between the current and target regions have also been measured. The data support predictions for the scaling behaviour of the angular correlations at high and for anti-correlations between the current and target regions over a large range in and in the Bjorken scaling variable x. Analytic QCD calculations and Monte Carlo models correctly describe the trends of the data at high , but show quantitative discrepancies. The data show differences between the correlations in deep inelastic scattering and annihilation. Received: 18 May 1999 / Published online: 3 November 1999     

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     

Saturation in diffractive deep inelastic eA scattering

In this paper, we investigate the saturation physics in diffractive deep inelastic electron-ion scattering. We estimate the energy and nuclear dependence of the ratio σ^diff/σ^tot and predict the x_ℙ and β behavior of the nuclear diffractive structure function F_2,A^D(3)(Q²,β,x_ℙ). Moreover, we analyze the ratio R^diff_A1,A2(Q²,β,x_ℙ)=F_2,A1^D(3)/F_2,A2^D(3), which probes the nuclear dependence of the structure of the pomeron. We show that saturation physics predicts that approximately 37% of the events observed at eRHIC should be diffractive.     

Photon polarisation in diffractive deep inelastic scattering

The distribution of a suitably defined azimuthal angle in diffractive deep inelastic scattering contains information on the polarisation of the exchanged photon. In particular it allows one to constrain the longitudinal diffractive structure function. We investigate the potential of such bounds in general and for particular diffractive final states. Received: 17 September 1997 / Published online: 23 February 1998     

Deep inelastic electron-pomeron scattering at HERA

The idea that the pomeron has partonic structure similar to any other hadron has been given strong support by recent measurements of the diffractive structure function at HERA. We present a detailed theoretical analysis of the diffractive structure function under the assumption that the diffractive cross section can be factorized into a pomeron emission factor and the deep inelastic scattering cross section of the pomeron. We pay particular attention to the kinematic correlations implied by this picture, and suggest the measurement of an angular correlation which should provide a first test of the whole picture. We also present two simple phenomenological models for the quark and gluon structure of the pomeron, which are consistent with various theoretical ideas and which give equally good fits to recent measurements by the H1 collaboration, when combined with the pomeron emission factor of Donnachie and Landshoff. We predict that a large fraction of diffractive deep inelastic events will contain charm, and discuss how improved data will be able to distinguish the models.     

Electroweak radiative corrections to deep inelastic scattering at HERA. Charged current scattering

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