Search for QCD-instanton induced events in deep inelastic ep scattering at HERA

A search for QCD-instanton-induced events in deep inelastic ep scattering has been performed with the ZEUS detector at the HERA collider, using data corresponding to an integrated luminosity of 38 pb(-1). A kinematic range defined by cuts on the photon virtuality, Q(2) gt 120 GeV2, and on the Bjorken scaling variable, x gt 10(-3), has been investigated. The QCD- instanton induced events were modelled by the Monte Carlo generator QCDINS. A background-independent, conservative 95% confidence level upper limit for the instanton cross section of 26 pb is obtained, to be compared with the theoretically expected value of 8.9 pb

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 38.3 pb(-1). The moments for Q(2) gt 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

Search for contact interactions, large extra dimensions and finite quark radius in ep collisions at HERA

A search for physics beyond the Standard Model has been performed with high-Q(2) neutral current deep inelastic scattering events recorded with the ZEUS detector at HERA. Two data sets, e(+) p → e(+) X and e(-) p → e(-) X, with respective integrated luminosities of 112 pb(-1) and 16 pb(-1), were analyzed. The data reach Q(2) values as high as 40000 GeV2. No significant deviations from Standard Model predictions were observed. Limits were derived on the effective mass scale in eeqq contact interactions. the ratio of leptoquark mass to the Yukawa coupling for heavy leptoquark models and the mass scale parameter in models with large extra dimensions. The limit on the quark charge radius, in the classical form factor approximation, is 0.85 x 10(-16) cm

Measurement of beauty production in deep inelastic scattering at HERA

The beauty production cross section for deep inelastic scattering events with at least one hard jet in the Breit frame together with a union has been measured, for photon virtualities Q(2) gt 2 GeV2, with the ZEUS detector at HERA using integrated luminosity of 72 pb(-1) The total visible cross section is sigma(bb)- (ep → e jet muX) = 40.9 +/- 5.7 (stat.) (+6.0)(-4.4) (syst) pb. The next-to- leading order QCD prediction lies about 2.5 standard deviations below the data. The differential cross sections are in general consistent with the NLO QCD predictions; however at low values of Q(2), Bjorken x, and muon transverse momentum, and high values of jet transverse energy and muon pseudorapidity, the prediction is about two standard deviations below the data. (C) 2004 Published by Elsevier B.

Scaled momentum spectra in deep inelastic scattering at HERA

Charged particle production has been studied in neutral current deep inelastic ep scattering with the ZEUS detector at HERA using an integrated luminosity of 0.44 fb(-1). Distributions of scaled momenta in the Breit frame are presented for particles in the current fragmentation region. The evolution of these spectra with the photon virtuality, Q(2), is described in the kinematic region 10 < Q(2) < 41000 Ge V-2. Next-to-leading-order and modified leading-log-approximation QCD calculations as well as predictions from Monte Carlo models are compared to the data. The results are also compared to e(+)e(-) annihilation data. The dependences of the pseudorapidity distribution of the particles on Q(2) and on the energy in the gamma p system, W, are presented and interpreted in the context of the hypothesis of limiting fragmentation

Inclusive dijet cross sections in neutral current deep inelastic scattering at HERA

Single- and double-differential inclusive dijet cross sections in neutral current deep inelastic ep scattering have been measured with the ZEUS detector using an integrated luminosity of 374 pb(-1). The measurement was performed at large values of the photon virtuality, Q (2), between 125 and 20 000 GeV2. The jets were reconstructed with the k (T) cluster algorithm in the Breit reference frame and selected by requiring their transverse energies in the Breit frame, E-jet (T,B), to be larger than 8 GeV. In addition, the invariant mass of the dijet system, M-jj,M- was required to be greater than 20 GeV. The cross sections are described by the predictions of next-to-leading-order QCD

A QCD analysis of ZEUS diffractive data

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. (C) 2010 Elsevier B.V. All rights reserved

Photoproduction of Events with Rapidity Gaps between Jets at HERA.

The photoproduction of dijet events, where the two jets with the highest transverse energy are separated by a large gap in pseudorapidity, have been studied with the ZEUS detector using an integrated luminosity of 39 pb -1. Rapidity-gap events are defined in terms of the energy flow between the jets, such that the total summed transverse energy in this region is less than some value ETCUT. The data show a clear excess over the predictions of standard photoproduction models. This is interpreted as evidence for a strongly interacting exchange of a color-singlet object. Monte Carlo models which include such a color-singlet exchange are able to describe the data. © Springer-Verlag Berlin Heidelberg 2007

Measurement of Neutral Current Cross Sections at High Bjorken-x with the ZEUS Detector at HERA.

A new method is employed to measure the neutral current cross section up to Bjorken-x values of one with the ZEUS detector at HERA using an integrated luminosity of 65.1 pb-1 for e+p collisions and 16.7 pb-1 for e-p collisions at sqrt{s}=318 GeV and 38.6 pb-1 for e+p collisions at sqrt{s}=300 GeV. Cross sections have been extracted for Q2 >= 648 GeV2 and are compared to predictions using different parton density functions. For the highest x bins, the data have a tendency to lie above the expectations using recent parton density function parametrizations.Comment: 13 pages, 14 figures, 6 tables, DESY-06-116, submitted to Eur. Phys. Journal

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 at HERA. 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.Comment: 10 pages, 4 figures, 4 table