115 research outputs found

    Search for a narrow charmed baryonic state decaying to D*(+/-)p(-/+) in ep collisions at HERA

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    A resonance search has been made in the D*±p∓ invariant-mass spectrum with the ZEUS detector at HERA using an integrated luminosity of 126 pb-1. The decay channels D*+ -→ D0πs+ → (K -π+)πs+ and D*+ → D0πs+ → (K-π+π+π-) πs+ (and the corresponding antiparticle decays) were used to identify D*± mesons. No resonance structure was observed in the D*± p∓ mass spectrum from more than 60 000 reconstructed D*± mesons. The results are not compatible with a report of the H1 Collaboration of a charmed pentaquark, Θc0. © Springer-Verlag / Società Italiana di Fisica 2004

    Multijet cross sections in charged current e(+/-)p scattering at HERA

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    Jet cross sections were measured in charged-current deep inelastic e±p scattering at high boson virtualities Q2 with the ZEUS detector at HERA II using an integrated luminosity of 0.36fb-1. Differential cross sections are presented for inclusive-jet production as functions of Q2, Bjorken x and the jet transverse energy and pseudorapidity. The dijet invariant mass cross section is also presented. Observation of three- and four-jet events in charged-current e±p processes is reported for the first time. The predictions of next-to-leading-order (NLO) QCD calculations are compared to the measurements. The measured inclusive-jet cross sections are well described in shape and normalization by the NLO predictions. The data have the potential to constrain the u and d valence-quark distributions in the proton if included as input to global fits. © 2008 The American Physical Society

    Evidence for a narrow baryonic state decaying to K0s-p and K0s-pbar in deep inelastic scattering at HERA

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    A resonance search has been made in the K(S)(0)p and K-S(0)(p) over bar invariant-mass spectrum measured with the ZEUS detector at HERA using an integrated luminosity of 121 pb(-1). The search was performed in the central rapidity region of inclusive deep inelastic scattering at an ep centre-of-mass energy of 300-318 GeV for exchanged photon virtuality, Q(2), above 1 GeV2. Recent results from fixed-target experiments give evidence for a narrow baryon resonance decaying to K(+)n and K(S)(0)p, interpreted as a pentaquark. The results presented here support the existence of such state, with a mass of 1521.5 +/- 1.5(stat.)(-1.7)(+2.8)(syst.) MeV and a Gaussian width consistent with the experimental resolution of 2 MeV The signal is visible at high Q(2) and, for Q(2) gt 20 GeV2, contains 221 +/- 48 events. The probability of a similar signal anywhere in the range 1500-1560 MeV arising from fluctuations of the background is below 6 x 10(-5)

    High-Q2 neutral current cross sections in e+ p deep inelastic scattering at √s = 318 GeV

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    Cross sections for e(+)p neutral current deep inelastic scattering have been measured at a center-of-mass energy of roots=318 GeV with the ZEUS detector at HERA using an integrated luminosity of 63.2 pb(-1). The double-differential cross section, d(2)sigma/dxdQ(2), is presented for 200 GeV2 lt Q(2) lt 30 000 GeV2 and for 0.005 lt x lt 0.65. The single- differential cross sections dsigma/dQ(2), dsigma/dx and dsigma/dy are presented for Q(2) gt 200 GeV2. The effect of Z- boson-exchange is seen in dsigma/dx measured for Q(2) gt 10 000 GeV2. The data presented here were combined with ZEUS e(+)p neutral current data taken at roots=300 GeV and the structure function F-2(em) was extracted. All results agree well with the predictions of the Standard Model

    Combined measurement and QCD analysis of the inclusive e(+/-)p scattering cross sections at HERA

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    A combination is presented of the inclusive deep inelastic cross sections measured by the H1 and ZEUS Collaborations in neutral and charged current unpolarised e ± p scattering at HERA during the period 1994-2000. The data span six orders of magnitude in negative four-momentum-transfer squared, Q 2, and in Bjorken x. The combination method used takes the correlations of systematic uncertainties into account, resulting in an improved accuracy. The combined data are the sole input in a NLO QCD analysis which determines a new set of parton distributions, HERAPDF1.0, with small experimental uncertainties. This set includes an estimate of the model and parametrisation uncertainties of the fit result

    Measurement of high-Q (2) charged current deep inelastic scattering cross sections with a longitudinally polarised positron beam at HERA

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    Measurements of the cross sections for charged current deep inelastic scattering in e+p collisions with a longitudinally polarised positron beam are presented. The measurements are based on a data sample with an integrated luminosity of 132 pb-1 collected with the ZEUS detector at HERA at a centre-of-mass energy of 318 GeV. The total cross section is presented at positive and negative values of the longitudinal polarisation of the positron beams. The single-differential cross-sections dσ/dQ2, dσ/dx and dσ/dy are presented for Q2>200 GeV2. The reduced cross-section σ̃ is presented in the kinematic range 200<Q2<60000 GeV2 and 0.006<x<0.562. The measurements agree well with the predictions of the Standard Model. The results are used to determine a lower limit on the mass of a hypothetical right-handed W boson. © 2010 The Author(s)

    Measurement of D-*+/- meson production in e(+/-)p scattering at low Q(2)

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    The production of D* ± (2010) mesons in e± p scattering in the range of exchanged photon virtuality 0.05 < Q2 < 0.7   GeV2 has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb-1. The decay channels D* + → D0 π+ with D0 → K- π+ and corresponding antiparticle decay were used to identify D* mesons and the ZEUS beampipe calorimeter was used to identify the scattered electron. Differential D* cross sections as functions of Q2, inelasticity, y, transverse momentum of the D* meson, pT (D*), and pseudorapidity of the D* meson, η (D*), have been measured in the kinematic region 0.02 < y < 0.85, 1.5 < pT (D*) < 9.0   GeV and | η (D*) | < 1.5. 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. © 2007 Elsevier B.V. All rights reserved

    Measurement of neutral current cross sections at high Bjorken-x with the ZEUS detector at HERA

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    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 √s = 318 GeV and 38.6 pb-1 for e+p collisions at √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. © Springer-Verlag Berlin Heidelberg 2007

    Leading neutron energy and p(T) distributions in deep inelastic scattering and photoproduction at HERA

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    The production of energetic neutrons in ep collisions has been studied with the ZEUS detector at HERA. The neutron energy and pT2 distributions were measured with a forward neutron calorimeter and tracker in a 40   pb-1 sample of inclusive deep inelastic scattering (DIS) data and a 6   pb-1 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 pT2 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. © 2007 Elsevier B.V. All rights reserved
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