35 research outputs found

    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)

    Exclusive electroproduction of two pions at HERA

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    The exclusive electroproduction of two pions in the mass range 0.4<M ππ<2.5 GeV has been studied with the ZEUS detector at HERA using an integrated luminosity of 82 pb<sup>-1</sup>. The analysis was carried out in the kinematic range of 2<Q <sup>2</sup><80 GeV<sup>2</sup>, 32<W<180 GeV and |t|<0.6 GeV<sup>2</sup>, where Q<sup>2</sup> is the photon virtuality, W is the photon-proton centre-of-mass energy and <i>t</i> is the squared four-momentum transfer at the proton vertex. The two-pion invariant-mass distribution is interpreted in terms of the pion electromagnetic form factor, |F(M ππ )|, assuming that the studied mass range includes the contributions of the ρ, ρ′ and ρ′′ vector-meson states. The masses and widths of the resonances were obtained and the Q<sup>2</sup> dependence of the cross-section ratios δ(ρ′→ππ)/δ(ρ) and δ(ρ′′→)/δ(ρ) was extracted. The pion form factor obtained in the present analysis is compared to that obtained in e<sup>+</sup> e<sup>−</sup>→π<sup>+</sup> π<sup>−</sup>

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

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    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<sup>−1</sup>. The measurement was performed at large values of the photon virtuality, Q 2, between 125 and 20 000 GeV<sup>2</sup>. The jets were reconstructed with the k<sub>T</sub> cluster algorithm in the Breit reference frame and selected by requiring their transverse energies in the Breit frame, E<sup>jet</sup><sub>TB</sub>, to be larger than 8 GeV. In addition, the invariant mass of the dijet system, M<sub>jj</sub>, was required to be greater than 20 GeV. The cross sections are described by the predictions of next-to-leading-order QCD

    Measurement of dijet photoproduction for events with a leading neutron at HERA

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    Differential cross sections for dijet photoproduction and this process in association with a leading neutron, e ++p→e ++jet+jet+X(+n), 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, x L, and of its transverse momentum squared, pT2. 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 x L 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. © 2009 Elsevier B.V

    Measurement of isolated photon production in deep inelastic ep scattering ZEUS Collaboration

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    Isolated photon production in deep inelastic ep scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 320 pb - 1. Measurements were made in the isolated-photon transverse-energy and pseudorapidity ranges 4 5 GeV. Differential cross sections are presented for inclusive isolated photon production as functions of Q 2, x, E Tγ and η γ. Leading-logarithm parton-shower Monte Carlo simulations and perturbative QCD predictions give a reasonable description of the data over most of the kinematic range. © 2010 Elsevier B.V

    Angular correlations in three-jet events in ep collisions at HERA

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    Three-jet production in deep inelastic ep scattering and photoproduction was investigated with the ZEUS detector at HERA using an integrated luminosity of up to 127  pb<sup>-1</sup>. Measurements of differential cross sections are presented as functions of angular correlations between the three jets in the final state and the proton-beam direction. These correlations provide a stringent test of perturbative QCD and show sensitivity to the contributions from different color configurations. Fixed-order perturbative calculations assuming the values of the color factors C<sub>F</sub>, C<sub>A</sub>, and T<sub>F</sub> as derived from a variety of gauge groups were compared to the measurements to study the underlying gauge group symmetry. The measured angular correlations in the deep inelastic ep scattering and photoproduction regimes are consistent with the admixture of color configurations as predicted by SU(3) and disfavour other symmetry groups, such as SU(N) in the limit of large N

    Scaled momentum spectra in deep inelastic scattering at HERA

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    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, Q2, is described in the kinematic region 10 < Q2< 41000Ge V2. 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 Q2 and on the energy in the p system, W, are presented and interpreted in the context of the hypothesis of limiting fragmentation

    Measurement of the energy dependence of the total photon-proton cross section at HERA

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    The energy dependence of the photon-proton total cross section, σtotγp, was determined from e+p scattering data collected with the ZEUS detector at HERA at three values of the center-of-mass energy, W, of the γp system in the range 19

    Search for single-top production in <i>ep</i> collisions at HERA

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    A search for single-top production,&lt;i&gt;ep&lt;/i&gt;&#8594; &lt;i&gt;et X&lt;/i&gt;, has been performed with the ZEUS detector at HERA using data corresponding to an integrated luminosity of 0.37 fb&lt;sup&gt;−1&lt;/sup&gt;. No evidence for top production was found, consistent with the expectation from the Standard Model. Limits were computed for single-top production via flavour changing neutral current transitions involving a neutral electroweak vector boson, &#947; or Z. The result was combined with a previous ZEUS result yielding a total luminosity of 0.50 fb&lt;sup&gt;−1&lt;/sup&gt;. A 95% credibility level upper limit of 0.13 pb was obtained for the cross section at the centre-of-mass energy of &#8730;s = 315 GeV
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