48 research outputs found

    Direct evidence for W exchange in charmed meson decay

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    Using the ARGUS detector at DORIS II, we have observed a signal of 36.7±8.0 events in the decay channel D 0 →K s 0 φ . In the same data sample, we have observed the well established decay D 0 →K s 0 π + π − , and find the ratio, Br(D) 0 → K s 0 φ) Br(D) 0 → K s 0 π + π − ) , to be 0.186±0.052. The substantial value of (0.99±0.32±0.17)% then derived for the branching ratio for D 0 → K 0 φ gives direct evidence that W exchange contributes D 0 decay

    BaBar Technical Design Report

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    Letter of intent for the study of CP violation and heavy flavor physics at PEP-II

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    Dijet cross-sections in photoproduction at HERA

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    Dijet production by almost real photons has been studied at HERA with the ZEUS detector. Jets have been identified using the cone algorithm, A cut on x(gamma)(OBS), the fraction of the photon energy participating in the production of the two jets of highest transverse energy, is used to define cross sections sensitive to the parton distributions in the proton and in the photon, The oss dependence of the dijet cross sections on pseudorapidity has been measured for x(gamma)(OBS)greater than or equal to 0.75 and x(gamma)(OBS)<0.75. The former is sensitive to the gluon momentum density in the proton. The latter is sensitive to the gluon in the photon. The cross sections are corrected for detector acceptance and compared to leading order QCD calculations

    Measurement of the neutral current cross-section and F(2) structure function for deep inelastic e + p scattering at HERA

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    The cross section and the proton structure function for neutral current deep inelastic scattering have been measured with the ZEUS detector at HERA using an integrated luminosity of 30 pb. The data were collected in 1996 and 1997 at a centre-of-mass energy of 300 GeV. They cover the kinematic range GeV and . The variation of with x and is well described by next-to-leading-order perturbative QCD as implemented in the DGLAP evolution equations

    Comparison of ZEUS data with standard model predictions for e+ p ---> e+ X scattering at high x and Q**2

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    Using the ZEUS detector at HERA, we have studied the reaction e+p → e+X for Q2 > 5000 GeV2 with a 20.1 pb-1 data sample collected during the years 1994 to 1996. For Q2 below 15000 GeV2, the data are in good agreement with Standard Model expectations. For Q2 > 35000 GeV2, two events are observed while 0.145 ± 0.013 events are expected. A statistical analysis of a large ensemble of simulated Standard Model experiments indicates that with probability 6.0%, an excess at least as unlikely as that observed would occur above some Q2 cut. For x > 0.55 and y > 0.25, four events are observed where 0.91 ± 0.08 events are expected. A statistical analysis of the two-dimensional distribution of the events in x and y yields a probability of 0.72% for the region x > 0.55 and y > 0.25 and a probability of 7.8% for the entire Q2 > 5000 GeV2 data sample. The observed excess above Standard Model expectations is particularly interesting because it occurs in a previously unexplored kinematic region. © Springer-Verlag 1997

    Comparison of energy flows in deep inelastic scattering events with and without a large rapidity gap

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    Energy flows in deep inelastic electron-proton scattering are investigated at a centre-of-mass energy of 269 GeV for the range Q2 ≥ 10 GeV2 using the ZEUS detector. A comparison is made between events with and without a large rapidity gap between the hadronic system and the proton direction. The energy flows, corrected for detector acceptance and resolution, are shown for these two classes of events in both the HERA laboratory frame and the Breit frame. From the differences in the shapes of these energy flows we conclude that QCD radiation is suppressed in the large-rapidity-gap eents compared to the events without a large rapidity gap

    Measurement of alpha-s from jet rates in deep inelastic scattering at HERA

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    Jet production in deep inelastic scattering for 120 < Q2 < 3600 GeV2 has been studied using data from an integrated luminosity of 3.2 pb-1 collected with the ZEUS detector at HERA. Jets are identified with the JADE algorithm. A cut on the angular distribution of parton emission in the γ*-parton centre-of-mass system minimises the experimental and theoretical uncertainties in the determination of the jet rates. The jet rates, when compared to (αs2) perturbative QCD calculations, allow a precise determination of αs(Q) in three Q2-intervals. The values are consistent with a running of αs(Q), as expected from QCD. Extrapolating to Q = MZ0 yields αs(MZ0) = 0.117 ± 0.005 (stat) -0.005+0.004(systexp) ± 0.007 (systtheory)

    Measurement of elastic J / psi photoproduction at HERA

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    The reaction a p --> J/psi p has been studied in epsilon p interactions using the ZEUS detector at HERA. The cross section for elastic J/psi photoproduction has been measured as a function of the photon-proton centre of mass energy W in the range 40 J/psi p), is observed to rise steeply with W. A fit to the data presented in this paper to determine the parameter delta in the form sigma(ap-->J/psi p) proportional to W-delta yields the value delta=0.92+/-0.14+/-0.10. The differential cross section d sigma/d\t\ is presented over the range \t\ < 1.0 GEV(2) where t is the square of the four-momentum exchanged at the proton vertex. d sigma/d\t\ falls exponentially with a slope parameter of 4.6+/-0.4(-0.6)(+0.4) GEV(-2). The measured decay angular distrubitions are consistent with 8-channel helicity conservation
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