Accelerator Neutrinos

Abstract:

We report the results of a search for the anomalous production of a massive particle decaying to four electrons via two Z0 bosons in 1.1fb-1 of pp̄ collisions at s=1.96TeV collected by the CDF II detector at Fermilab. We employ optimized electron identification criteria to maximize acceptance and efficiency. We estimate the backgrounds in the invariant mass range 500-1000GeV/c2 to be 0.028±0.009(stat)±0.011(syst) events. We observe zero events in this search region. Assuming a Randall-Sundrum graviton production model, we set 95% C.L. limits on σ×BF(G→Z0Z0)<4- 6pb, depending on the graviton mass. © 2008 The American Physical Society.

Abstract:

A first measurement is reported of the longitudinal proton structure function F L (x, Q 2) at the ep collider HERA. It is based on inclusive deep inelastic e + p scattering cross section measurements with a positron beam energy of 27.5   GeV and proton beam energies of 920, 575 and 460   GeV. Employing the energy dependence of the cross section, F L is measured in a range of squared four-momentum transfers 12 ≤ Q 2 ≤ 90   GeV 2 and low Bjorken x 0.00024 ≤ x ≤ 0.0036. The F L values agree with higher order QCD calculations based on parton densities obtained using cross section data previously measured at HERA. © 2008 Elsevier B.V. All rights reserved.

Abstract:

We present first evidence for the production of single top quarks in the D0 detector at the Fermilab Tevatron pp̄ collider. The standard model predicts that the electroweak interaction can produce a top quark together with an antibottom quark or light quark, without the antiparticle top-quark partner that is always produced from strong-coupling processes. Top quarks were first observed in pair production in 1995, and since then, single top-quark production has been searched for in ever larger data sets. In this analysis, we select events from a 0.9fb-1 data set that have an electron or muon and missing transverse energy from the decay of a W boson from the top-quark decay, and two, three, or four jets, with one or two of the jets identified as originating from a b hadron decay. The selected events are mostly backgrounds such as W+jets and tt̄ events, which we separate from the expected signals using three multivariate analysis techniques: boosted decision trees, Bayesian neural networks, and matrix-element calculations. A binned likelihood fit of the signal cross section plus background to the data from the combination of the results from the three analysis methods gives a cross section for single top-quark production of σ(pp̄→tb+X,tqb+X)=4.7±1.3pb. The probability to measure a cross section at this value or higher in the absence of signal is 0.014%, corresponding to a 3.6 standard deviation significance. The measured cross section value is compatible at the 10% level with the standard model prediction for electroweak top-quark production. We use the cross section measurement to directly determine the Cabibbo-Kobayashi-Maskawa quark mixing matrix element that describes the Wtb coupling and find |Vtbf1L|=1.31-0.21+0.25, where f1L is a generic vector coupling. This model-independent measurement translates into 0.68<|Vtb|≤1 at the 95% C.L. in the standard model. © 2008 The American Physical Society.