10 Search Results

We present a measurement of forward-backward asymmetry in top quark-antiquark production in proton-antiproton collisions in the final state containing a lepton and at least four jets. Using a dataset corresponding to an integrated luminosity of 5.4 fb^-1, collected by the D0 experiment at the Fermilab Tevatron Collider, we measure the t{bar t} forward-backward asymmetry to be (9.2 ± 3.7)% at the reconstruction level. When corrected for detector acceptance and resolution, the asymmetry is found to be (19.6 ± 6.5)%. We also measure a corrected asymmetry based on the lepton from a top quark decay, found to be (15.2 ± 4.0)%.more » The results are compared to predictions based on the next-to-leading-order QCD generator mc@nlo. The sensitivity of the measured and predicted asymmetries to the modeling of gluon radiation is discussed.« less

We present a measurement of the top-quark mass with tt events using a data sample corresponding to an integrated luminosity of 5.7 fb ^-1 of pp collisions at the Fermilab Tevatron with √s = 1.96 TeV and collected by the CDF II Detector. We select events having no identified charged leptons, large missing transverse energy, and four, five, or six jets with at least one jet containing a secondary vertex consistent with the decay of a b quark. This analysis considers events from the semileptonic tt decay channel, including events that contain tau leptons, which are usually not included inmore » the top-quark mass measurements. The measurement uses as kinematic variables the invariant mass of two jets consistent with the mass of the W boson, and the invariant masses of two different three-jet combinations. We fit the data to signal templates of varying top-quark masses and background templates, and measure a top-quark mass of M_top = 172.3 ± 2.4 (stat) ± 1.0 (syst) GeV/c².« less

We present a search for new physics in events with two high p_T leptons of the same electric charge, using data with an integrated luminosity of 6.1 fb_-1. The observed data are consistent with standard model predictions. We set 95% C.L. lower limits on the mass of doubly-charged scalars decaying to like-sign dileptons, m_H^±± > 190 - 245 GeV/c², depending on the decay mode and coupling.

Using data corresponding to 6.0 fb^-1 of pp collisions at √s = 1.96 TeV collected by the CDF II detector, we present a cross section measurement of top-quark pair production with an additional radiated photon, ttγ. The events are selected by looking for a lepton (ell), a photon (γ), significant transverse momentum imbalance (E_T), large total transverse energy, and three or more jets, with at least one identified as containing a b quark (b). The ttγ sample requires the photon to have 10 GeV or more of transverse energy, and to be in the central region. Using an event selectionmore » optimized for the ttγ candidate sample we measure the production cross section of tt (σ_tt), and the ratio of cross sections of the two samples. Control samples in the dilepton+photon and lepton+photon+E_T, channels are constructed to aid in decay product identification and background measurements. We observe 30 ttγ candidate events compared to the standard model expectation of 26.9 ± 3.4 events. We measure the ttγ cross section (σ_tt) to be 0.18 ± 0.08 pb, and the ratio of σ_ttγ to σ_tt to be 0.024 ± 0.009. Assuming no tty production, we observe a probability of 0.0015 of the background events alone producing 30 events or more, corresponding to 3.0 standard deviations.« less

In a tokamak, the electron distribution deviates from a Maxwellian because magnetically untrapped electrons run away parallel to the applied electric field. A new method for calculating the dynamics of the electron distribution is presented. This method is novel because it can treat systems with comparable numbers of trapped and untrapped electrons, and an electric field comparable with (but smaller than) the Dreicer field. The electron distribution function and the plasma resistivity are presented for representative values.

Herein the modification of the weak-interaction theory recently proposed by Okubo is compared with the current-current weak-interaction theory generalized to include local scalar or tensor interactions for $$K_{\ell2}$$ and $$K_{\ell3}$$ decays. In contrast to the latter, the existence of Okubo's tensor or scalar densities is difficult to verify from $$K_{\ell2}$$ and $$K_{\ell3}$$ experiments unless one has a reliable theoretical model for the pure V-A vertices. In particular, we discuss various possible explanations for the $$K_{\ell3}$$ parameters.

The K_l3 scalar form factor is studied by the hard-pion method in the framework of a (3,3*)+(3*,3)-symmetry-breaking model using a modified pole-dominance approximation. A set of consistency relations is found which provides a test of the reliability of the quadratic-smoothness assumption as well as of the symmetry-breaking model. In particular, we find that the solution for the symmetry-breaking parameters which fit the data is inconsistent with the quadratic-smoothness assumption. In addition, the status of other theoretical models is briefly reviewed.

Here we examine the consequences of the hypothesis that, for some range of energies, the second-order terms in the perturbation expansion of the Fermi theory are small but non-negligible corrections to the first-order terms, and responsible for reactions which violate first-order selection rules. We compute matrix elements through second order in the weak coupling constant G and introduce subtraction constants which are necessary to render the matrix elements finite to this order. From this point of view we consider a number of weak-interaction problems, including the experimental parameters of μ decay, the universality of the μ-decay coupling constant and themore » Fermi β-decay coupling constant, K_l3 decays, and Δ⁢S = ΔQ and Δ⁢S = 2 semileptonic decays.« less

A unified theory of the weak and electromagnetic interactions of leptons and hadrons is constructed. The underlying symmetry group is taken to be the SU⁡(2) generated by the weak lepton currents and the hadronic Cabibbo currents. This symmetry is destroyed by the spontaneous breakdown mechanism. In our theory, the weak coupling constant is the same as the electromagnetic coupling constant, and the mass of the charged intermediate boson is 37.4 GeV.

We investigate chiral-symmetry breaking within the Kπ system when a Veneziano model is assumed for various off-mass-shell extrapolations. Here, we derive expressions for the matrix elements of σ terms and the divergence of the strangeness-changing current and discuss the implications to chiral-symmetry-breaking interactions. Application of our results to the $$\mathrm{K}$$_{$$\mathcal{l}$$3} form factors gives $$(\frac{fk}{fπf+(0)})$$ =1.35 and the relation $$(\frac{fk}{fπ})$$² = cos⁡[α_K* ^$$\prime$$ (0) (μ_K² - μ_π²)].