957 research outputs found

    Determining VtbV_{tb} at Electron-Positron Colliders

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    Verifying Vtb≃1V_{tb} \simeq 1 is critical to test the three generation assumption of the Standard Model. So far our best knowledge of VtbV_{tb} is inferred either from the 3Γ—33\times 3 unitarity of CKM matrix or from single top-quark productions upon the assumption of universal weak couplings. The unitarity could be relaxed in new physics models with extra heavy quarks and the universality of weak couplings could also be broken if the WtbWtb coupling is modified in new physics models. In this work we propose to measure VtbV_{tb} in the process of e+eβˆ’β†’ttΛ‰e^+ e^- \to t\bar{t} without prior knowledge of the number of fermion generations or the strength of the WtbWtb coupling. Using an effective Lagrangian approach, we perform a model-independent analysis of the interactions among electroweak gauge bosons and the third generation quarks, i.e. the WtbWtb, ZttΛ‰Zt\bar{t} and ZbbΛ‰Zb\bar{b} couplings. The electroweak symmetry of the Standard Model specifies a pattern of deviations of the ZZ-tLt_L-tLt_L and WW-tLt_L-bLb_L couplings after one imposes the known experimental constraint on the ZZ-bLb_L-bLb_L coupling. We demonstrate that, making use of the predicted pattern and the accurate measurements of top-quark mass and width from the energy threshold scan experiments, one can determine VtbV_{tb} from the cross section and the forward-backward asymmetry of top-quark pair production at an {\it unpolarized} electron-positron collider.Comment: publish versio

    Combined Effect of QCD Resummation and QED Radiative Correction to W boson Observables at the Tevatron

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    A precise determination of the W boson mass at the Fermilab Tevatron requires a theoretical calculation in which the effects of the initial-state multiple soft-gluon emission and the final-state photonic correction are simultaneously included . Here, we present such a calculation and discuss its prediction on the transverse mass distribution of the W boson and the transverse momentum distribution of its decay charged lepton, which are the most relevant observables for measuring the W boson mass at hadron colliders.Comment: 10 pages, 3 Postscript figures, uses revtex4.st

    Signatures of Extra Gauge Bosons in the Littlest Higgs Model with T-parity at Future Colliders

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    We study the collider signatures of a T-odd gauge boson WHW_{H} pair production in the Littlest Higgs Model with T-parity (LHT) at Large Hadron Collider (LHC) and Linear Collider (LC). At the LHC, we search for the WHW_{H} boson using its leptonic decay, i.e. ppβ†’WH+WHβˆ’β†’AHAHβ„“+Ξ½β„“β„“β€²βˆ’Ξ½Λ‰β„“β€²pp\to W_{H}^{+}W_{H}^{-}\to A_{H}A_{H}\ell^{+}\nu_{\ell}\ell^{\prime-}\bar{\nu}_{\ell^{\prime}}, which gives rise to a collider signature of \ell^{+}\ell^{\prime-}+\met. We demonstrate that the LHC not only has a great potential of discovering the WHW_{H} boson in this channel, but also can probe enormous parameter space of the LHT. Due to four missing particles in the final state, one cannot reconstruct the mass of WHW_{H} at the LHC. But such a mass measurement can be easily achieved at the LC in the process of e+eβˆ’β†’WH+WHβˆ’β†’AHAHW+Wβˆ’β†’AHAHjjjje^{+}e^{-}\to W_{H}^{+}W_{H}^{-}\to A_{H}A_{H}W^{+}W^{-}\to A_{H}A_{H}jjjj. We present an algorithm of measuring the mass and spin of the WHW_{H} boson at the LC. Furthermore, we illustrate that the spin correlation between the WW boson and its mother particle (WHW_{H}) can be used to distinguish the LHT from other new physics models.Comment: version to appear in PRD (a few references added

    Observing the Dark Scalar Doublet and its Impact on the Standard-Model Higgs Boson at Colliders

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    If the Standard Model of particle interactions is extended to include a second scalar doublet [H+,(H0+iA0)/2][H^{+},(H^{0}+iA^{0})/\sqrt{2}], which is odd under an unbroken Z_{2} discrete symmetry, it may be called the darkdark scalar doublet, because its lightest neutral member, say H^{0}, is one posssible component for the dark matter of the Universe. We discuss the general phenomenology of the four particles of this doublet, without assuming that H^{0} is the dominant source of dark matter. We also consider the impact of this darkdark scalar doublet on the phenomenology of the SM Higgs boson h.Comment: PRD versio

    Resummation Effects in the Search of SM Higgs Boson at Hadron Colliders

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    We examine the soft-gluon resummation effects, including the exact spin correlations among the final state particles, in the search of the Standard Model Higgs boson, via the process ggβ†’Hβ†’WW/ZZβ†’4leptons,attheTevatronandtheLHC.AcomparisonbetweentheresummationandtheNextβˆ’toβˆ’Leadingorder(NLO)calculationisperformedafterimposingvariouskinematicscutssuggestedintheliteraturefortheHiggsbosonsearch.Forthegg\to H\to WW/ZZ \to 4 leptons, at the Tevatron and the LHC. A comparison between the resummation and the Next-to-Leading order (NLO) calculation is performed after imposing various kinematics cuts suggested in the literature for the Higgs boson search. For the H\to ZZmode,theresummationeffectsincreasetheacceptanceofthesignaleventsbyabout25variouskinematicsdistributionsofthefinalstateleptons.Forthe mode, the resummation effects increase the acceptance of the signal events by about 25%, as compared to the NLO prediction, and dramatically alter various kinematics distributions of the final state leptons. For the H\to WW$ mode, the acceptance rates of the signal events predicted by the resummation and NLO calculations are almost the same, but some of the predicted kinematical distributions are quite different. Thus, to precisely determine the properties of the Higgs boson at hadron colliders, the soft-gluon resummation effects have to be taken into account.Comment: The version to appear in PR
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