957 research outputs found

### Determining $V_{tb}$ at Electron-Positron Colliders

Verifying $V_{tb} \simeq 1$ is critical to test the three generation
assumption of the Standard Model. So far our best knowledge of $V_{tb}$ is
inferred either from the $3\times 3$ 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 $Wtb$ coupling
is modified in new physics models. In this work we propose to measure $V_{tb}$
in the process of $e^+ e^- \to t\bar{t}$ without prior knowledge of the number
of fermion generations or the strength of the $Wtb$ 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 $Wtb$, $Zt\bar{t}$ and $Zb\bar{b}$ couplings. The electroweak symmetry
of the Standard Model specifies a pattern of deviations of the $Z$-$t_L$-$t_L$
and $W$-$t_L$-$b_L$ couplings after one imposes the known experimental
constraint on the $Z$-$b_L$-$b_L$ 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 $V_{tb}$ 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

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

We study the collider signatures of a T-odd gauge boson $W_{H}$ 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 $W_{H}$
boson using its leptonic decay, i.e. $pp\to W_{H}^{+}W_{H}^{-}\to
A_{H}A_{H}\ell^{+}\nu_{\ell}\ell^{\prime-}\bar{\nu}_{\ell^{\prime}}$, 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
$W_{H}$ 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 $W_{H}$ at the LHC. But such a mass measurement can be
easily achieved at the LC in the process of $e^{+}e^{-}\to
W_{H}^{+}W_{H}^{-}\to A_{H}A_{H}W^{+}W^{-}\to A_{H}A_{H}jjjj$. We present an
algorithm of measuring the mass and spin of the $W_{H}$ boson at the LC.
Furthermore, we illustrate that the spin correlation between the $W$ boson and
its mother particle ($W_{H}$) 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

If the Standard Model of particle interactions is extended to include a
second scalar doublet $[H^{+},(H^{0}+iA^{0})/\sqrt{2}]$, which is odd under an
unbroken Z_{2} discrete symmetry, it may be called the $dark$ 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 $dark$ 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

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 $gg\to H\to WW/ZZ \to 4 leptons, at the
Tevatron and the LHC. A comparison between the resummation and the
Next-to-Leading order (NLO) calculation is performed after imposing various
kinematics cuts suggested in the literature for the Higgs boson search. For the$H\to ZZ$mode, the resummation effects increase the acceptance of the signal
events by about 25%, as compared to the NLO prediction, and dramatically alter
various kinematics distributions of the final state leptons. For the$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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