3,857 research outputs found
Determining at Electron-Positron Colliders
Verifying is critical to test the three generation
assumption of the Standard Model. So far our best knowledge of is
inferred either from the 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 coupling
is modified in new physics models. In this work we propose to measure
in the process of without prior knowledge of the number
of fermion generations or the strength of the 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 , and couplings. The electroweak symmetry
of the Standard Model specifies a pattern of deviations of the --
and -- couplings after one imposes the known experimental
constraint on the -- 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 from the
cross section and the forward-backward asymmetry of top-quark pair production
at an {\it unpolarized} electron-positron collider.Comment: publish versio
Simple non-Abelian extensions of the standard model gauge group and the diboson excesses at the LHC
The ATLAS collaboration reported excesses at around 2 TeV in the di-boson
production decaying into hadronic final states. We consider the possibility of
explaining the excesses with extra gauge bosons in two simple non-Abelian
extensions of the Standard Model. One is the so-called models with a
symmetry structure of and the other is
the models with an extended symmetry of . The and bosons emerge after the electroweak symmetry is
spontaneously broken. Two patterns of symmetry breaking in the models
are considered in this work: one is , the other is . The symmetry breaking of the model is
. We perform a global
analysis of and phenomenology in ten new physics models,
including all the channels of decay. Our study shows that
the leptonic mode and the dijet mode of decays impose a
very stringent bound on the parameter space in several new physics models. Such
tight bounds provide a useful guide for building new physics models to address
on the diboson anomalies. We also note that the Left-Right and Lepton-Phobic
models can explain the excess if the deviation in
the pair around 2~TeV were confirmed to be a fluctuation of the SM
backgrounds.Comment: Publish version; title changed as suggested by journal Edito
A General Analysis of Wtb anomalous Couplings
We investigate new physics effects on the Wtb effective couplings in a
model-independent manner. The new physics effects are summarized as four
independent couplings , , and . Using
single-top-quark productions and W-helicity fraction measurements at the LHC
and Tevatron, we perform a global fit to impose constraints on top quark
effective couplings. We introduce a set of parameters , , and
to study the correlations among Wtb effective couplings. We show that (i)
improving the measurements of and is important in
constraining the correlation of and ; (ii)
and are anti-correlated, which is sensitive to all the
experiments; (iii) and are also anti-correlated, which is
sensitive to the W-helicity measurements; (iv) the correlation between
and is sensitive to the precision of , and
measurements. The effective Wtb couplings are studied in three kinds of
new physics models: models, vector-like
quark models and Littlest Higgs model with and without T-parity. The Wtb
couplings in the left-right model and the un-unified model are sensitive to the
ratio of gauge couplings when the new heavy gauge boson's mass () is
less than several hundred GeV, but the constraint is loose if TeV.
The Wtb couplings in vector-like quark models and the Littlest Higgs models are
sensitive to the mixing angles of new heavy particles and SM particles. We also
include the constraints of the oblique T-parameter and Zbb couplings which
impose much tighter constraints on the mixing angles. We show that the Wtb
coupling constraints become relevant if the precision of single top production
cross section measurements could be reduced to 1\% relative to the SM
predictions in future.Comment: Chin. Phys. C in pres
Resolving the Degeneracy in Single Higgs Production with Higgs Pair Production
The Higgs boson production can be affected by several anomalous couplings,
e.g. and anomalous couplings. Precise measurement of
production yields two degenerate parameter spaces of and ; one
parameter space exhibits the SM limit while the other does not. Such a
degeneracy could be resolved by Higgs boson pair production. In this work we
adapt the strategy suggested by the ATLAS collaboration to explore the
potential of distinguishing the degeneracy at the 14 TeV LHC. If the
anomalous coupling is induced only by the operator , then the non-SM-like band could be excluded with an integrated luminosity
of . Making use of the fact that the Higgs boson pair
is mainly produced through an -wave scattering, we propose an analytical
function to describe the fraction of signal events surviving a series of
experimental cuts for a given invariant mass of Higgs boson pair. The function
is model independent and can be applied to estimate the discovery potential of
various NP models
Oscillation of solutions of some higher order linear differential equations
In this paper, we deal with the order of growth and the hyper order of solutions of higher order linear differential equations where and are entire functions or polynomials. Some results are obtained which improve and extend previous results given by Z.-X. Chen, J. Wang, T.-B. Cao and C.-H. Li
Signature of Pseudo Nambu-Goldstone Higgs boson in its Decay
If the Higgs boson is a pseudo Nambu-Goldstone boson (PNGB), the
contact interaction induced by the invariants of the
non-linear sigma model is free from its nonlinearity effects. The process
can be used to eliminate the universal effects of heavy
particles, which can fake the nonlinearity effects of the PNGB Higgs boson in
the process (,\ ). We demonstrate that the
ratio of the signal strength of and
is good to distinguish the signature of the PNGB Higgs boson from Higgs
coupling deviations
Probing dark particles indirectly at the CEPC
When dark matter candidate and its parent particles are nearly degenerate, it
would be difficult to probe them at the Large Hadron Collider directly. We
propose to explore their quantum loop effects at the CEPC through the golden
channel process . We use a renormalizable toy model
consisting of a new scalar and a fermion to describe new physics beyond the
Standard Model. The new scalar and fermion are general multiplets of the
symmetry, and couple to the muon lepton through Yukawa
interaction. We calculate their loop contributions to anomalous
and couplings which can be applied to many new
physics models. The prospects of their effects at the CEPC are also examined
assuming a 0.002 accuracy in the cross section measurement
Soft Gluon Resummation in -channel single top quark production at the LHC
We present a detailed phenomenological study of the multiple soft gluon
radiation for the -channel single top and anti-top quark production at the
Large Hadron Collider (LHC). By applying the transverse momentum dependent
factorization formalism, large logarithms introduced by small total transverse
momentum of the single-top (anti-top) plus one-jet final state system
are resummed to all orders in the expansion of the strong interaction coupling
at the accuracy of Next-to-Leading Logarithm. We discuss various kinematical
distributions which are sensitive to this effect and find that soft gluon
radiation become more important when the final state jet is required to be in
the forward region. We show that the main difference from PYTHIA prediction
lies on the inclusion of the exact color coherence effect between the initial
and final states in our resummation calculation. We further propose to apply
the experimental observable to test the effect of multiple gluon
radiation in the single-top and anti-top events. The bottom quark mass effect
and jet rapidity distribution are also discussed.Comment: 22 pages. arXiv admin note: substantial text overlap with
arXiv:1801.09656, arXiv:1811.0142
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