176 research outputs found
Probing the indefinite CP nature of the Higgs Boson through decay distributions in the process
The recently discovered scalar resonance at the LHC is now almost confirmed
to be a Higgs Boson, whose CP properties are yet to be established. At the ILC
with and without polarized beams, it may be possible to probe these properties
at high precision. In this work, we study the possibility of probing departures
from the pure CP-even case, by using the decay distributions in the process
, with mainly decaying into a
pair. We have compared the case of a minimal extension of the SM case (Model I)
with an additional pseudoscalar degree of freedom, with a more realistic case
namely the CP-violating Two-Higgs Doublet Model (Model II) that permits a more
general description of the couplings. We have considered the ILC with
\,GeV and integrated luminosity of . Our
main findings are that even in the case of small departures from the CP-even
case, the decay distributions are sensitive to the presence of a CP-odd
component in Model II, while it is difficult to probe these departures in Model
I unless the pseudoscalar component is very large. Noting that the proposed
degrees of beam polarization increases the statistics, the process demonstrates
the effective role of beam polarization in studies beyond the Standard Model.
Further, our study shows that an indefinite CP Higgs would be a sensitive
laboratory to physics beyond the SM.Comment: 14 pages using revtex, 10 figures, corresponds to version accepted
for publication in Phys. Rev. D.; compared to v1, discussion extended, figure
added, table added, section reorganize
Top Yukawa coupling measurement with indefinite CP Higgs in
We consider the issue of the top quark Yukawa coupling measurement in a model
in dependent and general case with the inclusion of CP-violation in the
coupling. Arguably the best process to study this coupling is the associa ted
production of Higgs boson along with a pair in a machine like the
International Linear Collider (ILC). While detailed analyses of the sensitivity
of the measurement assuming a Standard Model (SM) - like coupling are available
in the context of ILC, conclude that th e coupling could be pinned down at
about 10\% level with modest luminosity, our investigations show that the
scenario could be different in case of a more general coupling. The modified
Lorentz structure resulting in a changed functional dependence of the cross
section on the couplin g, along with the difference in the cross section itself
leads to considerable deviation in the sensitivity. Our studies with an ILC of
center of mass energies of 500 GeV, 800 GeV and 1000 GeV show that moderate
CP-mixing in the Higgs sector could change the sensitivity to about 20\ %,
while it could be worsened to 75\% in cases which could accommodate more
dramatic changes in the coupling. While detailed considerations of the decay
distributions point to a need for a relook at the analysis strategy followed
for the case of SM such as for a model independent analysis of the top quark
Yukawa coupling measurement. This study strongly suggests that, a joint
analysis of the CP properties and the Yukawa coupling measurement would be the
way forward at the ILC and that caution must be excercised in the measurem ent
of the Yukawa couplings and the conclusions drawn from it.Comment: 18 pages, 7 figures, uses revte
Study of CP Property of the Higgs at a Photon Collider using
We study possible effects of CP violation in the Higgs sector on
production at a -collider. These studies are performed in a
model-independent way in terms of six form-factors which
parametrize the CP mixing in the Higgs sector, and a strategy for their
determination is developed. We observe that the angular distribution of the
decay lepton from produced in this process is independent of any CP
violation in the vertex and hence best suited for studying CP mixing in
the Higgs sector. Analytical expressions are obtained for the angular
distribution of leptons in the c.m. frame of the two colliding photons for a
general polarization state of the incoming photons. We construct combined
asymmetries in the initial state lepton (photon) polarization and the final
state lepton charge. They involve CP even ('s) and odd ('s) combinations
of the mixing parameters. We study limits up to which the values of and
, with only two of them allowed to vary at a time, can be probed by
measurements of these asymmetries, using circularly polarized photons. We use
the numerical values of the asymmetries predicted by various models to
discriminate among them. We show that this method can be sensitive to the
loop-induced CP violation in the Higgs sector in the MSSM.Comment: 15 pages, 9 figures added one referenc
QCD corrections to decay-lepton polar and azimuthal angular distributions in e+e- -> t tbar in the soft-gluon approximation
QCD corrections to order alpha_s in the soft-gluon approximation to angular
distributions of decay charged leptons in the process e+e- -> t tbar followed
by semileptonic decay of t or tbar, are obtained in the e+e- centre-of-mass
frame. As compared to distributions in the top rest frame, these have the
advantage that they would allow direct comparison with experiment without the
need to reconstruct the top rest frame. The results also do not depend on the
choice of a spin quantization axis for t or tbar. Analytic expression for the
triple distribution in the polar angle of t and polar and azimuthal angles of
the lepton is obtained. Analytic expression is also derived for the
distribution in the charged-lepton polar angle. Numerical values are discussed
for total c.m. energies of 400 GeV, 800 GeV and 1500 GeV.Comment: 21 pages, Latex, 6 figures included in the submission. To appear in
Pramana - Journal of Physics; expanded version of hep-ph/0011321, v
Signals of additional Z boson in e+e-\to W+W^- at the ILC with polarized beams
We consider the possibility of fingerprinting the presence of heavy
additional Z' bosons that arise naturally in extensions of the standard model
such as E_6 models and left-right symmetric models, through their mixing with
the standard model Z boson. By considering a class of observables including
total cross sections, energy distributions and angular distributions of decay
leptons we find significant deviation from the standard model predictions for
these quantities with right-handed electrons and left-handed positrons at
\sqrt{s}=800 GeV. The deviations being less pronounced at smaller centre of
mass energies as the models are already tightly constrained. Our work suggests
that the ILC should have a strong beam polarization physics program
particularly with these configurations. On the other hand, a forward backward
asymmetry and lepton fraction in the backward direction are more sensitive to
new physics with realistic polarization due to interesting interplay with the
neutrino t- channel diagram. This process complements the study of fermion pair
production processes that have been considered for discrimination between these
models.Comment: 23 pages, 9 figures, uses plain latex; substantially improved
discussion, references added, version accepted for publication in JHE
in NonCommutative Standard Model
We study the top quark decay to b quark and W boson in the NonCommutative
Standard Model (NCSM). The lowest contribution to the decay comes from the
terms quadratic in the matrix describing the noncommutative (NC) effects while
the linear term is seen to identically vanish because of symmetry. The NC
effects are found to be significant only for low values of the NC
characteristic scale.Comment: 11 page Latex file containing 2 eps figures (redrawn). More
discussion included. To appear in PR
CP-violating dipole form factors of the top quark and tau lepton in scalar leptoquark models
We calculate the CP-violating electric and weak dipole form factors of the
top quark and the tau lepton in models with scalar leptoquarks coupling only to
the third generation of quarks and leptons. We obtain numerical values of the
real and imaginary parts of these form factors at various energies for
different values of leptoquark masses and couplings. The existing limits on the
tau electric and weak dipole form factors allows us to put a limit on the
masses and couplings of such leptoquarks and therefore on the top electric and
weak dipole form factors. We also discuss constraints on the form factors
coming from indirect limits on leptoquark masses and couplings deduced from LEP
results on Z properties.Comment: 22 pages, LaTeX, includes 14 figures in postscript file
Polarized Single Top Production at Leptonic Colliders from Broken R Parity Interactions Incorporating CP Violation
The contribution from the R parity violating interactions in the associated
production of a top quark with a charm antiquark is examined for high energy
leptonic colliders. We concentrate on the reactions associated with the
semileptonic top decay. A set of characteristic dynamical distributions for the
signal events is evaluated and the results contrasted against those from the
standard model W-boson pair production background. Next, we turn to a study of
a CP-odd observable, associated with the top spin, which leads to an asymmetry
in the energy distribution of the emitted charged leptons for the pair of
CP-conjugate final states, and . A
non vanishing asymmetry arises from a CP-odd phase, embedded in the R parity
violating coupling constants, through interference terms between the R parity
violating amplitudes at both the tree and loop levels.Comment: revtex file. 17 pages. 7 postscript figures. 1 table. The revised
version includes an estimate of experimental uncertainties. Submitted to
Phys. Rev.
Probing strongly interacting W's at the ILC with polarized beams
We study the possibility of fingerprinting a strongly interacting boson
sector which is consistent with present day LHC searches at the ILC with
longitudinal as well as transversely polarized electron and positron beams. We
account for the final state interaction using a suitable Omn\`es formalism in
terms of a plausible resonance description, and carry out thorough analyses of
cross sections, asymmetries and angular distributions of the . We carry
out a comparison with other extensions of the Standard Model, where heavy
additional bosons arise naturally. We also consider the effect of the
strong final state interaction on a correlation that depends on
, where the are the azimuthal angles of decay
leptons, and find that it is a useful discriminant.Comment: 25 pages latex using JHEP style files, 14 figures; v2 is a slightly
expanded version of v1, reference added, discussions improved, some figures
have been changed; corresponds to version accepted for publication in JHE
CP violation
The salient features of CP-violating interactions in the standard electroweak
theory and in a few of its popular extensions are discussed. Moreover a brief
overview is given on the status and prospects of searches for CP
non-conservation effects in low and high energy experiments.Comment: 28 pages, Lectures given at the 37th Winter School on Particle
Physics, Schladming, Austria, 199
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