15 research outputs found
The top quark right coupling in the tbW-vertex
The most general parametrization of the tbW vertex includes a right coupling V-R that is zero at tree level in the standard model. This quantity may be measured at the Large Hadron Collider where the physics of the top decay is currently investigated. This coupling is present in new physics models at tree level and/or through radiative corrections, so its measurement can be sensitive to non-standard physics. In this paper we compute the leading electroweak and QCD contributions to the top V-R coupling in the standard model. This value is the starting point in order to separate the standard model effects and, then, search for new physics. We also propose observables that can be addressed at the LHC in order to measure this coupling. These observables are defined in such a way that they do not receive tree level contributions from the standard model and are directly proportional to the right coupling. Bounds on new physics models can be obtained through the measurements of these observables
Majorana neutrinos production at LHeC in an effective approach
We investigate the possibility of detecting Majorana neutrinos at the Large
Hadron-electron Collider (LHeC), an electron-proton collision mode at CERN. We
study the () final states which are,
due to leptonic number violation, a clear signature for intermediate Majorana
neutrino contributions. Such signals are not possible if the heavy neutrinos
have Dirac nature. The interactions between Majorana neutrinos and the Standard
Model (SM) particles are obtained from an effective lagrangian approach. We
present our results for the total cross section as a function of the neutrino
mass, the effective couplings and the new physics scale. We also show the
discovery region as a function of the Majorana neutrino mass and the effective
couplings. Our results show that the LHeC may be able to discover Majorana
neutrinos with masses lower than GeV and GeV for electron beams
settings of GeV and GeV, respectively.Comment: 15 pages, 11 pdf figures. arXiv admin note: text overlap with
arXiv:1110.095
Tau EDM at Low Energies
Low energy tau pair production, at B factories and on top of the
resonances, allows for a detailed investigation on the CP violation at the
electromagnetic tau pair production vertex. High statistic available at low
energies offers the opportunity for an independent analysis of CP-violation in
the lepton physics. We show that stringent and independent bounds on the
electric dipole moment, competitive with the high energy measurements,
can be established in low energies experiments.Comment: Talk at the Seventh International Workshop on Tau Lepton Physics
(TAU02), Santa Cruz, Ca, USA, Sept 2002, 5 pages, LaTeX, 1 eps figur
The top right coupling in the aligned two-Higgs-doublet model
We compute the right top quark coupling in the aligned two-Higgs-doublet
model. In the Standard Model the real part of this coupling is dominated by
QCD-gluon-exchange diagram, but the imaginary part, instead, is purely
electroweak at one loop. Within this model we show that values for the
imaginary part of the coupling up to one order of magnitude larger than the
electroweak prediction can be obtained. For the real part of the electroweak
contribution we find that it can be up to three orders of magnitude larger than
the standard model one. We also present detailed results of the one loop
analytical computation
Anomalous top magnetic couplings
The real and imaginary parts of the one-loop electroweak contributions to the left and right tensorial anomalous couplings of the tbW vertex in the Standard Model (SM) are computed
Bounds on the Tau Magnetic Moments: Standard Model and Beyond
We obtain new bounds for the magnetic dipole moments of the tau lepton. These
limits on the magnetic couplings of the tau to the electroweak gauge bosons
(gamma, W, Z) are set in a model independent way using the most general
effective Lagrangian with the SU(2)_L x U(1)_Y symmetry. Comparison with data
from the most precise experiments at high energies shows that the present
limits are more stringent than the previous published ones. For the anomalous
magnetic moment the bounds are, for the first time, within one order of
magnitude of the standard model prediction.Comment: 8 pages, 1 figure; to appear in the proceedings of the 6th
International Workshop on Tau Lepton Physics, 18-21 September (2000),
Victoria (Canada
Model independent bounds on the tau lepton electromagnetic and weak magnetic moments
Using LEP1, SLD and LEP2 data, for tau lepton production, and data from CDF,
D0 and LEP2, for W decays into tau leptons, we set model independent limits on
non-standard electromagnetic and weak magnetic moments of the tau lepton. The
most general effective Lagrangian giving rise to tau moments is used without
further assumptions. Precise bounds () on the non-standard model
contributions to tau electromagnetic (), tau Z-magnetic
() and tau W-magnetic ()
dipole moments are set from the analysis.Comment: 19 pages, 2 figures, elsart.sty. Revised version with new data.
Accepted for publication in Nucl. Phys.
Top quark anomalous tensor couplings in the two-Higgs-doublet models
We compute the one loop right and left anomalous tensor couplings (g(R) and g(L), respectively) for the top quark, in the aligned two-Higgs-doublet model. They are the magnetic-like couplings in the most general parameterization of the tbW vertex. We find that the aligned two-Higgs doublet model, that includes as particular cases some of the most studied extensions of the Higgs sector, introduces new electroweak contribution's and provides theoretical predictions that are very sensitive to both new scalar masses and the neutral scalar mixing angle. For a largo area in the parameters space we obtain significant deviations in both the real and the imaginary parts of the couplings gR and gL, compared to the predictions given by the electroweak sector of the Standard Model. The most important ones are those involving the imaginary part of the left coupling g(L) and the real part of the right coupling gR. The real part of g(L), and the imaginary part of gR also show an important sensitivity to new physics scenarios. The model can also account for new CP violation effects via the introduction of complex alignment parameters that have important consequences on the values for the imaginary parts of the couplings. The top anomalous tensor couplings will be measured at the LHC and at future colliders providing a complementary insight on new physics, independent from the bounds in top decays coming from B physics and b -> s gamma
Top quark tensor couplings
We compute the real and imaginary parts of the one-loop electroweak
contributions to the left and right tensorial anomalous couplings of the
vertex in the Standard Model (SM). For both tensorial couplings we find that
the real part of the electroweak SM correction is close to 10 of the leading
contribution given by the QCD gluon exchange. We also find that the electroweak
real and imaginary parts for the anomalous right coupling are almost of the
same order of magnitude. The one loop SM prediction for the real part of the
left coupling is close to the 3 discovery limit derived from
. Besides, taking into account that the predictions of
new physics interactions are also at the level of a few percents when compared
with the one loop QCD gluon exchange, these electroweak corrections should be
taken into account in order to disentangle new physics effects from the
standard ones. These anomalous tensorial couplings of the top quark will be
investigated at the LHC in the near future where sensitivity to these
contributions may be achieved.Comment: 16 pages, 2 figure
EDM observables for tau production with polarized beams
The Tau-lepton electric dipole moment (EDM) can be measured at Super B/Flavor factories operating with polarized electron beams at energies near and on top of the resonances. In particular, new CP-odd observables, independent from others already considered, will allow to put stringent bounds on the EDM