1,491 research outputs found
On finite--temperature and --density radiative corrections to the neutrino effective potential in the early Universe
Finite-temperature and -density radiative corrections to the neutrino
effective potential in the otherwise CP-symmetric early Universe are considered
in the real-time approach of Thermal Field Theory. A consistent perturbation
theory endowed with the hard thermal loop resummation techniques developed by
Braaten and Pisarski is applied. Special attention is focused on the question
whether such corrections can generate any nonzero contribution to the
CP-symmetric part of the neutrino potential, if the contact approximation for
the W-propagator is used.Comment: 11 pages, revtex styl
Bounds on New Physics from the New Data on Parity Violation in Atomic Cesium
We assume the latest experimental determination of the weak charge of atomic
cesium and analyze its implications for possible new physics. We notice that
the data would imply positive upper and lower bounds on the new physics
contribution to the weak charge, . The required new physics should
be of a type not severely constrained by the high energy precision data. A
simplest possibility would be new neutral vector bosons almost un-mixed to the
and with sizeable couplings to fermions. The lower positive bound would
however forbid zero or negative and exclude not only the standard
model but also models with sequential , in particular simple-minded
towers of -like excitations from extra-dimensions. The bound would also
imply an upper limit on the mass within the models allowed.
Conclusions are also derived for models of four-fermion contact interactions.Comment: 11 pages, Latex, 1 PS figure, final version for Physics Letter
Flat Directions in Three-Generation Free-Fermionic String Models
In quasi-realistic string models that contain an anomalous U(1) the non-zero
Fayet-Iliopoulos term triggers the shifting of the original vacuum to a new one
along some flat direction, so that SUSY is preserved but the gauge group is
partially broken. The phenomenological study of these models thus requires as a
first step the mapping of the space of flat directions. We investigate F- and
D-flat directions in several three-generation SU(3)_C x SU(2)_L x U(1)_Y
free-fermionic string models and discuss the typical scenarios that generically
arise. When they exist, we systematically construct the flat directions that
preserve hypercharge, only break Abelian group factors, and can be proven to
remain F-flat to all orders in the non-renormalizable superpotential.Comment: 59 page
The 331 model with right-handed neutrinos
We explore some more consequences of the electroweak
model with right-handed neutrinos. By introducing the mixing angle
, the {\it exact} physical eigenstates for neutral gauge bosons are
obtained. Because of the mixing, there is a modification to the coupling
proportional to . The data from the -decay allows us to fix the
limit for as . >From the neutrino
neutral current scatterings, we estimate a bound for the new neutral gauge
boson mass in the range 300 GeV, and from symmetry-breaking hierarchy a
bound for the new charged and neutral (non-Hermitian) gauge bosons are obtained.Comment: Slight changes in section 5, Latex, 16 page
An SU(5)Z_{13} Grand Unification Model
We propose an SU(5) grand unified model with an invisible axion and the
unification of the three coupling constants which is in agreement with the
values, at , of , , and . A discrete,
anomalous, symmetry implies that the Peccei-Quinn symmetry is an
automatic symmetry of the classical Lagrangian protecting, at the same time,
the invisible axion against possible semi-classical gravity effects. Although
the unification scale is of the order of the Peccei-Quinn scale the proton is
stabilized by the fact that in this model the standard model fields form the
SU(5) multiplets completed by new exotic fields and, also, because it is
protected by the symmetry.Comment: 14 pages, more typos correcte
Radiatively Induced Neutrino Masses and Large Higgs-Neutrino Couplings in the Standard Model with Majorana Fields
The Higgs sector of the Standard Model with one right-handed neutrino per
family is systematically analyzed. In a model with intergenerational
independent mixings between families, we can account for very light neutrinos
acquiring Majorana masses radiatively at the first electroweak loop level. We
also find that in such a scenario the Higgs coupling to the light-heavy
neutrinos and to the heavy-heavy ones may be remarkably enhanced with
significant implications for the production of these heavy neutrinos at high
energy colliders.Comment: Making the text of an old paper electronically availabl
Recent Developments in Precision Electroweak Physics
Developments in precision electroweak physics in the two years since the
symposium are briefly summarized.Comment: Update on recent developments, prepared for the publication of the
Proceedings of Alberto Sirlin Symposium, New York University, October 2000.
10 pages, 1 figur
Comment On ``Grand Unification and Supersymmetric Threshold"
Barbieri and Hall have argued that threshold effects at the scale of
grand-unification wipe out predictions on the SUSY scale, M_S. Using triviality
arguments we give upper bounds on ultraheavy particles, while proton stability
gives lower bounds on the mass of the higgs color-triplet. We find no useful
lower bound on the supermultiplet, but if the strong coupling constant
is as large as recent experiments suggest, unification in the minimal SUSY
SU(5) model requires that the masses be and that the
color octet and weak triplet be split in mass by a factor of 100.Comment: 6 pages (revised
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