9,867 research outputs found
Flavor Physics in SO(10) GUTs with Suppressed Proton decay Due to Gauged Discrete Symmetry
Generic SO(10) GUT models suffer from the problem that Planck scale induced
non-renormalizable proton decay operators require extreme suppression of their
couplings to be compatible with present experimental upper limits. One way to
resolve this problem is to supplement SO(10) by simple gauged discrete
symmetries which can also simultaneously suppress the renormalizable R-parity
violating ones when they occur and make the theory "more natural". Here we
discuss the phenomenological viability of such models. We first show that for
both classes of models, e.g the ones that use or to
break B-L symmetry, the minimal Higgs content which is sufficient for proton
decay suppression is inadequate for explaining fermion masses despite the
presence of all apparently needed couplings. We then present an extended model, with three {\bf 10} and three {\bf 45}-Higgs, where is free of
this problem. We propose this as a realistic and "natural" model for fermion
unification and discuss the phenomenology of this model e.g. its predictions
for neutrino mixings and lepton flavor violation.Comment: 21 pages, 2 figure
Unification of standard and exotic matter through a symmetry
We consider a scenario in which the discrete weak symmetry between quarks and
leptons is extended to the weak force by introducing exotic partners. We
conjecture that there exists a hidden discrete symmetry , defining a
group, between standard and exotic quarks and leptons. The unified model
is discussed, where
the unifying discrete symmetry extends over particles and forces. It is shown
that the lighter neutral and charged weak bosons generated upon spontaneous
symmetry breaking have the same properties as those of the standard model.
Cabbibo-Kobayashi-Maskawa unitarity is used to set a mass of order 2.8 TeV for
the nonstandard weak bosons, which do not exhibit quark-lepton universality. A
grand unified theory of type, with decay of exotic matter into standard matter and no
decay of the ordinary nucleon, is put forward.Comment: 6 pages, no figure
New vector-scalar contributions to neutrinoless double beta decay and constraints on R-parity violation
We show that in minimal supersymmetric standard model (MSSM) with R-parity
breaking as well as in the left-right symmetric model, there are new observable
contributions to neutrinoless double beta decay arising from hitherto
overlooked diagrams involving the exchange of one W boson and one scalar boson.
In particular, in the case of MSSM, the present experimental bounds on
neutrinoless double beta decay lifetime improves the limits on certain R-parity
violating couplings by about two orders of magnitude. It is shown that similar
diagrams also lead to enhanced rates for conversion in
nuclei, which are in the range accessible to ongoing experiments.Comment: Latex file; 9 pages; 3 figures available on reques
Radion Cosmology in Theories with Universal Extra Dimensions
We discuss cosmology of models with universal extra dimensions, where the
Standard Model degrees of freedom live in a dimensional brane, with
compact and small extra spatial dimensions. In these models, the simplest way
to obtain the conventional 4-dimensional Planck scale starting with a low
string scale is to have also some larger extra dimensions, where only gravity
propagates. In such theories, dimensional reduction generically leads to at
least two radion fields, one associated with the total volume of the extra
spatial dimensions, and the other with the ratio of the sizes of small and
large extra dimensions. In this paper, we discuss the impact of the radion
fields on cosmology. We emphasize various aspects of radion physics such as
radion coupling to the Standard Model fields, bare and dressed radion masses
during inflation, dynamical stabilization of radions during and after
inflation, radion decay life time and its late dominance in thermal history of
the Universe as well as its quantum fluctuations during inflation. We argue
that models where the radion plays the role of an inflaton or the inflaton is a
brane scalar field, run into problems. We then present a successful inflation
model with bulk scalar fields that seems to have all the desired properties. We
also briefly discuss the possibility of radion as a cold dark matter candidate.Comment: 37 pages + 3 figure
The Higgs Sector on a Two-Sheeted Space Time
We present a general formalism based on the framework of non-commutative
geometry, suitable to the study the standard model of electroweak interactions,
as well as that of more general gauge theories. Left- and right-handed chiral
fields are assigned to two different sheets of space-time (a discretized
version of Kaluza-Klein theory). Scalar Higgs fields find themselves treated on
the same footing as the gauge fields, resulting in spontaneous symmetry
breaking in a natural and predictable way. We first apply the formalism to the
Standard Model, where one can predict the Higgs mass and the top Yukawa
coupling. We then study the left-right symmetric model, where we show that this
framework imposes constraints on the type and coefficients of terms appearing
in the Higgs potential.Comment: 24 pages, uses revtex
Supernova Constraints on a Superlight Gravitino
In supergravity models with low supersymmetry breaking scale the gravitinos
can be superlight with mass in the micro-eV to keV range. In such a case,
gravitino emission provides a new cooling mechanism for protoneutron stars and
therefore can provide constraints on the mass of the superlight gravitino. This
happens because the coupling to matter of superlight gravitinos is dominated by
its goldstino component, whose coupling to matter is inversely proportional to
the scale of supersymmetry breaking and increases as the gravitino mass
decreases. Present observations therefore provide lower limits on the gravitino
mass. Using recently revised goldstino couplings, we find that the two dominant
processes in supernova cooling are and
. They lead to lower limits on the
supersymmetry breaking scale from 160 to 500 GeV for core
temperatures 30 to 60 MeV and electron chemical potentials 200 to 300 MeV. The
corresponding lower limits on the gravitino mass are eV.Comment: Latex 6 pages; one figure; UTEXAS-HEP-97-19, UMD-PP-98-07,
SMU-HEP-97-1
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