495 research outputs found

### Radiative processes (tau -> mu gamma, mu -> e gamma and muon g-2) as probes of ESSM/SO(10)

The Extended Supersymmetric Standard Model (ESSM), motivated on several
grounds, introduces two vectorlike families (16 + 16-bar) of SO(10)) with
masses of order one TeV. It is noted that the successful predictions of prior
work on fermion masses and mixings, based on MSSM embedded in SO(10), can be
retained rather simply within the ESSM extension. These include an
understanding of the smallness of V_{cb} ~ 0.04 and the largeness of nu_mu -
nu_tau oscillation angle, sin^2 2 theta_{nu_mu nu_tau}^{osc} ~ 1. We analyze
the new contributions arising through the exchange of the vectorlike families
of ESSM to radiative processes including tau -> mu gamma, mu -> e gamma, b -> s
gamma, EDM of the muon and the muon (g-2). We show that ESSM makes significant
contributions especially to the decays tau -> mu gamma and mu -> e gamma and
simultaneously to muon (g-2). For a large and plausible range of relevant
parameters, we obtain: a_mu^{ESSM} ~ +(10-40) times 10^{-10}, with a correlated
prediction that tau -> mu gamma should be discovered with an improvement in its
current limit by a factor of 3-20. The implications for mu -> e gamma are very
similar. The muon EDM is within reach of the next generation experiments. Thus,
ESSM with heavy leptons being lighter than about 700 GeV (say) can be probed
effectively by radiative processes before a direct search for these vectorlike
leptons and quarks is feasible at the LHC.Comment: 27 pages LaTex, 2 figure

### Up-Down Unification, Neutrino Masses and Rare Lepton Decays

In a recent paper, we showed that tree level up-down unification of fermion
Yukawa couplings is a natural consequence of a large class of supersymmetric
models. They can lead to viable quark masses and mixings for moderately large
values of $\tan\beta$ with interesting and testable predictions for CP
violation in the hadronic sector. In this letter, we extend our discussion to
the leptonic sector focusing on one particular class of these models, the
supersymmetric left-right model with the seesaw mechanism for neutrino masses.
We show that fitting the solar and the atmospheric neutrino data considerably
restricts the Majorana-Yukawa couplings of the leptons in this model and leads
to predictions for the decay $\tau\to \mu +\gamma$, which is found to be
accessible to the next generation of rare decay searches. We also show that the
resulting parameter space of the model is consistent with the requirements of
generating adequate baryon asymmetry through lepton-number violating decays of
the right-handed neutrino.Comment: 16 pages, latex, 6 figures, typos correcte

### Family Unification with SO(10)

Unification based on the group SO(10)^3 \times S_3 is studied. Each family
has its own SO(10) group, and the S_3 permutes the three families and SO(10)
factors. This is the maximal local symmetry for the known fermions. Family
unification is achieved in the sense that all known fermions are in a single
irreducible multiplet of the symmetry. The symmetry suppresses SUSY flavor
changing effects by making all squarks and sleptons degenerate in the symmetry
limit. Doublet-triplet splitting can arise simply, and non-trivial structure of
the quark and lepton masses emerges from the gauge symmetry, including the
"doubly lopsided" form.Comment: 11 pages, references adde

### Classification of Effective Neutrino Mass Operators

We present a classification of SU(3) x SU(2) x U(1) gauge invariant \Delta L
= 2 (L being lepton number) effective operators relevant for generating small
Majorana neutrino masses. Operators of dimension up to 11 have been included in
our analysis. This approach enables us to systematically identify interesting
neutrino mass models. It is shown that many of the well-known models fall into
this classification. In addition, a number of new models are proposed and their
neutrino phenomenology is outlined. Of particular interest is a large class of
models in which neutrinoless double beta decays arise at a lower order compared
to the neutrino mass, making these decays accessible to the current round of
experiments.Comment: 34 pages in RevTeX with 18 figure

### SUSY breaking based on Abelian gaugino kinetic term mixings

We present a SUSY breaking scenario based on Abelian gaugino kinetic term
mixings between hidden and observable sectors. If an extra U(1) gaugino in the
observable sector obtains a large mass through this mixing effect based on SUSY
breaking in the hidden sector, soft SUSY breaking parameters in the MSSM may be
affected by radiative effects due to this gaugino mass. New phenomenological
aspects are discussed in such a SUSY breaking scenario.Comment: latex, 12pages, 2figures, published versio

### Predictive Models of Large Neutrino Mixing Angles

Several experimental results could be interpreted as evidence that certain
neutrino mixing angles are large, of order unity. However, in the context of
grand unified models the neutrino angles come out characteristically to be
small, like the KM angles. It is shown how to construct simple grand-unified
models in which neutrino angles are not only large but completely predicted
with some precision. Six models are presented for illustration.Comment: 19 pages, LaTe

### Fermion masses in SO(10) with a single adjoint Higgs field

It has recently been shown how to break SO(10) down to the Standard Model in
a realistic way with only one adjoint Higgs. The expectation value of this
adjoint must point in the B-L direction. This has consequences for the possible
form of the quark and lepton mass matrices. These consequences are explored in
this paper, and it is found that one is naturally led to consider a particular
form for the masses of the heavier generations. This form implies typically
that there should be large (nearly maximal) mixing of the mu- and
tau-neutrinos. An explanation that does not involve large tan beta also emerges
for the fact that b and tau are light compared to the top quark.Comment: 20 pages, LaTeX, clarification of statements about multiple adjoint
Higgs fields in the context of superstring theor

### Lepton Flavor Violation and the Origin of the Seesaw Mechanism

The right--handed neutrino mass matrix that is central to the understanding
of small neutrino masses via the seesaw mechanism can arise either (i) from
renormalizable operators or (ii) from nonrenormalizable or super-renormalizable
operators, depending on the symmetries and the Higgs content of the theory
beyond the Standard Model. In this paper, we study lepton flavor violating
(LFV) effects in the first class of seesaw models wherein the \nu_R Majorana
masses arise from renormalizable Yukawa couplings involving a B-L = 2 Higgs
field. We present detailed predictions for \tau -> \mu + \gamma and \mu -> e +
\gamma branching ratios in these models taking the current neutrino oscillation
data into account. Focusing on minimal supergravity models, we find that for a
large range of MSSM parameters suggested by the relic abundance of neutralino
dark matter and that is consistent with Higgs boson mass and other constraints,
these radiative decays are in the range accessible to planned experiments. We
compare these predictions with lepton flavor violation in the second class of
models arising entirely from the Dirac Yukawa couplings. We study the
dependence of the ratio r \equiv B(\mu -> e+\gamma)/B(\tau ->\mu +\gamma) on
the MSSM parameters and show that measurement of r can provide crucial insight
into the origin of the seesaw mechanism.Comment: 20 pages, Revtex, 7 figure

### Lifting a Realistic SO(10) Grand Unified Model to Five Dimensions

It has been shown recently that the problem of rapid proton decay induced by
dimension five operators arising from the exchange of colored Higgsinos can be
simply avoided in grand unified models where a fifth spatial dimension is
compactified on an orbifold. Here we demonstrate that this idea can be used to
solve the Higgsino-mediated proton decay problem in any realistic SO(10) model
by lifting that model to five dimensions. A particular SO(10) model that has
been proposed to explain the pattern of quark and lepton masses and mixings is
used as an example. The idea is to break the SO(10) down to the Pati-Salam
symmetry by the orbifold boundary conditions. The entire four-dimensional
SO(10) model is placed on the physical SO(10) brane except for the gauge
fields, the 45 and a single 10 of Higgs fields, which are placed in the
five-dimensional bulk. The structure of the Higgs superpotential can be
somewhat simplified in doing so, while the Yukawa superpotential and mass
matrices derived from it remain essentially unaltered.Comment: 17 pages, version to be published in Phys. Rev. D with expanded
discussion of the suppression of dim-5 proton decay operator

### Closing the Windows on Mev Tau Neutrinos

In this note, we analyze various constraints on the ``visible'' decay modes
of a massive $\tau$ neutrino, $\nu_\tau\rightarrow\nu^\prime\,\gamma$ and
$\nu_\tau\rightarrow\nu^\prime\, e^+ e^-$, where $\nu^\prime$ is a light
neutrino. The BEBC beam dump experiment provides model-independent constraints
on these modes. The lifetime for the $\nu^\prime\, e^+e^-$ mode is constrained
to be $\tau_{\nu^\prime\, e^+e^-} \ge 0.18~(m_{\nu_\tau}/MeV)~sec.$ We point
out that the same experiment implies a similar constraint on the
$\nu^\prime\,\gamma$ mode. This results in a new upper limit on the transition
magnetic moment of $\nu_\tau$, $\mu_{\rm tran} \le 1.1 \times 10^{-9}
(MeV/m_{\nu_\tau})^2 \mu_B$. Furthermore, a limit on the electric charge of
$\nu_\tau$ may be obtained, $Q_{\nu_\tau} \le 4 \times 10^{-4}e$. Combining
these constraints with those arising from supernova observations and primordial
nucleosynthesis calculations, we show that these ``visible'' decays cannot be
the dominant decay modes of the $\tau$ neutrino.Comment: 8 pgs. LaTeX (1 uuencoded fig., also available on request),
Bartol-930XXX, JHU-TIPAC-930026, UM-TH-93-2

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