233 research outputs found
What if Is Small?
In the basis where the charged lepton mass matrix is diagonal, the
left-handed neutrino mass matrix is invariant under the permutation of the
second and third generations if, and only if, the reactor angle
is zero and the atmospheric mixing angle is maximal. In the
presence of the seesaw mechanism, this symmetry leads to an inverted hierarchy,
with . This inverted mass spectrum is doubly protected if the
right-handed neutrinos also have a 2-3 symmetry
Neutrino masses through see-saw mechanism in 3-3-1 models
Some years ago it was shown by Ma that in the context of the electroweak
standard model there are, at the tree level, only three ways to generate small
neutrino masses by the see-saw mechanism via one effective dimension-five
operator. Here we extend this approach to 3-3-1 chiral models showing that in
this case there are several dimension-five operators and we also consider their
tree level realization.Comment: RevTex, 7 pages and 4 .eps figures. Version published in Phys. Rev.
D. with a change in the titl
Lepton masses in a supersymmetric 3-3-1 model
We consider the mass generation for both charginos and neutralinos in a 3-3-1
supersymmetric model. We show that R-parity breaking interactions leave the
electron and one of the neutrinos massless at the tree level. However the same
interactions induce masses for these particles at the 1-loop level. Unlike the
similar situation in the MSSM the masses of the neutralinos are related to the
masses of the charginos.Comment: RevTex, 11 pages incluing 2 .eps figures. Extended published versio
Large Solar Neutrino Mixing in an Extended Zee Model
The Zee model, which employs the standard Higgs scalar () with its
duplicate () and a singly charged scalar (), can utilize two
global symmetries associated with the conservation of the numbers of and
, , where coincides
with the hypercharge while () is a new
conserved charge, which is identical to for the left-handed
leptons. Charged leptons turn out to have - and - mixing
masses, which are found to be crucial for the large solar neutrino mixing. In
an extended version of the Zee model with an extra triplet Higgs scalar (s),
neutrino oscillations are described by three steps: 1) the maximal atmospheric
mixing is induced by democratic mass terms supplied by with =2 that can
initiate the type II seesaw mechanism for the smallness of these masses; 2) the
maximal solar neutrino mixing is triggered by the creation of radiative masses
by with = 0; 3) the large solar neutrino mixing is finally induced by
a - mixing arising from the rotation of the radiative mass
terms as a result of the diagonalization that converts - and -
mixing masses into the electron mass.Comment: RevTex, 10 pages including one figure page, to be published in Int.
J. Mod. Phys. A (2002
Nucleation and Growth of GaN/AlN Quantum Dots
We study the nucleation of GaN islands grown by plasma-assisted
molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In
particular, we assess the variation of their height and density as a function
of GaN coverage. We show that the GaN growth passes four stages: initially, the
growth is layer-by-layer; subsequently, two-dimensional precursor islands form,
which transform into genuine three-dimensional islands. During the latter
stage, island height and density increase with GaN coverage until the density
saturates. During further GaN growth, the density remains constant and a
bimodal height distribution appears. The variation of island height and density
as a function of substrate temperature is discussed in the framework of an
equilibrium model for Stranski-Krastanov growth.Comment: Submitted to PRB, 10 pages, 15 figure
Can the Zee Model Explain the Observed Neutrino Data?
The eigenvalues and mixing angles in the Zee model are investigated
parameter-independently. When we require |\Delta m^2_{12}/\Delta m^2_{23}| \ll
1 in order to understand the solar and atmospheric data simultaneously, the
only solution is one which gives bi-maximal mixing. It is pointed out that the
observed values \sin^2 2\theta_{solar} \simeq 0.66 in the MSW LMA solution
cannot be explained within the framework of the Zee model, because we derive a
severe constraint on the value of \sin^2 2 \theta_{solar}, \sin^2 2
\theta_{solar} \geq 1 -(1/16)(\Delta m^2_{solar}/\Delta m^2_{atm})^2.Comment: Latex file, 10 pages, 1 figure, explanations and references added,
typos corrected, to be published in Phys.Rev.
Lepton Masses from a TeV Scale in a 3-3-1 Model
In this work, using the fact that in 3-3-1 models the same leptonic bilinear
contributes to the masses of both charged leptons and neutrinos, we develop an
effective operator mechanism to generate mass for all leptons. The effective
operators have dimension five for the case of charged leptons and dimension
seven for neutrinos. By adding extra scalar multiplets and imposing the
discrete symmetry we are able to generate realistic textures
for the leptonic mixing matrix. This mechanism requires new physics at the TeV
scale.Comment: RevTex, 13 pages. Extended version to be published in Physical Review
Escape from washing out of baryon number in a two-zero-texture general Zee model compatible with the large mixing angle MSW solution
We propose a two-zero-texture general Zee model, compatible with the large
mixing angle Mikheyev-Smirnov-Wolfenstein solution. The washing out of the
baryon number does not occur in this model for an adequate parameter range. We
check the consistency of a model with the constraints coming from flavor
changing neutral current processes, the recent cosmic microwave background
observation, and the Z-burst scenario.Comment: 22 pages, 2 eps figures, Type set revtex
Bilarge Neutrino Mixing and \mu - \tau Permutation Symmetry for Two-loop Radiative Mechanism
The presence of approximate electron number conservation and \mu-\tau
permutation symmetry of S_2 is shown to naturally provide bilarge neutrino
mixing. First, the bimaximal neutrino mixing together with U_{e3}=0 is
guaranteed to appear owing to S_2 and, then, the bilarge neutrino mixing
together with |U_{e3}|<<1 arises as a result of tiny violation of S_2. The
observed mass hierarchy of \Delta m^2_{\odot}<<\Delta m^2_{atm} is subject to
another tiny violation of the electron number conservation. This scenario is
realized in a specific model based on SU(3)_L x U(1)_N with two-loop radiative
mechanism for neutrino masses. The radiative effects from heavy leptons
contained in lepton triplets generate the bimaximal structure and those from
charged leptons, which break S_2, generate the bilarge structure together with
|U_{e3}|<<1. To suppress dangerous flavor-changing neutral current interactions
due to Higgs exchanges especially for quarks, this S_2 symmetry is extended to
a discrete Z_8 symmetry, which also ensures the absence of one-loop radiative
mechanism.Comment: 18 pages, 7 figures, to appear in Phys. Rev.
Two-loop Radiative Neutrino Mechanism in an Gauge Model
By using the - - symmetry, we construct an
gauge model that provides two-loop radiative neutrino
masses as well as one-loop radiative neutrino masses. The generic smallness of
two-loop neutrino masses leading to compared with one-loop
neutrino masses leading to successfully explains by invoking the - -
breaking. The Higgs scalar () that initiates radiative mechanisms is
unified into a Higgs triplet together with the standard Higgs scalar (,
) to form (, , ), which calls for three families
of lepton triplets: (, , ) (i = 1,2,3), where
denote heavy neutral leptons. The two-loop radiative mechanism is
found possible by introducing a singly charged scalar, which couples to
(i,j = 2,3).Comment: with 10 pages, revtex, including 2 figures, accepted for publication
in Phys. Rev. D (with undefined latex citation indices removed
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