403 research outputs found
New Lepton Family Symmetry and Neutrino Tribimaximal Mixing
The newly proposed finite symmetry Sigma(81) is applied to the problem of
neutrino tribimaximal mixing. The result is more satisfactory than those of
previous models based on A_4 in that the use of auxiliary symmetries (or
mechanisms) may be avoided. Deviations from the tribimaximal pattern are
expected, but because of its basic structure, only tan^2 (theta_12) may differ
significantly from 0.5 (say 0.45) with sin^2 (2 theta_23) remaining very close
to one, and theta_13 very nearly zero.Comment: 8 pages, no figur
Effective Lagrangian Approach to the Fermion Mass Problem
An effective theory is proposed, combining the standard gauge group
with a horizontal discrete
symmetry. By assigning appropriate charges under this discrete symmetry to the
various fermion fields and to (at least) two Higgs doublets, the broad spread
of the fermion mass and mixing angle spectrum can be explained as a result of
suppressed, non-renormalisable terms. A particular model is constructed which
achieves the above while simultaneously suppressing neutral Higgs-induced
flavour-changing processes.Comment: 21 pages, UM-P-93/81, latex file, 1 figure available on reques
Flavor Symmetry L_mu - L_tau and quasi-degenerate Neutrinos
Current data implies three simple forms of the neutrino mass matrix, each
corresponding to the conservation of a non-standard lepton charge. While models
based on L_e and L_e - L_mu - L_tau are well-known, little attention has been
paid to L_mu - L_tau. A low energy mass matrix conserving L_mu - L_tau implies
quasi-degenerate light neutrinos. Moreover, it is mu-tau symmetric and
therefore (in contrast to L_e and L_e - L_mu - L_tau) automatically predicts
maximal atmospheric neutrino mixing and zero U_{e3}. A see-saw model based on
L_mu - L_tau is investigated and testable predictions for the neutrino mixing
observables are given. Renormalization group running below and in between the
see-saw scales is taken into account in our analysis, both numerically and
analytically.Comment: 15 pages, 2 figures. Prepared for 5th International Conference on
Nonaccelerator New Physics (NANP 05), Dubna, Russia, 20-25 Jun 200
Phenomenology of the Gauge Boson
Assuming the existence of a gauge boson which couples to ,
we discuss the present experimental constraints on and from and . We also discuss the
discovery potential of at hadron colliders through its decay into pairs. In the scenario where all three charged leptons (and their
neutrinos) mix, lepton flavor nonconservation through becomes possible and
provides another experimental probe into this hypothesis.Comment: 19 pages, LaTeX, including 4 figure
Deviation from tri-bimaximal mixings in two types of inverted hierarchical neutrino mass models
An attempt is made to explore the possibility for deviations of solar mixing
angle () from tri-bimaximal mixings, without sacrificing the
predictions of maximal atmospheric mixing angle () and zero
reactor angle (). We find that the above conjecture can be
automatically realised in the inverted hierarchical neutrino mass model having
2-3 symmetry, in the basis where charged lepton mass matrix is diagonal. For
the observed ranges of and \bigtriangleup m^2_{23],
we calculate the predictions on for
different input values of the parameters in the neutrino mass matrix. We also
observe a possible crossing over from one type of inverted hierarchical model
having same CP parity (Type-IHA) to other type having opposite CP parity
(Type-IHB). Such neutrino mass matrices can be obtained from the canonical
seesaw formula using diagonal form of Dirac neutrino mass matrix and
non-diagonal texture of right-handed Majorana mass matrix, and may have
important implications in model building using discrete as well as non-abelian
symmetry groups.Comment: 13 pages, 7 figure
Predictive fermion mass matrix ansatzes in non-supersymmetric SO(10) grand unification
We investigate the status of predictive fermion mass ansatzes which make use
of the grand unification scale conditions , , and in non-supersymmetric SO(10) grand unification.
The gauge symmetry below an intermediate symmetry breaking scale is
assumed to be that of the standard model with either one Higgs doublet or two
Higgs doublets . We find in both cases that a maximum of 5 standard model
parameters may be predicted within experimental ranges. We find that
the standard model scenario predicts the low energy to be in
a range which includes its experimental mid-value 0.044 and which for a large
top mass can extend to lower values than the range resulting in the
supersymmetric case. In the two Higgs standard model case, we identify the
regions of parameter space for which unification of the bottom quark and tau
lepton Yukawa couplings is possible at grand unification scale. In fact, we
find that unification of the top, bottom and tau Yukawa couplings is possible
with the running b-quark mass within the preferred range provided is near the low end of its allowed
range. In this case, one may make 6 predictions which include
within its confidence limits. However unless the running mass , third generation Yukawa coupling unification requires the top mass to be
greater thanComment: 30 pages, 8 figures available on request from
[email protected], Late
Symmetries of the Standard Model without and with a Right-Handed Neutrino
Given the particle content of the standard model without and with a
right-handed neutrino, the requirement that all anomalies cancel singles out a
set of possible global symmetries which can be gauged. I review this topic and
propose a new gauge symmetry B - 3L_tau in the context of the minimal standard
model consisting of the usual three families of quarks and leptons plus just
one nu_R. The many interesting phenomenological consequences of this hypothesis
are briefly discussed.Comment: 7 pages, no figure, latex, sprocl.sty, talk at the Fifth Workshop on
High Energy Physics Phenomenology, Pune, Jan 9
Tri-bimaximal mixing, discrete family symmetries, and a conjecture connecting the quark and lepton mixing matrices
Neutrino oscillation experiments (excluding the LSND experiment) suggest a
tri-bimaximal form for the lepton mixing matrix. This form indicates that the
mixing matrix is probably independent of the lepton masses, and suggests the
action of an underlying discrete family symmetry. Using these hints, we
conjecture that the contrasting forms of the quark and lepton mixing matrices
may both be generated by such a discrete family symmetry. This idea is that the
diagonalisation matrices out of which the physical mixing matrices are composed
have large mixing angles, which cancel out due to a symmetry when the CKM
matrix is computed, but do not do so in the MNS case. However, in the cases
where the Higgs bosons are singlets under the symmetry, and the family symmetry
commutes with SU(2)L, we prove a no-go theorem: no discrete unbroken family
symmetry can produce the required mixing patterns. We then suggest avenues for
future research.Comment: 14 pages, no figures, RevTeX4, references adde
Hierarchical Quark Mass Matrices
I define a set of conditions that the most general hierarchical Yukawa mass
matrices have to satisfy so that the leading rotations in the diagonalization
matrix are a pair of (2,3) and (1,2) rotations. In addition to Fritzsch
structures, examples of such hierarchical structures include also matrices with
(1,3) elements of the same order or even much larger than the (1,2) elements.
Such matrices can be obtained in the framework of a flavor theory. To leading
order, the values of the angle in the (2,3) plane (s_{23}) and the angle in the
(1,2) plane (s_{12}) do not depend on the order in which they are taken when
diagonalizing. We find that any of the Cabbibo-Kobayashi-Maskawa matrix
parametrizations that consists of at least one (1,2) and one (2,3) rotation may
be suitable. In the particular case when the s_{13} diagonalization angles are
sufficiently small compared to the product s_{12}s_{23}, two special CKM
parametrizations emerge: the R_{12}R_{23}R_{12} parametrization follows with
s_{23} taken before the s_{12} rotation, and vice versa for the
R_{23}R_{12}R_{23} parametrization.Comment: LaTeX, 19 pages. References added, minor changes in text. Version
published in Phys. Rev.
Radiative Two Loop Inverse Seesaw and Dark Matter
Seesaw mechanism provides a natural explanation of light neutrino masses
through suppression of heavy seesaw scale. In inverse seesaw models the seesaw
scale can be much lower than that in the usual seesaw models. If terms inducing
seesaw masses are further induced by loop corrections, the seesaw scale can be
lowered to be in the range probed by experiments at the LHC without fine
tuning. In this paper we construct models in which inverse seesaw neutrino
masses are generated at two loop level. These models also naturally have dark
matter candidates. Although the recent data from Xenon100 put stringent
constraint on the models, they can be consistent with data on neutrino masses,
mixing, dark matter relic density and direct detection. These models also have
some interesting experimental signatures for collider and flavor physics.Comment: RevTex 14 pages 3 figures. Several references adde
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