الفهرس 
IntroductionOnly 14 pages are availabe for public view
Abstract
Summary
The masses in the quark sectors are set in strong hierarchy while the hierarchy
is smaller in the lepton sector. The mixing in the lepton sector is large (one mixing angle is close to maximal, one is large and the third is small), while the mixing in the quark sector is small. The horizontal or flavor symmetry
was proposed to give answers to all above mentioned observations. In this
thesis we study the lepton mixing and how to infer the flavor symmetry from
it. First we study the widely proposed mixing which is called tri-bimaximal
(TBM) and try to answer the question ”Is TBM an accidental symmetry”. The TBM mixing is not accidental if structures of the corresponding leptonic
mass matrices follow immediately from a certain (residual or broken) flavor
symmetry. We study the effect of the deviation of the mixing parameters
(angles and phase) from their TBM values on the TBM symmetry which is the
symmetry manifesting in the mass matrix. We show that possible deviations
from the TBM mixing can lead to strong modifications of the mass matrix and
strong violation of the TBM mass relations. As a result, the mass matrix may have an “anarchical” structure with random values of elements or it may have
some symmetry which differs from the TBM symmetry. Interesting examples
include matrices with texture zeros, matrices with certain “flavor alignment”
as well as hierarchical matrices with a two-component structure, where the
dominant and sub-dominant contributions have different symmetries. This
opens up new approaches to understand lepton mixing.
We study two methods to deal with flavor symmetry. The first is the
conventional way called (top - down) in which one can consider the invariance
of the Lagrangian under a certain group and finally try to obtain the desired
mixing and masses to check whether this group can be considered as the flavor
symmetry group or not. Following this method we study a model based on
the discrete group (27) ⋉ S2 account for the recent neutrino oscillation data
that deviate from TBM. Breaking of the (27) group with a certain vacuum
xialignment can lead to the observed deviations from TBM mixing, in particular nonzero 1-3 mixing and deviation of the 2-3 mixing from maximal. Some
correlations between mixing angle deviations from TBM resulted from the
model consistent with the recent neutrino data.
The other method is called (bottom - up) in which we study the residual
symmetry that manifests itself in the mass matrix and how to use it to obtain
the flavor symmetry group. The connection between the two methods is
studied also.
We then study another model based on the group (Z2)3 for the nonvanishing
value for the smallest mixing angle (θ13), we derive and find explicit
realizations of the (Z2)3 flavor symmetry which characterizes, for the neutrino mass matrix, uniquely a variant of the tripartite form, originally conceived to
lead to TBM mixing with θ13 = 0, so as to allow now for a non-tri-bimaximal
pattern with non-zero θ13. We impose this flavor symmetry in a setting including the charged leptons and we see that it can make room, through highe order terms involving new SM-singlet scalars, for the mass hierarchy of chargleptons. Moreover, within a type-I seesaw mechanism augmented with theflavor symmetry, certain patterns occurring in both the Dirac and the Majorananeutrino mass matrices can accommodate all types of mass hierarchies
in the effective neutrino mass matrix,but no lepton/baryon asymmetry can
be generated. Finally, we discuss how a type-II seesaw mechanism, when supplemented
with the flavor symmetry, could be used to interpret the observed
baryon asymmetry through leptogenesis.