8 research outputs found
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
Flavor Symmetry L_e - L_mu - L_tau, Atmospheric Neutrino Mixing and CP Violation in the Lepton Sector
The PMNS neutrino mixing matrix is given, in general, by the product of two
unitary matrices associated with the diagonalization of the charged lepton and
neutrino mass matrices, respectively. Assuming that the active flavor neutrinos
possess a Majorana mass matrix which is diagonalized by a bimaximal mixing
matrix, we give the allowed forms of the charged lepton mixing matrix and the
corresponding implied forms of the charged lepton mass matrix. We further
assume that the origin of bimaximal mixing is a weakly broken flavor symmetry
of the neutrino Majorana mass matrix, corresponding to the conservation of the
non-standard lepton charge L' = L_e - L_mu - L_tau. The latter does not
predict, in general, the atmospheric neutrino mixing to be maximal. We study
the impact of this fact on the allowed forms of the charged lepton mixing
matrix and on the neutrino mixing observables, analyzing the case of CP
nonconservation in detail. We find, in particular, that the allowed ranges of
U_{e3} and of the atmospheric neutrino mixing angle are typically larger when
one assumes L' conservation than in the case of exact initial bimaximal mixing.
The atmospheric neutrino mixing can take any value inside its currently allowed
range, while |U_{e3}|^2 can be as small as 0.007. We discuss under which
conditions the atmospheric neutrino mixing angle can be larger or smaller than
pi/4. We present also a simple see-saw realization of the implied light
neutrino Majorana mass matrix and consider leptogenesis in this scenario.Comment: 26 pages, 3 figures. Comments and references added, to appear in PR
More on deviation from bi-maximal neutrino mixing
We study the case of presenting the exact bi-maximal mixing form with
inducing the deviation from the bi-maximal mixing in the final form of
the Pontecorvo-Maki-Nakagawa-Sakata neutrino mixing, . We
will show that such possibility will lead to a democratic texture for the
charged lepton mass matrix and to a neutrino mass matrix with four null
entries.Comment: a scenario realizing our proposal is added, new references added, to
be published in J.Phys.
The mu - e Conversion in Nuclei, mu --> e gamma, mu --> 3e Decays and TeV Scale See-Saw Scenarios of Neutrino Mass Generation
We perform a detailed analysis of lepton flavour violation (LFV) within
minimal see-saw type extensions of the Standard Model (SM), which give a viable
mechanism of neutrino mass generation and provide new particle content at the
electroweak scale. We focus, mainly, on predictions and constraints set on each
scenario from mu --> e gamma, mu --> 3e and mu - e conversion in the nuclei. In
this class of models, the flavour structure of the Yukawa couplings between the
additional scalar and fermion representations and the SM leptons is highly
constrained by neutrino oscillation measurements. In particular, we show that
in some regions of the parameters space of type I and type II see-saw models,
the Dirac and Majorana phases of the neutrino mixing matrix, the ordering and
hierarchy of the active neutrino mass spectrum as well as the value of the
reactor mixing angle theta_{13} may considerably affect the size of the LFV
observables. The interplay of the latter clearly allows to discriminate among
the different low energy see-saw possibilities.Comment: Expressions for the factors |C_{me}|^2 and |C_{mu3e}|^2 in the mu-e
conversion and mu-->3e decay rates, eqs. (36) and (49), respectively,
corrected; results in subsections 2.2 and 2.3 quantitatively changed,
qualitatively remain the same; figures 2, 3, 4 and 5 replace
Models of Neutrino Masses and Mixings
We review theoretical ideas, problems and implications of neutrino masses and
mixing angles. We give a general discussion of schemes with three light
neutrinos. Several specific examples are analyzed in some detail, particularly
those that can be embedded into grand unified theories.Comment: 44 pages, 2 figures, version accepted for publication on the Focus
Issue on 'Neutrino Physics' edited by F.Halzen, M.Lindner and A. Suzuki, to
be published in New Journal of Physics
Discrete Flavour Groups, \theta_13 and Lepton Flavour Violation
Discrete flavour groups have been studied in connection with special patterns
of neutrino mixing suggested by the data, such as Tri-Bimaximal mixing (groups
A4, S4...) or Bi-Maximal mixing (group S4...) etc. We review the predictions
for sin(\theta_13) in a number of these models and confront them with the
experimental measurements. We compare the performances of the different classes
of models in this respect. We then consider, in a supersymmetric framework, the
important implications of these flavour symmetries on lepton flavour violating
processes, like \mu -> e gamma and similar processes. We discuss how the
existing limits constrain these models, once their parameters are adjusted so
as to optimize the agreement with the measured values of the mixing angles. In
the simplified CMSSM context, adopted here just for indicative purposes, the
small tan(beta) range and heavy SUSY mass scales are favoured by lepton flavour
violating processes, which makes it even more difficult to reproduce the
reported muon g-2 discrepancy.Comment: 45 pages, 16 figures, 3 tables; V3 submitted to add an acknowledgment
to a Networ