2,172 research outputs found
Maximal Neutrino Mixing from Discrete Symmetry in Extra Dimensions
I review the construction of a model for lepton masses based on the flavour
symmetry group A4 x U(1) reproducing the so-called tri-bimaximal lepton mixing
scheme, in eccelent agreement with current data. The model predicts a neutrino
spectrum of normal hierarchy type, not far from degenerate. A testable relation
between neutrino masses is obtained. I shortly discuss also general
requirements for models based on spontaneously broken flavour symmetries, in
order to get a maximal atmospheric mixing angle.Comment: 3 pages, 1 figure, contribution to proc. of the Int. Europhysics
Conf. on High Energy Physics (HEP2005) July 21th-27th 2005, Lisboa, Portuga
Pieces of the Flavour Puzzle
An overview of the flavour problem is presented, with emphasis on the
theoretical efforts to find a satisfactory description of fermion masses and
mixing angles.Comment: 26 pages, 7 figure
Are neutrino masses modular forms?
We explore a new class of supersymmetric models for lepton masses and mixing
angles where the role of flavour symmetry is played by modular invariance. The
building blocks are modular forms of level N and matter supermultiplets, both
transforming in representations of a finite discrete group Gamma_N. In the
simplest version of these models, Yukawa couplings are just modular forms and
the only source of flavour symmetry breaking is the vacuum expectation value of
a single complex field, the modulus. In the special case where modular forms
are constant functions the whole construction collapses to a supersymmetric
flavour model invariant under Gamma_N, the case treated so far in the
literature. The framework has a number of appealing features. Flavon fields
other than the modulus might not be needed. Neutrino masses and mixing angles
are simultaneously constrained by the modular symmetry. As long as
supersymmetry is exact, modular invariance determines all higher-dimensional
operators in the superpotential. We discuss the general framework and we
provide complete examples of the new construction. The most economical model
predicts neutrino mass ratios, lepton mixing angles, Dirac and Majorana phases
uniquely in terms of the modulus vacuum expectation value, with all the
parameters except one within the experimentally allowed range. As a byproduct
of the general formalism we extend the notion of non-linearly realised
symmetries to the discrete case.Comment: 40 pages, 3 figures; added comments and a new section with an example
of normal ordering of neutrino masses; to appear in the book "From my vast
repertoire: the legacy of Guido Altarelli", S. Forte, A. Levy and G. Ridolfi,
ed
Sum rules for asymptotic form factors in scattering
At very large energies and in gauge theories, the
trilinear gauge boson vertices relevant for scattering are
related in a simple way to the gauge boson self-energies. We derive these
relations, both from the requirement of perturbative unitarity and from the
Ward Identities of the theory. Our discussion shows that, in general, it is
never possible to neglect vector boson self-energies when computing the form
factors which parametrize the helicity amplitudes. The
exclusion of the self-energy contributions would lead to estimates of the
effects wrong by orders of magnitudes. We propose a simple way of including the
self-energy contributions in an appropriate definition of the form factors.Comment: 16 pages, latex file, 2 eps figures, need epsfig.st
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