J. I. Silva-Marcos

Leptonic invariants, neutrino mass-ordering and the octant of θ 23

A bstractWe point out that leptonic weak-basis invariants are an important tool for the study of the properties of lepton flavour models. In particular, we show that appropriately chosen invariants can give a clear indication of whether a particular lepton flavour model favours normal or inverted hierarchy for neutrino masses and what is the octant of θ23. These invariants can be evaluated in any conveniently chosen weak-basis and can also be expressed in terms of neutrino masses, charged lepton masses, mixing angles and CP violation phases.

DEGENERATE AND QUASIDEGENERATE MAJORANA NEUTRINOS

We study mixing and CP violation of three left-handed Majorana neutrinos in the limit of exactly degenerate masses, identify the weak-basis invariant relevant for CP violation and show that the leptonic mixing matrix is parametrized only by two angles and one phase. After the lifting of the degeneracy, this parametrization accommodates the present data on atmospheric and solar neutrinos, as well as double beta decay. Some of the leptonic mixing ansatze suggested in the literature correspond to special cases of this parametrization.

Flavour physics of the RS model with KK masses reachable at LHC

The version of the higher-dimensional Randall-Sundrum (RS) model with matter in the bulk, which addresses the gauge hierarchy problem, has additional attractive features. In particular, it provides an intrinsic geometrical mechanism that can explain the origin of the large mass hierarchies among the Standard Model fermions. Within this context, a good solution for the gauge hierarchy problem corresponds to low masses for the Kaluza-Klein (KK) excitations of the gauge bosons. Some scenarios have been proposed in order to render these low masses (down to a few TeV) consistent with precision electroweak measurements. Here, we give specific and complete realizations of this RS version with small KK masses, down to 1 TeV, which are consistent with the entire structure of the fermions in flavour space: (1) all the last experimental data on quark/lepton masses and mixing angles (including massive neutrinos of Dirac type) are reproduced, (2) flavour changing neutral current constraints are satisfied and (3) the effective suppression scales of non-renormalizable interactions (in the physical basis) are within the bounds set by low energy flavour phenomenology. Our result, on the possibility of having KK gauge boson modes as light as a few TeV, constitutes one of the first theoretical motivations for experimental searches of direct signatures at the LHC collider, of this interesting version of the RS model which accommodates fermion masses.

Universal strength for Yukawa couplings

Abstract We suggest that the Yukawa couplings which are responsible for the generation of quark masses and charged current mixings have all the same strength. It is shown that the observed pattern of mixing angles and the spectrum of quark masses is consistent with the hypothesis of universal strength for Yukawa couplings.

Neutrinos in a warped extra dimension

Amongst the diverse propositions for extra dimensional scenarios, the model of Randall and Sundrum (RS), which offers a solution for the long standing puzzle of the gauge hierarchy problem, has attracted considerable attention from both the theoretical and experimental points of view. In the context of the RS model with gauge bosons and fermions living in the bulk, a novel type of mechanism has arisen for interpreting the strong mass hierarchy of the Standard Model fermions. This purely geometrical mechanism is based on a type and flavor dependent localization of fermions along a warped extra dimension. Here, we find concrete realizations of this mechanism, reproducing all the present experimental data on masses and mixings of the entire leptonic sector. We consider the case of Dirac neutrino masses (due to an additional right handed neutrino) where the various constraints on RS parameter space are taken into account. The scenarios, elaborated in this paper, generate the entire lepton mass hierarchy and mixing, essentially, via the higher-dimensional mechanism, as the Yukawa coupling dependence is chosen to be minimal. In addition, from the above mechanism, we predict the lepton mixing angle 10−5sin θ1310−1, a neutrino mass spectrum with normal hierarchy and the smallest neutrino mass to lie in the range: 10−11eVmν110−2eV. A large part of the sin θ13 interval should be testable in future neutrino experiments.

Predicting VCKM with universal strength of Yukawa couplings

We study the question of calculability of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements within the framework of universal strength for Yukawa couplings (USY). We first classify all solutions leading to

Leptogenesis, Yukawa textures and weak basis invariants

m_u=m_d=0

New approach to the Yukawa puzzle

within USY and then suggest a highly predictive ansatz where all the moduli of the CKM matrix elements are correctly predicted in terms of quark mass ratios, with no free parameters.

Non-hermitian Yukawa couplings?

We show that a large class of sets of leptonic texture zeros considered in the literature imply the vanishing of certain CP-odd weak-basis invariants. These invariant conditions enable one to recognize a flavour model corresponding to a set of texture zeros, when written in an arbitrary weak-basis where the zeros are not manifest. We also analyse the role of texture zeros in allowing for a connection between leptogenesis and low-energy leptonic masses, mixing and CP violation. For some of the textures the variables relevant for leptogenesis can be fully determined in terms of low energy parameters and heavy neutrino masses.

What if the masses of the first two quark families are not generated by the standard model Higgs boson

We do a systematic analysis of the question of the calculability of CKM matrix elements in terms of quark-mass ratios, within the framework of the hypothesis of the universality of strength for Yukawa couplings (USY), where all Yukawa couplings have equal moduli, and the flavor dependence is only in their phases. We use the fact that the limit m{sub u}=m{sub d}=0 is especially simple in the USY, to construct the various {ital Ans{umlt a}tze}. It is shown that the experimentally observed CKM matrix can be obtained within USY {ital Ans{umlt a}tze} corresponding to simple relations among phases of Yukawa couplings. Within the USY, one finds a natural explanation why Cabibbo mixing is significantly larger than the other CKM mixings. In the most successful of the USY {ital Ans{umlt a}tze}, one obtains, in leading order, {vert_bar}V{sub us}{vert_bar}={radical}(m{sub d}/m{sub s}), {vert_bar}V{sub cb}{vert_bar}={radical}(2)(m{sub s}/m{sub b}), {vert_bar}V{sub ub}{vert_bar}=(1/{radical}(2)){radical}(m{sub d}m{sub s}/m{sub b}{sup 2}), and {vert_bar}V{sub td}{vert_bar}=3{vert_bar}V{sub ub}{vert_bar}. We study the behavior of this USY {ital Ansatz} under the renormalization group. {copyright} {ital 1997} {ital The American Physical Society}