G.C. Branco

Flavor physics of leptons and dipole moments

This chapter of the report of the “Flavor in the era of the LHC” Workshop discusses the theoretical, phenomenological and experimental issues related to flavor phenomena in the charged lepton sector and in flavor conserving CP-violating processes. We review the current experimental limits and the main theoretical models for the flavor structure of fundamental particles. We analyze the phenomenological consequences of the available data, setting constraints on explicit models beyond the standard model, presenting benchmarks for the discovery potential of forthcoming measurements both at the LHC and at low energy, and exploring options for possible future experiments.

Leptogenesis, CP violation and neutrino data: What can we learn?

A detailed analytic and numerical study of baryogenesis through leptogenesis is performed in the framework of the standard model of electroweak interactions extended by the addition of three right-handed neutrinos, leading to the seesaw mechanism. We analyze the connection between GUT-motivated relations for the quark and lepton mass matrices and the possibility of obtaining a viable leptogenesis scenario. In particular, we analyze whether the constraints imposed by SO(10) GUTs can be compatible with all the available solar, atmospheric and reactor neutrino data and, simultaneously, be capable of producing the required baryon asymmetry via the leptogenesis mechanism. It is found that the Just-So 2 and SMA solar solutions lead to a viable leptogenesis even for the simplest SO(10) GUT, while the LMA, LOW and VO solar solutions would require a different hierarchy for the Dirac neutrino masses in order to generate the observed baryon asymmetry. Some implications on CP violation at low energies and on neutrinoless double beta decay are also considered.

A bridge between CP violation at low energies and leptogenesis

We discuss the possibility of relating the size and sign of the observed baryon asymmetry of the universe to CP violation observable at low energies, in a framework where the observed baryon asymmetry is produced by leptogenesis through the out of the equilibrium decay of heavy Majorana neutrinos. We identify the CP violating phases entering in leptogenesis as well as those relevant for CP violation at low energies in the minimal seesaw model. We show that although in general there is no relation between these two sets of phases, there are specific frameworks in which such a connection may be established and we give a specific grand unification inspired example where such a connection does exist. We construct weak-basis invariants related to CP violation responsible for leptogenesis, as well as those relevant for CP violation at low energies.

Minimal Scenarios for Leptogenesis and CP Violation

The relation between leptogenesis and CP violation at low energies is analyzed in detail in the framework of the minimal seesaw mechanism. Working, without loss of generality, in a weak basis where both the charged lepton and the right-handed Majorana mass matrices are diagonal and real, we consider a convenient generic parametrization of the Dirac neutrino Yukawa coupling matrix and identify the necessary condition which has to be satisfied in order to establish a direct link between leptogenesis and CP violation at low energies. In the context of the LMA solution of the solar neutrino problem, we present minimal scenarios which allow for the full determination of the cosmological baryon asymmetry and the strength of CP violation in neutrino oscillations. Some specific realizations of these minimal scenarios are considered. The question of the relative sign between the baryon asymmetry and CP violation at low energies is also discussed.

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.

CP RESTRICTIONS ON QUARK MASS MATRICES

Abstract The general implications are studied of CP invariance on quark mass matrices within the standard SU(2) × U(1) gauge theory, with an arbitrary number of fermion families. In particular, a set of necessary conditions is derived for CP invariance that the quark mass matrices must satisfy when written in the weak eigenstate basis. These conditions are expressed in terms of traces of powers of the mass matrices. The violation of any of these conditions implies CP non-conservation by the charged current weak interactions. In the case of three generations, it is found that one of these conditions is also sufficient for CP invariance.

Relating the scalar flavour-changing neutral couplings to the CKM matrix

Abstract We build a class of two-Higgs-doublet models in which the flavour-changing couplings of the neutral scalars are related in an exact way to elements of the quark mixing matrix. In this framework, we explore the different possibilities for CP violation and find some interesting scenarios, like a realization of the superweak idea without CP violation in the B -meson system. In another scenario the neutral scalars can be very light, and their contributions to the B 0 - B 0 transitions can drastically alter the standard-model predictions for CP violation in that system.

The Higgs mass in a model with two scalar doublets and spontaneous CP violation

Abstract We examine the question of the value of the Higgs mass in models with two Higgs doublets and spontaneous CP violation. It is pointed out that in the presence of a softly broken permutation symmetry, the Higgs mass can be lower than one TeV, without having to resort to unnatural fine-tuning of parameters. This avoids a potential conflict with the validity of the perturbation expansion.

Geometrical T-violation

We investigate the possibility of having spontaneous T violation arising from complex vacuum expectation values with calculable phases, assuming geometrical values, entirely determined by the symmetry of the scalar potential.

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.