Made available in DSpace on 2015-05-14T12:14:11Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 354736 bytes, checksum: 7cf5aa1bfd0c39f4aa895f8ab4ba7e67 (MD5) Previous issue date: 2012-03-09 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / For a long time it is known that neutrinos have mass and are also able to oscilate between their flavor states. However, the Standard Model only contain massless neutrinos, what lead us to believe that, among other known issues of the model, it is not the final theory. If we want to give an explanation to the neutrino masses we would have to extend the Standard Model in such a way to naturally accommodate these tiny masses. A great number of mechanisms appeared for such matters and the Canonical Seesaw Mechanism was highly accepted for its simplicity and beautifulness. Simple because it demands the addition of a minimal set of fields possible to obtain the neutrino mass at the order of sub-eV. Beautiful because it requires the breaking of lepton number at the scale of Great Unification Theories, bringging effects of high energy theories to low energy ones. Nevertheless, its beauty has a price. It will be impossible for the Canonical Seesaw Mechanism to be tested in recent and future experiments, hence new mechanisms emerged with the possibility of being probed by the experiments. We propose a mechanism based on the Inverse Seesaw Mechanism, which gives rise to the neutrino mass at sub-eV relying on a tiny leptonic breaking scale μ ∼ KeV. The Inverse Seesaw is not able to explain, in a natural way, the smallness of the μ parameter and it is here that our modifications emerge. With the introduction of new scalar fields and assuming a Z5⊗Z2 symmetry it is possible to dinamicaly explain the smallness of μ and also recover the Canonical Seesaw formula for the neutrino masses. Along with that, the right-handed neutrinos are able to be at the eletroweak scale, hence it is possible to test the model in actual experiments. / Sabemos há vários anos que os neutrinos possuem massa, bem como oscilam entre seus estados de sabor. No entanto, o Modelo Padrão contém apenas neutrinos sem massa, o que nos leva a crer, juntamente com outros problemas conhecidos da teoria, que ele não é a teoria final. Se quisermos explicar as massas dos neutrinos, devemos estender o Modelo Padrão de tal forma a acomodar estas pequenas massas naturalmente. Muitos mecanismos com este fim surgiram e o Mecanismo Seesaw Canônico se destacou por sua simplicidade e beleza. Simples por exigir a menor modificação possível do Modelo Padrão para que ele seja realizado. Belo pois requer a quebra explícita do número leptônico em uma escala de energia da ordem da escala das Teorias de Grande Unificação, trazendo efeitos de teorias a altas energias para teorias a baixas energias. Contudo, sua beleza tem um preço. Em experimentos recentes e futuros é impossível que o Mecanismo Seesaw Canônico seja testado, consequentemente novos mecanismos surgiram com a possibilidade de que os experimentos possam comprová-los. Nossa proposta é baseada no Mecanismo Seesaw Inverso, cujo objetivo é gerar a massa dos neutrinos da ordem de sub-eV através de uma pequena escala de quebra do número leptônico μ ∼ KeV. O Seesaw Inverso não é capaz de explicar de uma forma natural a pequenez do parâmetro μ e é neste ponto que nossas modificações surgem. Com a introdução de novos campos escalares e assumindo uma simetria Z5 ⊗ Z2 é possível explicar dinamicamente a pequenez de μ e também recuperar a fórmula para as massas dos neutrinos obtidas no Seesaw Canônico. Juntamente com isso, os neutrinos de mão-direita podem ter massas até da ordem da escala Eletro-Fraca, portanto passa a ser possível que este modelo seja testado em experimentos atuais.

In this doctoral thesis, we study both low- and high-energy observables related to massive neutrinos. Neutrino oscillations have provided indisputable evidence in favour of non-zero neutrino masses and mixings. However, the original formulation of the Standard Model cannot account for these observations, which calls for the introduction of new Physics. Among many possibilities, we focus here on the inverse seesaw, a neutrino mass generation mechanism in which the Standard Model is extended with fermionic gauge singlets. This model offers an attractive alternative to the usual seesaw realisations since it can potentially have natural Yukawa couplings (O(1)) while keeping the new Physics scale at energies within reach of the LHC. Among the many possible effects, this scenario can lead to deviations from lepton flavour universality. We have investigated these signatures and found that the ratios R_K and R_π provide new, additional constraints on the inverse seesaw. We have also considered the embedding of the inverse seesaw in supersymmetric models. This leads to increased rates for various lepton flavour violating processes, due to enhanced contributions from penguin diagrams mediated by the Higgs and Z bosons. Finally, we also found that the new invisible decay channels associated with the sterile neutrinos present in the supersymmetric inverse seesaw could significantly weaken the constraints on the mass and couplings of a light CP-odd Higgs boson.

Dans cette thèse, nous étudions à la fois des observables de basse et de haute énergie liée à la présence de neutrinos massifs. Les oscillations de neutrinos ont apporté des preuves indiscutables en faveur de l'existence de masses non-nulles et de mélanges. Néanmoins, la formulation originale du Modèle Standard ne permet pas d'expliquer ces observations, d'où la nécessité d'introduire de nouveaux modèles. Parmi de nombreuses possibilités, nous nous concentrons ici sur le seesaw inverse, un mécanisme générant des neutrinos massifs par l'ajout de fermions singulets de jauge au Modèle Standard. Ce modèle offre une alternative attractive aux réalisations habituelles du seesaw puisqu'il a des couplages de Yukawa potentiellement naturels (O(1)) tout en conservant l'échelle de la nouvelle physique à des énergies accessibles au LHC. Parmi de nombreux effets, ce scénario peut générer de larges écarts à l'universalité leptonique. Nous avons étudié ces signatures et trouvé que les rapports R_K et R_π constituent de nouvelles contraintes pour le seesaw inverse. Nous nous sommes aussi intéressé à l'intégration de l'inverse seesaw dans différents modèles supersymétriques. Ceci conduit à une augmentation de la section efficace de divers processus violant la saveur leptonique du fait de contributions plus importantes venant des diagrammes pingouins comportant un boson de Higgs ou Z. Finalement, nous avons aussi trouvé que les nouveaux canaux de désintégration ouverts par la présence de neutrinos stériles dans les modèles de seesaw inverse supersymétriques peuvent significativement relaxer les contraintes sur la masse et les couplages d'un boson de Higgs CP-impair. / In this doctoral thesis, we study both low- and high-energy observables related to massive neutrinos. Neutrino oscillations have provided indisputable evidence in favour of non-zero neutrino masses and mixings. However, the original formulation of the Standard Model cannot account for these observations, which calls for the introduction of new Physics. Among many possibilities, we focus here on the inverse seesaw, a neutrino mass generation mechanism in which the Standard Model is extended with fermionic gauge singlets. This model offers an attractive alternative to the usual seesaw realisations since it can potentially have natural Yukawa couplings (O(1)) while keeping the new Physics scale at energies within reach of the LHC. Among the many possible effects, this scenario can lead to deviations from lepton flavour universality. We have investigated these signatures and found that the ratios R_K and R_π provide new, additional constraints on the inverse seesaw. We have also considered the embedding of the inverse seesaw in supersymmetric models. This leads to increased rates for various lepton flavour violating processes, due to enhanced contributions from penguin diagrams mediated by the Higgs and Z bosons. Finally, we also found that the new invisible decay channels associated with the sterile neutrinos present in the supersymmetric inverse seesaw could significantly weaken the constraints on the mass and couplings of a light CP-odd Higgs boson.

Experiments on neutrino oscillations have confirmed that neutrinos have small, but non-zero masses, and that the interacting neutrino states do not have definite masses, but are mixtures of such states.The seesaw models make up a group of popular models describing the small neutrino masses and the corresponding mixing.In these models, new, heavy fields are introduced and the neutrino masses are suppressed by the ratio between the electroweak scale and the large masses of the new fields. Usually, the new fields introduced have masses far above the electroweak scale, outside the reach of any foreseeable experiments, making these versions of seesaw models essentially untestable. However, there are also so-called low-scale seesaw models, where the new particles have masses above the electroweak scale, but within the reach of future experiments, such as the LHC.In quantum field theories, quantum corrections generally introduce an energy-scale dependence on all their parameters, described by the renormalization group equations. In this thesis, the energy-scale dependence of the neutrino parameters in two low-scale seesaw models, the low-scale type I and inverse seesaw models, are considered. Also, the question of whether the neutrinos are Majorana particles, \ie , their own antiparticles, has not been decided experimentally. Future experiments on neutrinoless double beta decay could confirm the Majorana nature of neutrinos. However, there could also be additional contributions to the decay, which are not directly related to neutrino masses. We have investigated the possible future bounds on the strength of such additional contributions to neutrinoless double beta decay, depending on the outcome of ongoing and planned experiments related to neutrino masses. / QC 20110812

The standard model of particle physics can excellently describe the vast majorityof data of particle physics experiments. However, in its simplest form, it cannot account for the fact that the neutrinos are massive particles and lepton flavorsmixed, as required by the observation of neutrino oscillations. Hence, the standardmodel must be extended in order to account for these observations, opening up thepossibility to explore new and interesting physical phenomena. There are numerous models proposed to accommodate massive neutrinos. Thesimplest of these are able to describe the observations using only a small numberof effective parameters. Furthermore, neutrinos are the only known existing particleswhich have the potential of being their own antiparticles, a possibility that isactively being investigated through experiments on neutrinoless double beta decay.In this thesis, we analyse these simple models using Bayesian inference and constraintsfrom neutrino-related experiments, and we also investigate the potential offuture experiments on neutrinoless double beta decay to probe other kinds of newphysics. In addition, more elaborate theoretical models of neutrino masses have beenproposed, with the seesaw models being a particularly popular group of models inwhich new heavy particles generate neutrino masses. We study low-scale seesawmodels, in particular the resulting energy-scale dependence of the neutrino parameters,which incorporate new particles with masses within the reach of current andfuture experiments, such as the LHC. / Standardmodellen för partikelfysik beskriver den stora majoriteten data från partikelfysikexperimentutmärkt. Den kan emellertid inte i sin enklaste form beskrivadet faktum att neutriner är massiva partiklar och leptonsmakerna är blandande,vilket krävs enligt observationerna av neutrinooscillationer. Därför måste standardmodellenutökas för att ta hänsyn till detta, vilket öppnar upp möjligheten att utforska nya och intressanta fysikaliska fenomen. Det finns många föreslagna modeller för massiva neutriner. De enklaste av dessakan beskriva observationerna med endast ett fåtal effektiva parametrar. Dessutom är neutriner de enda kända befintliga partiklar som har potentialen att vara sinaegna antipartiklar, en möjlighet som aktivt undersöks genom experiment på neutrinolöst dubbelt betasönderfall. I denna avhandling analyserar vi dessa enkla modellermed Bayesisk inferens och begränsningar från neutrinorelaterade experiment och undersöker även potentialen för framtida experiment på neutrinolöst dubbelt betasönderfall att bergänsa andra typer av ny fysik. Även mer avancerade teoretiska modeller för neutrinomassor har föreslagits, med seesawmodeller som en särskilt populär grupp av modeller där nya tunga partiklargenererar neutrinomassor. Vi studerar seesawmodeller vid låga energier, i synnerhetneutrinoparametrarnas resulterande energiberoende, vilka inkluderar nya partiklarmed massor inom räckh°all för nuvarande och framtida experiment såsom LHC. / <p>QC 20130830</p>