[en] The observation of neutrino masses and lepton mixing has highlighted the incompleteness of the Standard Model of particle physics. In conjunction with this discovery, new questions arise: why are the neutrino masses so small, which form has their mass hierarchy, why is the mixing in the quark and lepton sectors so different or what is the structure of the Higgs sector. In order to address these issues and to predict future experimental results, different approaches are considered. One particularly interesting possibility, are Grand Unified Theories such as SU(5) or SO(10). GUTs are vertical symmetries since they unify the SM particles into multiplets and usually predict new particles which can naturally explain the smallness of the neutrino masses via the seesaw mechanism. On the other hand, also horizontal symmetries, i.e., flavor symmetries, acting on the generation space of the SM particles, are promising. They can serve as an explanation for the quark and lepton mass hierarchies as well as for the different mixings in the quark and lepton sectors. In addition, flavor symmetries are significantly involved in the Higgs sector and predict certain forms of mass matrices. This high predictivity makes GUTs and flavor symmetries interesting for both, theorists and experimentalists. These extensions of the SM can be also combined with theories such as supersymmetry or extra dimensions. In addition, they usually have implications on the observed matter-antimatter asymmetry of the universe or can provide a dark matter candidate. In general, they also predict the lepton flavor violating rare decays μ → eγ, τ → μγ, and τ → eγ which are strongly bounded by experiments but might be observed in the future. In this thesis, we combine all of these approaches, i.e., GUTs, the seesaw mechanism and flavor symmetries. Moreover, our request is to develop and perform a systematic model building approach with flavor symmetries and to search for phenomenological implications. This provides a new perspective in model building since it allows us to screen models by its predictions on the theoretical and phenomenological side, i.e., we can apply further model constraints to single out a desired model. The results of our approach are, e.g., diverse lepton flavor and GUT models, a systematic scan of lepton flavor violation, new mass matrices, a new understanding of lepton mixing angles, a general extension of the idea of quark-lepton complementarity θ₁₂ ∼ π/4-ε/√(2) and for the first time the QLC relation in an SU(5) GUT. (orig.)

[en] We construct a class of supersymmetric SU(5) grand unified theory (GUT) models that produce nearly tribimaximal lepton mixing, the observed quark mixing matrix, and the quark and lepton masses, from discrete non-Abelian flavor symmetries. The SU(5) GUTs are formulated on five-dimensional throats in the flat limit and the neutrino masses become small due to the type-I seesaw mechanism. The discrete non-Abelian flavor symmetries are given by semidirect products of cyclic groups that are broken at the infrared branes at the tip of the throats. As a result, we obtain SU(5) GUTs that provide a combined description of non-Abelian flavor symmetries and quark-lepton complementarity.

[en] We systematically construct realistic mass matrices for the type-I seesaw mechanism out of more than 20x10¹² possibilities. We use only very generic assumptions from extended quark-lepton complementarity, i.e., the leptonic mixing angles between flavor and mass eigenstates are either maximal or parametrized by a single small quantity ε that is of the order of the Cabibbo angle ε≅θ_C. The small quantity ε also describes all fermion mass hierarchies. We show that special cases often considered in the literature, such as having a symmetric Dirac mass matrix or small mixing among charged leptons, constitute only a tiny fraction of our possibilities. Moreover, we find that in most cases the spectrum of right-handed neutrino masses is only mildly hierarchical. As a result, we provide for the charged leptons and neutrinos a selected list of 1981 qualitatively different Yukawa coupling matrices (or textures) that are parametrized by the Cabibbo angle and allow for a perfect fit to current data. In addition, we also briefly show how the textures could be generated in explicit models from flavor symmetries

[en] We present numerous simple models for fermion masses realizing quark-lepton complementarity, i.e., the solar mixing angle θ₁₂ obeys the sum-rule θ₁₂+θ_C∼π/4, where θ_C is the Cabibbo angle. As flavor symmetries, we use products of cyclic groups. The fermion mass hierarchies are generated by higher-dimension operators via the Froggatt-Nielsen mechanism. Neutrino masses emerge exclusively from the usual type-I seesaw mechanism. We focus on solutions that can naturally give close to tri-bimaximal lepton mixing with a very small reactor angle θ₁₃<<θ_C. Hierarchical fermion mass ratios and the mixing angles are predicted as powers of a single parameter of the order of θ_C. These models are rather general in the sense that large mixings can come from the charged leptons and/or neutrinos. Moreover, in the neutrino sector, both left- and right-handed neutrinos can mix maximally. This could be the result of quark-lepton unification in grand unified theories

[en] We present a systematic group space scan of discrete Abelian flavor symmetries for lepton mass models that produce nearly tribimaximal lepton mixing. In our models, small neutrino masses are generated by the type-I seesaw mechanism. The lepton mass matrices emerge from higher-dimension operators via the Froggatt-Nielsen mechanism and are predicted as powers of a single expansion parameter ε that is of the order of the Cabibbo angle θ_C ≅ 0.2. We focus on solutions that can give close to tribimaximal lepton mixing with a very small reactor angle θ₁₃ ∼ 0 and find several thousand explicit such models that provide an excellent fit to current neutrino data. The models are rather general in the sense that large leptonic mixings can come from the charged leptons and/or neutrinos. Moreover, in the neutrino sector, both left- and right-handed neutrinos can mix maximally. We also find a new relation θ₁₃∼< O(ε³) for the reactor angle and a new sum rule θ₂₃ ∼ π/4+ε/(2)^1/2 for the atmospheric angle, allowing the models to be tested in future neutrino oscillation experiments

[en] We survey the lepton flavor violation decay rates Br(μ→eγ), Br(τ→μγ), and Br(τ→eγ), in mSUGRA at the level of the fundamental Lagrangian for a broad class of non-trivial lepton mass matrices that give nearly tribimaximal lepton mixing. SUSY SU(5) GUT models with non-Abelian discrete flavor symmetries as feasible origin will be presented. Moreover, we study the lepton flavor violation branching ratios for the most general CP-violating forms of the mass matrices. We show that the branching ratios are roughly enhanced by an order of magnitude as compared to the real case. The branching ratios exhibit, however, a strong dependence on the choice of the phases. In particular, for general CP-phases, the LFV-rates appear to be largely uncorrelated with the possible high- and low-energy lepton mixing parameters

[en] Current neutrino data are consistent with the so-called tribimaximal mixing scenario, which predicts sin²θ₁₂=1/3, zero U_e3 and maximal θ₂₃. This implies a special form of the neutrino mass matrix. Introducing small breaking terms in this mass matrix generates deviations from the tribimaximal scheme and leads to testable correlations between the parameters. They depend on where the perturbation is located in the mass matrix. A special case of such perturbations are radiative corrections. Alternative deviations from tribimaximal mixing may stem from contributions of the charged lepton sector. If there is quark-lepton unification and it is the CKM matrix which corrects the tribimaximal mixing scheme, then almost maximal CP violation and sizable deviation from zero U_e3 are implied

[en] We study perturbations of exactly tri-bimaximal neutrino mixings under the assumption that they are coming solely from the charged lepton mass matrix. This may be plausible in scenarios where the mass generation mechanisms of neutrinos and charged leptons/quarks have a different origin. As a working hypothesis, we assume mass textures which may be generated by the Froggatt-Nielsen mechanism for the charged lepton and quark sectors, which generically leads to strong hierarchies, whereas the neutrino sector is exactly tri-bimaximal with a mild (normal) hierarchy. We find that in this approach, deviations from maximal atmospheric mixing can be introduced without affecting θ₁₃ and θ₁₂, whereas a deviation of θ₁₃ or θ₁₂ from its tri-bimaximal value will inevitably lead to a similar-sized deviation of the other parameter. Therefore, the already very precise knowledge of θ₁₂ points towards small sin²2θ₁₃≤0.01. The magnitude of this deviation can be controlled by the specific form of the charged lepton texture.

[en] We systematically investigate the parameter space of neutrino and charged lepton mass matrices for textures motivated by an extended quark-lepton complementarity. As the basic hypothesis, we postulate that all mixing angles in U_l and U_ν be either maximal or described by powers of a single small quantity ε≅θ_C. All mass hierarchies are described by this ε as well. In this study, we do not assume specific forms for U_l and U_ν, such as large mixing coming from the neutrino sector only. We perform a systematic scan of the 262144 generated mixing matrices for being compatible with current experimental data, and find a sample of 2468 possibilities. We then analyze and classify the effective charged lepton and neutrino mass textures, where we especially focus on a subset of models getting under pressure for small θ₁₃. In addition, we predict the mixing angle distributions from our sample of all valid textures, and study the robustness of this prediction. We also demonstrate how our procedure can be extended to predictions of the Dirac and Majorana phases in U_PMNS. For instance, we find that CP conservation in neutrino oscillations is preferred, and we can impose a lower bound on the mixing matrix element for 0νββ decay