The mu - e Conversion in Nuclei, mu --> e gamma, mu --> 3e Decays and TeV Scale See-Saw Scenarios of Neutrino Mass Generation

We perform a detailed analysis of lepton flavour violation (LFV) within minimal see-saw type extensions of the Standard Model (SM), which give a viable mechanism of neutrino mass generation and provide new particle content at the electroweak scale. We focus, mainly, on predictions and constraints set on each scenario from mu --> e gamma, mu --> 3e and mu - e conversion in the nuclei. In this class of models, the flavour structure of the Yukawa couplings between the additional scalar and fermion representations and the SM leptons is highly constrained by neutrino oscillation measurements. In particular, we show that in some regions of the parameters space of type I and type II see-saw models, the Dirac and Majorana phases of the neutrino mixing matrix, the ordering and hierarchy of the active neutrino mass spectrum as well as the value of the reactor mixing angle theta_{13} may considerably affect the size of the LFV observables. The interplay of the latter clearly allows to discriminate among the different low energy see-saw possibilities.Comment: Expressions for the factors |C_{me}|^2 and |C_{mu3e}|^2 in the mu-e conversion and mu-->3e decay rates, eqs. (36) and (49), respectively, corrected; results in subsections 2.2 and 2.3 quantitatively changed, qualitatively remain the same; figures 2, 3, 4 and 5 replace

Charged Lepton Flavour Violating Radiative Decays ℓi→ℓj+γ\ell_i \to \ell_j + \gamma in See-Saw Models with A4A_4 Symmetry

The charged lepton flavour violating (LFV) radiative decays, $\mu\to e+\gamma$, $\tau\to \mu+\gamma$ and $\tau\to e +\gamma$ are investigated in a class of supersymmetric $A_4$ models with three heavy right-handed (RH) Majorana neutrinos, in which the lepton (neutrino) mixing is predicted to leading order (LO) to be tri-bimaximal. The light neutrino masses are generated via the type I see-saw mechanism. The analysis is done within the framework of the minimal supergravity (mSUGRA) scenario, which provides flavour universal boundary conditions at the scale of grand unification $M_X \approx 2 \times 10^{16}$ GeV. Detailed predictions for the rates of the three LFV decays are obtained in two explicit realisations of the $A_4$ models due to Altarelli and Feruglio and Altarelli and Meloni, respectively.Comment: Results unchanged, minor improvements made; version accepted for publication in JHE

Fair scans of the seesaw. Consequences for predictions on LFV processes

Usual analyses based on scans of the seesaw parameter-space can be biassed since they do not cover in a fair way the complete parameter-space. More precisely, we show that in the common "R-parametrization", many acceptable R-matrices, compatible with the perturbativity of Yukawa couplings, are normally disregarded from the beginning, which produces biasses in the results. We give a straightforward procedure to scan the space of complex R-matrices in a complete way, giving a very simple rule to incorporate the perturbativity requirement as a condition for the entries of the R-matrix, something not considered before. As a relevant application of this, we show that the extended believe that BR(mu --> e, gamma) in supersymmetric seesaw models depends strongly on the value of theta_13 is an "optical effect" produced by such biassed scans, and does not hold after a careful analytical and numerical study. When the complete scan is done, BR(mu --> e, gamma) gets very insensitive to theta_13. Moreover, the values of the branching ratio are typically larger than those quoted in the literature, due to the large number of acceptable points in the parameter-space which were not considered before. Including (unflavoured) leptogenesis does not introduce any further dependence on theta_13, although decreases the typical value of BR(mu --> e, gamma).Comment: 22 pages, 5 figure

Sizeable \theta_13 from the Charged Lepton Sector in SU(5), (Tri-)Bimaximal Neutrino Mixing and Dirac CP Violation

The recent results from T2K and MINOS experiments point towards a relatively large value of the reactor angle \theta_13 in the lepton sector. In this paper we show how a large \theta_13 can arise from the charged lepton sector alone in the context of an SU(5) GUT. In such a scenario (tri-)bimaximal mixing in the neutrino sector is still a viable possibility. We also analyse the general implications of the considered scenario for the searches of CP violation in neutrino oscillations.Comment: 19 pages, 3 figures; version to be published in JHE

Neutrinoless Double Beta Decay in the Presence of Light Sterile Neutrinos

We investigate the predictions for neutrinoless double beta ((\u3b2\u3b2)0\u3bd -) decay effective Majorana mass |\u3008m\u3009| in the 3 + 1 and 3 + 2 schemes with one and two additional sterile neutrinos with masses at the eV scale. The two schemes are suggested by the neutrino oscillation interpretation of the reactor neutrino and Gallium \u201canomalies\u201d and of the data of the LSND and MiniBooNE experiments. We analyse in detail the possibility of a complete or partial cancellation between the different terms in |\u3008m\u3009|, leading to a strong suppression of |\u3008m\u3009|. We determine the regions of the relevant parameter spaces where such a suppression can occure. This allows us to derive the conditions under which the effective Majorana mass satisfies |\u3008m\u3009| > 0.01 eV, which is the range planned to be exploited by the next generation of (\u3b2\u3b2)0\u3bd -experiments. \ua9 SISSA 2013

Neutrino mixing and leptonic CP violation from S 4 flavour and generalised CP symmetries

We consider a class of models of neutrino mixing with S4 lepton flavour symmetry combined with a generalised CP symmetry, which are broken to residual Z2 and Z2 7 HCP\u3bd symmetries in the charged lepton and neutrino sectors, respectively, HCP\u3bd being a remnant CP symmetry of the neutrino Majorana mass term. In this set-up the neutrino mixing angles and CP violation (CPV) phases of the neutrino mixing matrix depend on three real parameters \u2014 two angles and a phase. We classify all phenomenologically viable mixing patterns and derive predictions for the Dirac and Majorana CPV phases. Further, we use the results obtained on the neutrino mixing angles and leptonic CPV phases to derive predictions for the effective Majorana mass in neutrinoless double beta decay

Renormalisation group corrections to neutrino mixing sum rules

Neutrino mixing sum rules are common to a large class of models based on the (discrete) symmetry approach to lepton flavour. In this approach the neutrino mixing matrix $U$ is assumed to have an underlying approximate symmetry form \tildeU_\nu, which is dictated by, or associated with, the employed (discrete) symmetry. In such a setup the cosine of the Dirac CP-violating phase $\delta$ can be related to the three neutrino mixing angles in terms of a sum rule which depends on the symmetry form of \tildeU_\nu. We consider five extensively discussed possible symmetry forms of \tildeU_\nu: i) bimaximal (BM) and ii) tri-bimaximal (TBM) forms, the forms corresponding to iii) golden ratio type A (GRA) mixing, iv) golden ratio type B (GRB) mixing, and v) hexagonal (HG) mixing. For each of these forms we investigate the renormalisation group corrections to the sum rule predictions for $\delta$ in the cases of neutrino Majorana mass term generated by the Weinberg (dimension 5) operator added to i) the Standard Model, and ii) the minimal SUSY extension of the Standard Model

Predictions for the Leptonic Dirac CP Violation Phase: a Systematic Phenomenological Analysis

We derive predictions for the Dirac phase $\delta$ present in the $3\times 3$ unitary neutrino mixing matrix $U = U_e^{\dagger} \, U_{\nu}$, where $U_e$ and $U_{\nu}$ are $3\times 3$ unitary matrices which arise from the diagonalisation respectively of the charged lepton and the neutrino mass matrices. We consider forms of $U_e$ and $U_{\nu}$ allowing us to express $\delta$ as a function of three neutrino mixing angles, present in $U$, and the angles contained in $U_{\nu}$. We consider several forms of $U_{\nu}$ determined by, or associated with, symmetries, tri-bimaximal, bimaximal, etc., for which the angles in $U_{\nu}$ are fixed. For each of these forms and forms of $U_e$ allowing to reproduce the measured values of the neutrino mixing angles, we construct the likelihood function for $\cos \delta$, using i) the latest results of the global fit analysis of neutrino oscillation data, and ii) the prospective sensitivities on the neutrino mixing angles. Our results, in particular, confirm the conclusion reached in earlier similar studies that the measurement of the Dirac phase in the neutrino mixing matrix, together with an improvement of the precision on the mixing angles, can provide unique information about the possible existence of symmetry in the lepton sector

Lepton flavor violating \u3c4 decays in TeV scale type i see-saw and Higgs triplet models

The lepton flavour violating (LFV) \u3c4 decays \u3c4 \u2192 (e, \u3bc)\u3b3 and \u3c4 \u2192 3\u3bc are investigated in the frameworks of the TeV scale type I see-saw and Higgs Triplet (or type II see-saw) models. Predictions for the rates of these processes are obtained. The implications of the existing stringent experimental upper bounds on the \u3bc \u2192 e + \u3b3 and \u3bc \u2192 3e decay branching ratios for the predictions of the \u3c4 \u2192 (e, \u3bc)\u3b3 and \u3c4 \u2192 3\u3bc decay rates are studied in detail. The possibilities to observe the indicated LFV \u3c4 decays in present and future experiments are analysed. \ua9 2013 SISSA, Trieste, Italy