A4A_4 Flavour Model for Dirac Neutrinos: Type I and Inverse Seesaw

We propose two different seesaw models namely, type I and inverse seesaw to realise light Dirac neutrinos within the framework of $A_4$ discrete flavour symmetry. The additional fields and their transformations under the flavour symmetries are chosen in such a way that naturally predicts the hierarchies of different elements of the seesaw mass matrices in these two types of seesaw mechanisms. For generic choices of flavon alignments, both the models predict normal hierarchical light neutrino masses with the atmospheric mixing angle in the lower octant. Apart from predicting interesting correlations between different neutrino parameters as well as between neutrino and model parameters, the model also predicts the leptonic Dirac CP phase to lie in a specific range -$\pi/3$ to $\pi/3$. While the type I seesaw model predicts smaller values of absolute neutrino mass, the inverse seesaw predictions for the absolute neutrino masses can saturate the cosmological upper bound on sum of absolute neutrino masses for certain choices of model parameters.Comment: 19 pages, 08 figures; matches published versio

Type III Seesaw and Dark Matter in a Supersymmetric Left-Right Model

We propose a new supersymmetric left right model with Higgs doublets carrying odd B-L charge, higgs bidoublet and heavy Higgs triplets with zero B-L charge and a set of sterile neutrinos which are singlet under the gauge group. We show that spontaneous parity violation can be achieved naturally in this model and the neutrino masses arise from the so called type III seesaw mechanism. We also discuss the possible phenomenology in the context of neutrino masses and dark matter

Charged Lepton Flavour Violation and Neutrinoless Double Beta Decay in Left-Right Symmetric Models with Type I+II Seesaw

We study the new physics contributions to neutrinoless double beta decay ($0\nu\beta \beta$) half-life and lepton flavour violation (LFV) amplitude within the framework of the minimal left-right symmetric model (MLRSM). Considering all possible new physics contributions to $0\nu\beta \beta$ and charged lepton flavour violation $\mu \rightarrow e \gamma, \mu \rightarrow 3e$ in MLRSM, we constrain the parameter space of the model from the requirement of satisfying existing experimental bounds. Assuming the breaking scale of the left-right symmetry to be $\mathcal{O}(1)$ TeV accessible at ongoing and near future collider experiments, we consider the most general type I+II seesaw mechanism for the origin of tiny neutrino masses. Choosing the relative contribution of the type II seesaw term allows us to calculate the right handed neutrino mass matrix as well as Dirac neutrino mass matrix as a function of the model parameters, required for the calculation of $0\nu\beta \beta$ and LFV amplitudes. We show that such a general type I+II seesaw structure results in more allowed parameter space compared to individual type I or type II seesaw cases considered in earlier works. In particular, we show that the doubly charged scalar masses $M_{\Delta}$ are allowed to be smaller than the heaviest right handed neutrino mass $M_N$ from the present experimental bounds in these scenarios which is in contrast to earlier results with individual type I or type II seesaw showing $M_{\Delta} > M_N$.Comment: 23 pages and 21 figures. minor revision, to appear in JHEP. arXiv admin note: text overlap with arXiv:1509.0180