We consider the most general dimension 5 effective Lagrangian that can be built using only Standard Model fields plus right-handed neutrinos, and find that there exists a term that provides electroweak moments (i.e., couplings to the 𝑍 and photon) for the right-handed neutrinos. Such term has not been described previously in the literature. We discuss its phenomenology and the bounds that can be derived from LEP results and from the observation of the cooling process of red giants and supernovae. MotivationNeutrino physics has been a hot topic of research and discussion in the last thirty years, and especially since we have compelling evidence that the structure of their masses is highly nontrivial (for a review on the subject, see [1,2]). The remarkable smallness of these masses, at least a factor 10 5 lighter than the electron mass, is usually regarded as an indication that new physics should be involved in their generation.The definite nature of this new physics, of course, depends on the specific model one wishes to consider, and there are plenty of them: the seesaw mechanism [2], which is one of the most popular proposals, the several models for radiative generation of masses [3] and many more. Precisely this abundance of proposals makes appealing the possibility of studying the new physics associated to neutrinos in a model-independent way. This can be done most easily if the new particles are very heavy, by eliminating them from our description of low-energy physics; the result is called an effective theory 1 , and has been for long a powerful tool for examining neutrino physics (see, for example, two early but interesting applications in [5,6]).However, in our current situation, and given the present knowledge and unknowns about neutrino masses, a piece of the puzzle has been mainly ignored: as we don't know if the neutrino masses are Dirac or Majorana, right-handed neutrinos might be among the light degrees of freedom of the theory. If they are, they must be included in the effective theory, or otherwise it will be incomplete. In any case, we don't know yet the nature of neutrino masses, so it seems sensible to include them for the sake of generality. This constitutes our starting point: we want to inspect an effective theory which can describe all possible neutrino mass structures, and see what insight it can cast upon new physics effects;1 For an example of the procedure, see, for instance, [4].

Neutrinoless double beta (0νββ) decay can in general produce electrons of either chirality, in contrast with the minimal Standard Model (SM) extension with only the addition of the Weinberg operator, which predicts two left-handed electrons in the final state. We classify the lepton number violating (LNV) effective operators with two leptons of either chirality but no quarks, ordered according to the magnitude of their contribution to 0νββ decay. We point out that, for each of the three chirality assignments, e L e L , e L e R and e R e R , there is only one LNV operator of the corresponding type to lowest order, and these have dimensions 5, 7 and 9, respectively. Neutrino masses are always induced by these extra operators but can be delayed to one or two loops, depending on the number of RH leptons entering in the operator. Then, the comparison of the 0νββ decay rate and neutrino masses should indicate the effective scenario at work, which confronted with the LHC searches should also eventually decide on the specific model elected by nature. We also list the SM additions generating these operators upon integration of the heavy modes, and discuss simple realistic examples of renormalizable theories for each case.

The minimal Standard Model extension with the Weinberg operator does accommodate the observed neutrino masses and mixing, but predicts a neutrinoless double beta (0νββ) decay rate proportional to the effective electron neutrino mass, which can be then arbitrarily small within present experimental limits. However, in general 0νββ decay can have an independent origin and be near its present experimental bound; whereas neutrino masses are generated radiatively, contributing negligibly to 0νββ decay. We provide a realization of this scenario in a simple, well defined and testable model, with potential LHC effects and calculable neutrino masses, whose twoloop expression we derive exactly. We also discuss the connection of this model to others that have appeared in the literature, and remark on the significant differences that result from various choices of quantum number assignments and symmetry assumptions. In this type of models lepton flavor violating rates are also preferred to be relatively large, at the reach of foreseen experiments.Interestingly enough, in our model this stands for a large third mixing angle, sin 2 θ 13 > ∼ 0.008, when µ → eee is required to lie below its present experimental limit.

We reconsider the possibility that Majorana masses for the three known neutrinos are generated radiatively by the presence of a fourth generation and one right-handed neutrino with Yukawa couplings and a Majorana mass term. We find that the observed light neutrino mass hierarchy is not compatible with low energy universality bounds in this minimal scenario, but all present data can be accommodated with five generations and two right-handed neutrinos. Within this framework, we explore the parameter space regions which are currently allowed and could lead to observable effects in neutrinoless double beta decay, $\mu - e$ conversion in nuclei and $\mu \rightarrow e \gamma$ experiments. We also discuss the detection prospects at LHC.Comment: 28 pages, 4 figures. Version to be published. Some typos corrected. Improved figures 3 and

We consider the neutrino sector of a Standard Model with four generations. While the three light neutrinos can obtain their masses from a variety of mechanisms with or without new neutral fermions, fourth-generation neutrinos need at least one new relatively light right-handed neutrino. If lepton number is not conserved this neutrino must have a Majorana mass term whose size depends on the underlying mechanism for lepton number violation. Majorana masses for the fourth generation neutrinos induce relative large two-loop contributions to the light neutrino masses which could be even larger than the cosmological bounds. This sets strong limits on the mass parameters and mixings of the fourth generation neutrinos.Comment: To be published. Few typos corrected, references update