Extracting Majorana Properties in the Throat of Neutrinoless Double Beta Decay

Assuming that neutrinos are Majorana particles, we explore what information can be inferred from future strong limits (i.e. non-observation) for neutrinoless double beta decay. Specifically we consider the case where the mass hierarchy is normal and the different contributions to the effective mass $\langle m \rangle_{ee}$ partly cancel. We discuss how this fixes the two Majorana CP phases simultaneously from the Majorana Triangle and how it limits the lightest neutrino mass $m_1$ within a narrow window. The two Majorana CP phases are in this case even better determined than in the usual case for larger $\langle m \rangle_{ee}$. We show that the uncertainty in these predictions can be significantly reduced by the complementary measurement of reactor neutrino experiments, especially the medium baseline version JUNO/RENO-50. We also estimate the necessary precision on $\langle m \rangle_{ee}$ to infer non-trivial Majorana CP phases and the upper limit \langle m \rangle_{ee} \lesssim 1\,\mbox{meV} sets a target for the design of future neutrinoless double beta decay experiments.Comment: 21 pages, 10 figure

Dark matter and U(1)' symmetry for the right-handed neutrinos

We consider a U(1)' gauge symmetry acting on three generations of right-handed neutrinos. The U(1)' symmetry is broken at the TeV scale and its remnant discrete symmetry makes one of the right-handed neutrinos stable. As a natural consequence of the anomaly cancellation, the neutrino mass matrix consists of a combination of Type I (TeV scale) seesaw and radiative correction. The stable right-handed neutrino communicates with the Standard Model via s-channel exchange of the Higgs field and the U(1)' gauge boson, so that the observed relic density for dark matter is obtained in a wide range of the parameter space. The experimental signatures in collider and other experiments are briefly discussed.Comment: 16 pages, 4 figure

Revisiting Large Neutrino Magnetic Moments

Current experimental sensitivity on neutrino magnetic moments is many orders of magnitude above the Standard Model prediction. A potential measurement of next-generation experiments would therefore strongly request new physics beyond the Standard Model. However, large neutrino magnetic moments generically tend to induce large corrections to the neutrino masses and lead to fine-tuning. We show that in a model where neutrino masses are proportional to neutrino magnetic moments. We revisit, discuss and propose mechanisms that still provide theoretical consistent explanations for a potential measurement of large neutrino magnetic moments. We find only two viable mechanisms to realize large transition magnetic moments for Majorana neutrinos only.Comment: 12 pages, 7 figure

Discovery reach for non-standard interactions in a neutrino factory

We study the discovery reach for Non-Standard Interactions (NSIs) in a neutrino factory experiment. After giving a theoretical, but model-independent, overview of the most relevant classes of NSIs, we present detailed numerical results for some of them. Our simulations take into account matter effects, uncertainties in the neutrino oscillation parameters, systematical errors, parameter correlations, and degeneracies. We perform scans of the parameter space, and show that a neutrino factory has excellent prospects of detecting NSIs originating from new physics at around 1 TeV, which is a scale favored by many extensions of the standard model. It will also turn out that the discovery reach depends strongly on the standard and non-standard CP violating phases in the Lagrangian.Comment: RevTeX 4, 10 pages, 5 figures, extended discussion of systematical errors and of existing bounds, matches published versio

Consistency test of neutrinoless double beta decay with one isotope

We discuss a consistency test which makes it possible to discriminate unknown nuclear background lines from neutrinoless double beta decay with only one isotope. By considering both the transition to the ground state and to the first excited $0^+$ state, a sufficiently large detector can reveal if neutrinoless double beta decay or some other nuclear physics process is at work. Such a detector could therefore simultaneously provide a consistency test for a certain range of Majorana masses and be sensitive to lower values of the effective Majorana mass.Comment: 1+12 pages, 4 figures; v2: discussion enhanced, figures improved, matches journal versio

See-saw Mechanisms for Dirac and Majorana Neutrino Masses

We investigate the see-saw mechanism for generally non-fine-tuned $n \times n$ mass matrices involving both Dirac and Majorana neutrinos. We specifically show that the number of naturally light neutrinos cannot exceed half of the dimension of the considered mass matrix. Furthermore, we determine a criterion for mass matrix textures leading to light Dirac neutrinos with the see-saw mechanism. Especially, we study $4 \times 4$ and $6 \times 6$ mass matrix textures and give some examples in order to highlight these types of textures. Next, we present a model scheme based on non-Abelian and discrete symmetries fulfilling the above mentioned criterion for light Dirac neutrinos. Finally, we investigate the connection between symmetries and the invariants of a mass matrix on a formal level.Comment: 12 pages, RevTeX. Final version to be published in Phys. Rev.

The Inverse Seesaw in Conformal Electro-Weak Symmetry Breaking and Phenomenological Consequences

We study the inverse seesaw mechanism for neutrino masses and phenomenological consequences in the context of conformal electro-weak symmetry breaking. The main difference to the usual case is that all explicit fermion mass terms including Majorana masses for neutrinos are forbidden. All fermion mass terms arise therefore from vacuum expectation values of suitable scalars times some Yukawa couplings. This leads to interesting consequences for model building, neutrino mass phenomenology and the Dark Matter abundance. In the context of the inverse seesaw we find a favoured scenario with heavy pseudo-Dirac sterile neutrinos at the TeV scale, which in the conformal framework conspire with the electro-weak scale to generate keV scale warm Dark Matter. The mass scale relations provide naturally the correct relic abundance due to a freeze-in mechanism. We demonstrate also how conformal symmetry decouples the right-handed neutrino mass scale and effective lepton number violation. We find that lepton flavour violating processes can be well within the reach of modern experiments. Furthermore, interesting decay signatures are expected at the LHC.Comment: 12 pages, 6 figures, new particles, Journal Version with minor changes and new citation

Minimal Radiative Neutrino Masses

We conduct a systematic search for neutrino mass models which only radiatively produce the dimension-5 Weinberg operator. We thereby do not allow for additional symmetries beyond the Standard Model gauge symmetry and we restrict ourselves to minimal models. We also include stable fractionally charged and coloured particles in our search. Additionally, we proof that there is a unique model with three new fermionic representations where no new scalars are required to generate neutrino masses at loop level. This model further has a potential dark matter candidate and introduces a general mechanism for loop-suppression of the neutrino mass via a fermionic ladderComment: final version as published in JHE

Confronting Flavour Symmetries and extended Scalar Sectors with Lepton Flavour Violation Bounds

We discuss the tension between discrete flavour symmetries and extended scalar sectors arising from lepton flavour violation experiments. The key point is that extended scalar sectors will generically lead to flavour changing neutral currents, which are strongly constrained by experiments. Due to the large parameter space in the scalar sector such models will, however, usually have no big problems with existing and future bounds (even though the models might be constrained). This changes considerably once a flavour symmetry is imposed in addition: Due to the symmetry, additional relations between the different couplings arise and cancellations become impossible in certain cases. The experimental bounds will then constrain the model severely and can easily exclude it. We consider two examples which show how these considerations are realized. The same logic should apply to a much wider class of models.Comment: 19 pages, 2 figures; Introduction extended, typos corrected, charged lepton sector of model 2 corrected; matches journal versio