Majorana neutrinos, <Emphasis Type="Italic">CP</Emphasis> violation,neutrinoless double beta and tritium beta decays

If the present or upcoming searches for neutrinoless double beta ((ββ)_0ν) decay give a positive result, the Majorana nature of massive neutrinos will be established. From the determination of the value of the (ββ)_0ν-decay effective Majorana mass parameter (|〈m〉|), it would be possible to obtain information on the type of neutrino mass spectrum. Assuming 3-ν mixing and massive Majorana neutrinos, we discuss the information that a measurement of, or an upper bound on, |〈m〉| can provide on the value of the lightest neutrino mass m₁. With additional data on the neutrino masses obtained in ³H β-decay experiments, it might be possible to establish whether the CP symmetry is violated in the lepton sector. This would require very high precision measurements. If CP invariance holds, the allowed patterns of the relative CP parities of the massive Majorana neutrinos would be determined.     

Remarks on the Zee model of neutrino mixing (μ →e+ γ, νH→ νL + γ, etc.)

Some features of the Zee model of lepton number nonconservation are discussed. In particular, the predictions of the model for the μ →e+ γ decay rate and the neutrino radiative life-times (ν_H → ν_L + γ) are analyzed in detail.     

Low-scale neutrino seesaw mechanism and scalar dark matter

Recently the AMS-02 experiment has released the data of positron fraction with a very small statistical error. Because of the small error, it is no longer easy to fit the data with single dark matter for a fixed diffusion model and dark matter profile. In this paper, we propose a new interpretation of the data: that it originates from decay of two-component dark matter. This interpretation gives a rough threshold of the lighter DM component. When DM decays into leptons, the positron fraction in the cosmic rays depends on the flavor of the final states, and this is fixed by imposing a non-Abelian discrete symmetry on our model. By assuming two gauge-singlet fermionic decaying DM particles, we show that a model with non-Abelian discrete flavor symmetry, e.g. $T_{13}$ , can give a much better fitting to the AMS-02 data compared with a single-component dark matter scenario. Few dimension-six operators of the universal leptonic decay of DM particles are allowed in our model, since its decay operators are constrained by the $T_{13}$ symmetry. We also show that the lepton masses and mixings are consistent with current experimental data, due to the flavor symmetry.     

Addendum to: “The SNO solar neutrino data, neutrinoless double beta-decay and neutrino mass spectrum”: [Phys. Lett. B 544 (2002) 239]

We update our earlier study [Phys. Lett. B 544 (2002) 239], which was inspired by the 2002 SNO data, on the implications of the results of the solar neutrino experiments for the predictions of the effective Majorana mass in neutrinoless double beta-decay, |m|. We obtain predictions for |m| using the values of the neutrino oscillation parameters, obtained in the analyzes of the presently available solar neutrino data, including the just published data from the salt phase of the SNO experiment, the atmospheric neutrino and CHOOZ data and the first data from the KamLAND experiment. The main conclusion reached in the previous study [Phys. Lett. B 544 (2002) 239] of the existence of significant lower bounds on |m| in the cases of neutrino mass spectrum of inverted hierarchical (IH) and quasi-degenerate (QD) type is strongly reinforced by fact that combined solar neutrino data (i) exclude the possibility of cos2θ=0 at more than 5 s.d., (ii) determine as a best fit value cos2θ=0.40, and (iii) imply at 95% C.L. that cos2θ0.22, θ being the solar neutrino mixing angle. For the IH and QD spectra we get using, e.g., the 90% C.L. allowed ranges of values of the oscillation parameters, |m|0.010 eV and |m|0.043 eV, respectively. We also comment on the possibility to get information on the neutrino mass spectrum and on the CP-violation in the lepton sector due to Majorana CP-violating phases.     

A fresh look at neutrino oscillations

We analyze the present experimental situation on neutrino oscillations in the standard weak interaction theory with three neutrinos. Sizeable oscillations ν_e ? ν_τ are compatible with, and perhaps indicated by, present data.     

Measuring the P-odd asymmetry in electroproduction of the δ(32, 32 resonance at low energies

We show that measurements of the P-odd asymmetry in the reaction $\text{e}{}^{\mathrm{?}}\text{+}\text{N}\text{→}\text{e}{}^{\mathrm{?}}\text{+δ(}\text{2}\text{2}\text{,}\text{3}\text{2}$ at low energies might determine the combination $\text{α}\text{～}\text{+(1.0–1.2γ}\text{β}\text{～}$ of the isovector phenomenological couplings, characterizing the parity nonconserving electron-quark neutral current interaction at presently accessible energies. They may provide also a very accurate determination of the parameter sin²θ_W of the standard electroweak gauge theory.     

Higgs decays in the low scale type I see-saw model

The couplings of the low scale type I see-saw model are severely constrained by the requirement of reproducing the correct neutrino mass and mixing parameters, by the non-observation of lepton number and charged lepton flavour violating processes and by electroweak precision data. We show that all these constraints still allow for the possibility of an exotic Higgs decay channel into a light neutrino and a heavy neutrino with a sizable branching ratio. We also estimate the prospects to observe this decay at the LHC and discuss its complementarity to the indirect probes of the low scale type I see-saw model from experiments searching for the μ→eγ$\mu \to e\gamma$ decay.     

Model-independent analysis of the weak interaction in e− -N elastic scattering experiments

We discuss what model-independent information about the parity non-conserving electron-quark neutral current interaction can be obtained in the experiments on elastic scattering of longitudinally polarized electrons on unpolarized nucleons. Two possible forms of the neutron electric form factor G_E^γn(q²) are used in our analysis. The interpretation of the P-odd asymmetry, measured in these experiments, in terms of electron-quark neutral current coupling constants depends on the form chosen for G_E^γn(q²). We note also that the polarized e^?-p elastic scattering experiments might be used in principle to study the q² dependence of G_E^γn(q²).     

Lepton mixing, μ → e + γ decay and neutrino oscillations

The μ → eγ decay is investigated in a gauge theory with lepton mixing under the assumption that in nature there exist heavy leptons. It is shown that for lepton masses of the order of a few GeV the μ → eγ decay probability may well be close to its experimentally determined upper limit. The relation between such a decay process and neutrino oscillations is briefly considered.