Neutrino oscillations and signals in β and 0ν2β experiments

Assuming Majorana neutrinos, we infer from oscillation data the expected values of the parameters mνe and mee probed by β and 0ν2β-decay experiments. If neutrinos have a ‘normal hierarchy’ we get the 90% CL ranges |mee| = (0.5÷5)meV, and discuss in which cases future experiments can test this possibility. For ‘inverse hierarchy’, we get |mee| = (10 ÷ 57)meV and mνe = (40 ÷ 57)meV. The 0ν2β data imply that almost degenerate neutrinos are lighter than 0.95 h eV at 90% CL, competitive with the β-decay bound. We critically reanalyse the data that were recently used to claim an evidence for 0ν2β, and discuss their implications. Finally, we review the predictions of flavour models for mee and θ13. The most successful extensions of the SM to date suggest that neutrinos are Majorana particles, with masses around v/MGUT ∼ meV, where v = 174 GeV (the electroweak scale) and MGUT ∼ 2 · 1016 GeV (the grand unification scale). If this indication is correct, the goal of future neutrino experiments will be the reconstruction of the neutrino Majorana mass matrix, namely the measurement of 3 masses, 3 mixing angles and 3 CP-violating phases. Within this framework, data on solar and atmospheric neutrinos [1, 2, 3] are interpreted as pieces of evidence for effects due to 2 mixing angles and 2 squared mass differences, and the LSND anomaly [4] cannot be explained. We are far from knowing all 9 neutrino parameters. Furthermore, only 6 of them might be measured by oscillations; thus, we will eventually need “non-oscillation” experiments to access the remaining ones. In this work, we discuss how future study of neutrinoless double beta decay (0ν2β) and tritium end-point spectrum (β) could contribute to this goal. In particular, we carefully study the expected signals for these experiments on the basis of our present knowledge. We follow two different strategies of investigation: we first pursue a purely phenomenological approach, and next we add speculative ingredients from models of quark and lepton masses. In our view, neither of these approaches is entirely satisfactory: the first one allows us to derive safe but not extremely strong restrictions; the second one can give stronger but unsafe restrictions. Therefore these two approaches are “complementary”. In section 1 we obtain the ranges of parameters measured by oscillation experiments, and summarize how future experiments are expected to improve on these quantities. In section 2 we work out the connections between oscillation and non-oscillation (β and 0ν2β-decay) experiments. We improve on On leave from dipartimento di Fisica dell’Università di Pisa and INFN.