The Nature of Massive Neutrinos

TL;DR

This paper reviews the current understanding of massive neutrinos, including their mixing, properties, models of mass generation, and the potential of neutrinoless double beta decay experiments to reveal their fundamental nature.

Contribution

It provides a comprehensive summary of experimental data, theoretical models, and future research directions regarding massive neutrinos and their role in particle physics.

Findings

Current data support 3-neutrino mixing with oscillations.

Models like see-saw and Higgs triplet explain neutrino mass generation.

Neutrinoless double beta decay can reveal neutrino mass nature and hierarchy.

Abstract

The compelling experimental evidences for oscillations of solar, reactor, atmospheric and accelerator neutrinos imply the existence of 3-neutrino mixing in the weak charged lepton current. The current data on the 3-neutrino mixing parameters are summarised and the phenomenology of 3-$\nu$ mixing is reviewed. The properties of massive Majorana neutrinos and of their various possible couplings are discussed in detail. Two models of neutrino mass generation with massive Majorana neutrinos - the type I see-saw and the Higgs triplet model, are briefly reviewed. The problem of determining the nature - Dirac or Majorana, of massive neutrinos is considered. The predictions for the effective Majorana mass $\meff$ in neutrinoless double beta ($\betabeta-$) decay in the case of 3-neutrino mixing and massive Majorana neutrinos are summarised. The physics potential of the experiments, searching for $\betabeta-$decay for providing information on the type of the neutrino mass spectrum, on the absolute scale of neutrino masses and on the Majorana CP-violation phases in the PMNS neutrino mixing matrix, is also briefly discussed. The opened questions and the main goals of future research in the field of neutrino physics are outlined.