Neutrino masses: From fantasy to facts

TL;DR

The paper reviews the evidence for neutrino masses and oscillations, discusses theoretical models involving sterile neutrinos, and explores alternative explanations like flavor-changing interactions, highlighting recent experimental and theoretical developments.

Contribution

It introduces simple theoretical scenarios linking neutrino properties to lepton-number symmetry and examines alternative non-mass-based explanations for neutrino anomalies.

Findings

Neutrino oscillations strongly suggest non-zero masses.

Sterile neutrinos can reconcile solar, atmospheric, and LSND data.

Flavor-changing interactions could mimic oscillation signals without neutrino mass.

Abstract

Theory suggests the existence of neutrino masses, but little more. Facts are coming close to reveal our fantasy: solar and atmospheric neutrino data strongly indicate the need for neutrino conversions, while LSND provides an intriguing hint. The simplest ways to reconcile these data in terms of neutrino oscillations invoke a light sterile neutrino in addition to the three active ones. Out of the four neutrinos, two are maximally-mixed and lie at the LSND scale, while the others are at the solar mass scale. These schemes can be distinguished at neutral-current-sensitive solar & atmospheric neutrino experiments. I discuss the simplest theoretical scenarios, where the lightness of the sterile neutrino, the nearly maximal atmospheric neutrino mixing, and the generation of $\Delta {m^2}_\odot$ & $\Delta {m^2}_{atm}$ all follow naturally from the assumed lepton-number symmetry and its breaking. Although the most likely interpretation of the present data is in terms of neutrino-mass-induced oscillations, one still has room for alternative explanations, such as flavour changing neutrino interactions, with no need for neutrino mass or mixing. Such flavour violating transitions arise in theories with strictly massless neutrinos, and may lead to other sizeable flavour non-conservation effects, such as $\mu \to e + \gamma$, $\mu-e$ conversion in nuclei, unaccompanied by neutrino-less double beta decay.