[Paper Review] Neutrino Mass: theory, data and interpretation

J. W. F. Valle|ArXiv.org|Jul 4, 1999

Neutrino Physics Research8 references3 citations

TL;DR

This paper presents a theoretical framework for neutrino masses based on lepton-number symmetry and its breaking, proposing a four-neutrino scheme with three active and one sterile neutrino to reconcile solar, atmospheric, and LSND neutrino data. It shows that maximal mixing at the LSND scale and solar-scale mass differences emerge naturally, with distinguishable signatures in neutral-current-sensitive experiments.

ABSTRACT

In these two lectures I describe first the theory of neutrino mass and then discuss the implications of recent data (including 708--day data Super--Kamiokande data) which strongly indicate the need for neutrino conversions to account for the solar and atmospheric neutrino observations. I also mention the LSND data, which provides an intriguing hint. The simplest ways to reconcile all 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 $Δ{m^2}_\odot$ & $Δ{m^2}_{atm}$ 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 $μ o e + γ$, $μ-e$ conversion in nuclei, unaccompanied by neutrino-less double beta decay.

Motivation & Objective

• To interpret the growing experimental evidence for neutrino oscillations from solar, atmospheric, and LSND data.
• To address the theoretical challenge of explaining small neutrino masses and large mixing angles in a unified framework.
• To explore the viability of a four-neutrino model including a light sterile neutrino to reconcile conflicting data sets.
• To distinguish between neutrino mass-induced oscillations and alternative flavor-violating interactions without neutrino mass.
• To identify observable signatures in neutral-current-sensitive solar and atmospheric neutrino experiments.

Proposed method

• Formulates a four-neutrino mixing scheme with two maximally-mixed pairs: one at the LSND scale and one at the solar mass scale.
• Applies lepton-number symmetry and its spontaneous breaking to naturally generate the required mass splittings Δm²_⊙ and Δm²_atm.
• Analyzes the implications of the 708-day Super-Kamiokande data for solar and atmospheric neutrino deficits.
• Considers the LSND anomaly as a hint for sterile neutrino contributions to short-baseline oscillations.
• Evaluates the phenomenological viability of flavor-changing neutrino interactions as an alternative to neutrino mass.
• Identifies distinguishable signals in neutral-current processes that could confirm the four-neutrino scenario.

Experimental results

Research questions

• RQ1 Can a four-neutrino model with a light sterile neutrino reconcile the solar, atmospheric, and LSND neutrino anomalies?
• RQ2 How do lepton-number symmetry and its breaking naturally generate the observed mass splittings and mixing patterns?
• RQ3 What experimental signatures distinguish the four-neutrino oscillation scenario from alternative explanations involving flavor-violating interactions?
• RQ4 Can neutral-current-sensitive experiments detect differences between the solar and atmospheric neutrino oscillation sectors?
• RQ5 What constraints do the 708-day Super-Kamiokande data impose on the four-neutrino mixing framework?

Key findings

• The four-neutrino scheme with two maximally-mixed pairs—one at the LSND scale and one at the solar scale—provides a consistent explanation for solar, atmospheric, and LSND data.
• The model naturally generates the observed Δm²_⊙ ≈ 10⁻⁵ eV² and Δm²_atm ≈ 10⁻³ eV² through lepton-number symmetry breaking.
• The lightness of the sterile neutrino is explained by the same symmetry mechanism that generates the other mass scales.
• The nearly maximal atmospheric neutrino mixing is a direct consequence of the assumed symmetry structure.
• Neutral-current-sensitive experiments can distinguish the four-neutrino scenario from alternative flavor-violating models.
• Alternative explanations involving flavor-changing neutrino interactions without neutrino mass remain viable but predict additional effects such as μ → e + γ and μ-e conversion in nuclei.