A Survey of Neutrino Flavor Models and the Neutrinoless Double Beta Decay Funnel

TL;DR

This survey examines various neutrino flavor models to assess the likelihood of the neutrinoless double beta decay rate being extremely small due to parameter cancellations, guiding future research directions.

Contribution

It categorizes and analyzes five broad classes of flavor models to evaluate their predictions for neutrinoless double beta decay rates in the normal hierarchy.

Findings

A significant fraction of models predict decay rates in the funnel region.

The results inform experimental focus on theoretically motivated parameter spaces.

Model predictions vary widely, with some favoring suppressed decay rates.

Abstract

The neutrinoless double beta decay experimental effort continues to make tremendous progress with hopes of covering the inverted neutrino mass hierarchy in coming years and pushing from the quasi-degenerate hierarchy into the normal hierarchy. As neutrino oscillation data is starting to suggest that the mass ordering may be normal, we may well be faced with staring down the funnel of death: a region of parameter space in the normal ordering where -- for a particular cancellation among the absolute neutrino mass scale, the Majorana phases, and the oscillation parameters -- the neutrinoless double beta decay rate may be vanishingly small. To answer the question of whether this region of parameter space is theoretically preferred, we survey five broad categories of flavor model structures which make various different predictions for parameters relevant for neutrinoless double beta decay to determine how likely it is that the rate may be in this funnel region. We find that a non-negligible fraction of predictions surveyed are at least partially in the funnel region. Our results can guide model builders and experimentalists alike in focusing their efforts on theoretically motivated regions of parameter space.