Neutrino Mass Ordering from Oscillations and Beyond: 2018 Status and Future Prospects

TL;DR

This review discusses current methods and recent findings on neutrino mass ordering, highlighting the strong evidence for the normal hierarchy and exploring future experimental prospects and novel approaches for determination.

Contribution

It provides a comprehensive overview of existing tools, recent global fit results, and future prospects, including novel methods like 21 cm cosmology and relic neutrino detection.

Findings

Bayesian analysis shows strong evidence for normal ordering at 3.5 sigma.

Current oscillation data primarily drive the preference for normal hierarchy.

Future methods like 21 cm cosmology and supernova neutrinos could further clarify the mass ordering.

Abstract

The ordering of the neutrino masses is a crucial input for a deep understanding of flavor physics, and its determination may provide the key to establish the relationship among the lepton masses and mixings and their analogous properties in the quark sector. The extraction of the neutrino mass ordering is a data-driven field expected to evolve very rapidly in the next decade. In this review, we both analyze the present status and describe the physics of subsequent prospects. Firstly, the different current available tools to measure the neutrino mass ordering are described. Namely, reactor, long-baseline (accelerator and atmospheric) neutrino beams, laboratory searches for beta and neutrinoless double beta decays and observations of the cosmic background radiation and the large scale structure of the universe are carefully reviewed. Secondly, the results from an up-to-date comprehensive global fit are reported: the Bayesian analysis to the 2018 publicly available oscillation and cosmological data sets provides \emph{strong} evidence for the normal neutrino mass ordering versus the inverted scenario, with a significance of 3.5 standard deviations. This preference for the normal neutrino mass ordering is mostly due to neutrino oscillation measurements. Finally, we shall also emphasize the future perspectives for unveiling the neutrino mass ordering. In this regard, apart from describing the expectations from the aforementioned probes, we also focus on those arising from alternative and novel methods, as 21~cm cosmology, core-collapse supernova neutrinos and the direct detection of relic neutrinos.