[Paper Review] Neutrino Mass Ordering in 2018: Global Status

P.F. de Salas, Stefano Gariazzo+3 more|arXiv (Cornell University)|Jun 28, 2018

Astrophysics and Cosmic Phenomena80 references19 citations

TL;DR

This 2018 review evaluates the global status of neutrino mass ordering using Bayesian analysis of oscillation and cosmological data, finding strong evidence (3.5σ) for normal mass ordering over inverted ordering, primarily driven by neutrino oscillation measurements. It outlines current and future experimental probes, including reactor, long-baseline, and cosmological observations, as well as novel methods like 21 cm cosmology and relic neutrino detection.

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.

Motivation & Objective

• To assess the current global status of neutrino mass ordering using the latest publicly available data.
• To evaluate the relative strength of evidence for normal versus inverted neutrino mass ordering.
• To review existing and upcoming experimental probes for determining the neutrino mass hierarchy.
• To explore novel and alternative methods such as 21 cm cosmology and direct relic neutrino detection for future mass ordering determination.
• To provide a comprehensive overview of the physics and prospects for resolving the neutrino mass ordering in the coming decade.

Proposed method

• Conduct a Bayesian global fit to 2018 publicly available oscillation and cosmological data sets.
• Integrate results from reactor neutrino experiments, long-baseline accelerator and atmospheric neutrino beams, and beta decay and neutrinoless double beta decay searches.
• Include constraints from cosmic microwave background radiation and large-scale structure observations.
• Quantify the significance of preference for normal versus inverted mass ordering using statistical significance (in σ units).
• Assess contributions from different experimental probes to the overall evidence for mass ordering.
• Explore future prospects by analyzing expected sensitivities of current and emerging methods, including 21 cm cosmology and core-collapse supernova neutrino detection.

Experimental results

Research questions

• RQ1 What is the current global statistical evidence for normal versus inverted neutrino mass ordering based on 2018 data?
• RQ2 Which experimental probes contribute most significantly to determining the neutrino mass ordering?
• RQ3 How do reactor, long-baseline, and cosmological observations collectively constrain the neutrino mass hierarchy?
• RQ4 What are the prospects for resolving the neutrino mass ordering using emerging techniques such as 21 cm cosmology and direct relic neutrino detection?
• RQ5 How do laboratory searches for beta decay and neutrinoless double beta decay contribute to the determination of the mass ordering?

Key findings

• The Bayesian analysis of 2018 data provides strong evidence for the normal neutrino mass ordering, with a significance of 3.5 standard deviations.
• The preference for normal mass ordering is primarily driven by neutrino oscillation measurements, not cosmological data.
• Reactor neutrino experiments, long-baseline accelerator and atmospheric neutrino beams, and cosmological observations all contribute to the global constraint on mass ordering.
• Future prospects include enhanced sensitivity from ongoing and planned experiments in oscillation physics, beta decay, and neutrinoless double beta decay.
• Novel methods such as 21 cm cosmology and core-collapse supernova neutrino detection are expected to provide additional, complementary constraints.
• Direct detection of relic neutrinos remains a long-term prospect but is considered a promising future avenue for mass ordering determination.