A new way to determine the neutrino mass hierarchy at reactors

TL;DR

This paper introduces a novel technique for reactor neutrino experiments, enabling a robust 5 sigma determination of the neutrino mass hierarchy within six years, with high precision on key oscillation parameters.

Contribution

A new analysis method that significantly reduces the time and energy resolution requirements for determining the neutrino mass hierarchy at reactors.

Findings

Achieves 5 sigma hierarchy discrimination in less than six years.

Allows precise measurement of the $ ext{Δ}m^2_{31}$ degeneracy at below 1% accuracy.

Demonstrates robustness against systematic errors and background effects.

Abstract

The determination of the neutrino mass ordering is currently pursued by several experiments and proposals. A very challenging one is its evaluation from reactor experiments based on the tiny interference effect between the $\Delta m^2_{31}$ and $\Delta m^2_{32}$ oscillations. Current analyses require several years of data taking and an extreme energy resolution to achieve anyhow less than 5 $\sigma$. Referring to the JUNO experimental conditions we developed a completely new technique that would provide a robust 5 $\sigma$ measurement in less than six years of running. The two orderings could be discriminated at the price of allowing for two different values of $\Delta m^2_{31}$. This degeneracy on $\Delta m^2_{31}$ (around $12\times 10^{-5}$ eV$^2$) can however be measured at an unprecedented accuracy of much less than 1\%, i.e. $10^{-5}$ eV$^2$, within the same analysis. Analogies with the usual $\chi^2$ analysis, where the $\Delta m^2_{31}$ degeneracy is much more important, are discussed. Evaluation and inclusion of systematic errors and backgrounds have been performed, the most relevant among them being the addition of the two remote reactor plants 250 km away. Baselines of each contributing reactor core and its spatial resolution have been taken into account. Possible results after two years of running and the foreseen initially-reduced available reactor power have been studied, too. These results confirm the very positive perspectives for JUNO to determine the mass ordering in a vacuum-oscillation dominated region.