Towards testing the unitarity of the 3X3 lepton flavor mixing matrix in a precision reactor antineutrino oscillation experiment

TL;DR

This paper investigates how precision reactor antineutrino experiments can test the unitarity of the lepton mixing matrix, considering effects of sterile neutrinos and providing methods to measure deviations from unitarity.

Contribution

It introduces a framework to test the unitarity of the 3x3 lepton mixing matrix in the presence of sterile neutrinos using reactor experiments, with analytical results for different sterile neutrino scenarios.

Findings

Zero-distance effects can indicate non-unitarity.

Active neutrino mixing angles can be accurately measured.

Analytical formulas for different sterile neutrino scenarios are provided.

Abstract

The 3X3 Maki-Nakagawa-Sakata-Pontecorvo (MNSP) lepton flavor mixing matrix may be slightly non-unitary if the three active neutrinos are coupled with sterile neutrinos. We show that it is in principle possible to test whether the relation |V_{e1}|^2 + |V_{e2}|^2 + |V_{e3}|^2 = 1 holds or not in a precision reactor antineutrino oscillation experiment, such as the recently proposed Daya Bay II experiment. We explore three categories of non-unitary effects on the 3X3 MNSP matrix: 1) the indirect effect in the (3+ 3) flavor mixing scenario where the three heavy sterile neutrinos do not take part in neutrino oscillations; 2) the direct effect in the (3+\mathbbm{1}) scenario where the light sterile neutrino can oscillate into the active ones; and 3) the interplay of both of them in the (3+\mathbbm{1}+ 2) scenario. We find that both the zero-distance effect and flavor mixing factors of different oscillation modes can be used to determine or constrain the sum of |V_{e1}|^2, |V_{e2}|^2 and |V_{e3}|^2 and its possible deviation from one, and the active neutrino mixing angles \theta_{12} and \theta_{13} can be cleanly extracted even in the presence of light or heavy sterile neutrinos. Some useful analytical results are obtained for each of the three scenarios.