A framework for testing leptonic unitarity by neutrino oscillation experiments

TL;DR

This paper develops a nearly model-independent framework to test low-scale leptonic unitarity violation using neutrino oscillation experiments, distinguishing it from high-scale violations through characteristic features like flavor universality and zero-distance transitions.

Contribution

It introduces a framework that isolates low-scale unitarity violation effects in neutrino oscillations, independent of sterile sector details, and proposes experimental constraints using JUNO-like setups.

Findings

The oscillation probability becomes insensitive to sterile sector details for mass squared differences ≥ 0.1 eV².

Presence of a constant leaking probability distinguishes low-scale from high-scale unitarity violation.

Potential modifications due to matter effects are discussed at first order.

Abstract

If leptonic unitarity is violated by new physics at an energy scale much lower than the electroweak scale, which we call low-scale unitarity violation, it has different characteristic features from those expected in unitarity violation at high-energy scales. They include maintaining flavor universality and absence of zero-distance flavor transition. We present a framework for testing such unitarity violation at low energies by neutrino oscillation experiments. Starting from the unitary 3 active plus $N$ (arbitrary integer) sterile neutrino model we show that by restricting the active-sterile and sterile-sterile neutrino mass squared differences to $\gtrsim$ 0.1 eV$^2$ the oscillation probability in the $(3+N)$ model becomes insensitive to details of the sterile sector, providing a nearly model-independent framework for testing low-scale unitarity violation. Yet, the presence of the sterile sector leaves trace as a constant probability leaking term, which distinguishes low-scale unitarity violation from the high-scale one. The non-unitary mixing matrix in the active neutrino subspace is common for the both cases. We analyze how severely the unitarity violation can be constrained in $\nu_{e}$-row by taking a JUNO-like setting to simulate medium baseline reactor experiments. Possible modification of the features of the $(3+N)$ model due to matter effect is discussed to first order in the matter potential.