Non-unitary neutrino mixing in the NO$\nu$A near detector data

TL;DR

This paper investigates non-unitary neutrino mixing using NO$ u$A near detector data, finding no evidence for non-unitarity and setting constraints on the parameters, thus supporting the standard three-neutrino paradigm.

Contribution

It provides the first constraints on non-unitary neutrino mixing parameters using NO$ u$A near detector data, testing deviations from unitarity in short baseline oscillations.

Findings

Non-unitary mixing does not improve fit to data over standard model.

Constraints on non-unitary parameters: $eta_{00}>0.911$, $|eta_{10}|<0.020$, $eta_{11}>0.952$ at 90% C.L.

Combined analysis confirms no significant deviation from unitarity.

Abstract

The $\nu_\mu \to \nu_e$ oscillation probability over short baseline ($\lesssim 1$~km) would be negligible in case the mixing matrix for three active neutrinos is unitary. However, in case of non-unitary mixing of three neutrinos, this probability would be non-negligible due to the so-called "zero distance" effect. Hence, the near detector of the accelerator experiments such as NO$\nu$A can provide strong constraints on the parameters of the non-unitary mixing with very large statistics. By analyzing the NO$\nu$A near detector data we find that the non-unitary mixing does not improve fits to the $\nu_e$ or $\nu_\mu$ events over the standard unitary mixing. This leads to constraints on the non-unitary parameters: $\alpha_{00}>0.911$, $|\alpha_{10}|<0.020$ and $\alpha_{11}>0.952$ at 90\% C.L. A combined analysis with the near and far detector data does not change these constraints significantly.