# Testing unitarity of the $3\times 3$ neutrino mixing matrix in an atomic system

## TL;DR

This paper proposes using atomic transition photon spectra to test the unitarity of the 3x3 neutrino mixing matrix, which could be violated in seesaw models with heavy neutrino mixing.

## Contribution

It introduces a novel atomic transition method to probe unitarity violation in the neutrino mixing matrix through photon spectrum analysis.

## Key findings

- Photon spectrum distortion indicates unitarity violation.
- Certain frequencies can minimize degeneracy effects.
- Experimental setup considerations are discussed.

## Abstract

Unitarity of the $3\times 3$ lepton flavor mixing matrix $V$ is unavoidably violated in a seesaw mechanism if its new heavy degrees of freedom are slightly mixed with the active neutrino flavors. We propose to use the atomic transition process ${\left| \rm e \right> \to\left|\rm g \right> + \gamma + \nu^{}_{i} + \overline{\nu}^{}_{j}}$ (for $i, j = 1, 2, 3$), where $\left|\rm e \right>$ and $\left|\rm g \right>$ stand respectively for the excited and ground levels of an atomic system, to probe or constrain the unitarity-violating effects of $V$. We find that the photon spectrum of this transition will be distorted by the effects of $V V^\dagger \neq {\bf 1}$ and $V^\dagger V \neq {\bf 1}$ as compared with the $V V^\dagger = V^\dagger V = {\bf 1}$ case. We locate certain frequencies in the photon spectrum to minimize the degeneracy of effects of the unitarity violation and uncertainties of the flavor mixing parameters themselves. The requirements of a nominal experimental setup to test the unitarity of $V$ are briefly discussed.