Testing the unitarity of the light neutrino mixing matrix

TL;DR

This paper introduces a collider-based method to test the unitarity of the neutrino mixing matrix by analyzing energy-dependent cross section deviations, providing bounds from LEP-II data and future collider sensitivities.

Contribution

It proposes a novel collider test for neutrino mixing unitarity violations, applicable to models with heavy neutrino states, and derives model-independent bounds using existing data.

Findings

LEP-II data constrains unitarity-violating effects

Future colliders can significantly improve sensitivity

Method exploits energy growth of cross sections due to unitarity violation

Abstract

We propose a novel test of the unitarity of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix at collider experiments. Our approach exploits the incomplete cancellation between $t$-channel neutrino exchange and $s$-channel gauge-boson contributions that arises in the presence of violation of the flavor-diagonal PMNS unitarity conditions, leading to an anomalous growth of the cross section with energy. Such effects are generic in extensions of the Standard Model in which light neutrinos mix with heavier states, and can manifest at colliders as long as the characteristic energy of the process remains below the mass threshold of the new degrees of freedom. After briefly reviewing these scenarios, we employ our strategy to derive model-independent bounds on flavor diagonal unitarity-violating effects using LEP-II data. We then present sensitivity projections for future lepton and hadron colliders, demonstrating that they are well suited to probe the unitarity of the neutrino mixing matrix with this method.