Searching for heavy neutrinos in $e^+ e^- \to W^+ W^-$: it is all about unitarity

TL;DR

This paper investigates the potential to detect heavy neutrinos in electron-positron collisions producing W boson pairs, emphasizing the importance of using exact unitarity in mixing schemes for accurate predictions.

Contribution

It compares linearized and exact unitarity mixing schemes, highlighting the inaccuracies of the former and the experimental signatures predicted by the latter.

Findings

Linearized mixing approximation yields incorrect results.

Exact unitarity mixing predicts observable signatures.

Accurate modeling of neutrino mixing is crucial for collider searches.

Abstract

We study the process $e^+e^- \to W^+W^-$ with the aim of estimating the prospects for observing heavy neutrinos contributions at future $e^+e^-$-colliders. In this work, we consider two implementations of heavy-light neutrino mixing: a linearized mixing approximation applied in popular models and an exact unitary mixing scheme. We conclude that the approximate realization leads to physically incorrect results for this process, while exact unitary mixing provides some signatures that can be experimentally checked.