Heavy Neutral Leptons at Muon Colliders

TL;DR

This paper explores the potential of future muon colliders to detect heavy neutral leptons (HNLs), which are key to understanding neutrino masses, by analyzing their projected sensitivities in the TeV energy range.

Contribution

It provides a detailed study of the sensitivity of muon colliders to heavy neutral leptons with masses above 100 GeV, focusing on their decay channels and mixing parameters.

Findings

Projected sensitivities reach the TeV scale in the |U_ell|^2 - m_N plane.

Muon colliders could significantly improve detection prospects for HNLs.

Full-reconstructable decay channels enhance search capabilities.

Abstract

The future high-energy muon colliders, featuring both high energy and low background, could play a critical role in our searches for new physics. The smallness of neutrino mass is a puzzle of particle physics. Broad classes of solutions to the neutrino puzzles can be best tested by seeking the partners of SM light neutrinos, dubbed as heavy neutral leptons (HNLs), at muon colliders. We can parametrize HNLs in terms of the mass $m_N$ and the mixing angle with $\ell$-flavor $U_\ell$. In this work, we focus on the regime $m_N > O(100)$ GeV and study the projected sensitivities on the $|U_\ell|^2 - m_N$ plane with the full-reconstructable HNL decay into a hadronic $W$ and a charged lepton. The projected reach in $|U_\ell|^2$ leads to the best sensitivities in the TeV realm.