Searching for Heavy Neutral Leptons at A Future Muon Collider

TL;DR

This paper evaluates the potential of future TeV-scale muon colliders to discover and distinguish heavy neutral leptons, providing estimates of production rates, backgrounds, and sensitivity to mixing parameters in a model-independent manner.

Contribution

It offers the first comprehensive analysis of HNL detection prospects at muon colliders across a wide mass range, including discrimination between Majorana and Dirac types.

Findings

Exclusion limits on HNL-SM neutrino mixing can reach as low as 10^{-6}.

Muon colliders can differentiate between Majorana and Dirac HNLs over a large parameter space.

HNL production and decay rates are estimated for collider energies of 3 and 10 TeV.

Abstract

As the planning stages for a high energy muon collider enter a more concrete era, an important question arises as to what new physics could be uncovered. A TeV-scale muon collider is also a vector boson fusion (VBF) factory with a very clean background, and as such it is a promising environment to look for new physics that couples to the electroweak (EW) sector. In this paper, we explore the ability of a future TeV-scale muon collider to search for Majorana and Dirac Heavy Neutral Leptons (HNLs) produced via EW bosons. Employing a model-independent, conservative approach, we present an estimation of the production and decay rate of HNLs over a mass range between 200 GeV and 9.5 TeV in two benchmark collider proposals with $\sqrt{s}=3,\,10$ TeV, as well as an estimation of the dominant Standard Model (SM) background. We find that exclusion limits for the mixing between the HNLs and SM neutrinos can be as low as $\mathcal{O}(10^{-6})$. Additionally, we demonstrate that a TeV-scale muon collider allows for the ability to discriminate between Majorana and Dirac type HNLs for a large range of mixing values.