Heavy Neutral Lepton at Same-Sign Muon Collider

TL;DR

This paper investigates the potential of the proposed high-energy same-sign muon collider, $$TRISTAN, to discover heavy neutral leptons through lepton-flavor-violating and lepton-number-violating processes, potentially surpassing current bounds.

Contribution

It introduces the first detailed study of HNL signatures at a same-sign muon collider, highlighting its superior sensitivity over existing constraints for certain mass ranges.

Findings

Collider sensitivity exceeds current bounds for 5-10 TeV HNLs.

LNV process analogous to neutrinoless double beta decay offers evidence for Majorana neutrinos.

LFV process provides a new probe of lepton flavor-changing currents.

Abstract

We explore the discovery potential of heavy neutral leptons (HNLs), motivated by models addressing the origin of neutrino masses, at the proposed high-energy same-sign muon collider known as $\mu$TRISTAN. The study focuses on two complementary HNL-mediated signatures: (i) the lepton-flavor-violating (LFV) channel $\mu^+\mu^+ \to W^+\tau^+\bar\nu_\mu$ and (ii) the lepton-number-violating (LNV) channel $\mu^{+}\mu^{+} \to W^{+}W^{+}$. The LNV process is the muon analogue of inverse neutrinoless double beta decay and, if observed, would provide strong evidence for Majorana neutrinos, while the LFV process offers a novel probe of flavor-changing neutral currents in the lepton sector. At the $\mu$TRISTAN collider with $\sqrt{s} \sim \mathcal{O}(10)~\text{TeV}$, the resulting sensitivity to the HNL mixing with muon and tau neutrinos, as a function of mass, can surpass current bounds from the measurements of electroweak precision observables over a broad mass range. In particular, for the mixing with muon neutrinos, the collider bound can improve by an order of magnitude for $5$-$10$ TeV HNLs.