Heavy Neutral Lepton at Same-Sign Muon Collider

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.

Comments: 18 pages, 6 figures, and 3 tables. Matches the version accepted for publication in PRD