Heavy neutral lepton at same-sign muon collider

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 μ TRISTAN . The study focuses on two complementary HNL-mediated signatures: (i) the lepton-flavor-violating (LFV) channel μ + μ + → W + τ + ν ¯ μ and (ii) the lepton-number-violating (LNV) channel μ + μ + → 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 μ TRISTAN collider with s ∼ O ( 10 ) 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.