Heavy neutral leptons at muon colliders

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 ℓ-flavor U_ℓ. In this work, we focus on the regime m_N> O(100) GeV and study the projected sensitivities on the |U_ℓ|² − m_N plane with the full-reconstructable HNL decay into a hadronic W and a charged lepton. The projected reach in |U_ℓ|² leads to the best sensitivities in the TeV realm.