Probing Heavy Neutral Leptons at Muon Collider

The smallness of neutrino mass is a puzzle of particle physics, and a broad class of solutions can be best tested by seeking the partners of SM light neutrinos. These neutrino partners, dubbed as heavy neutral leptons (HNL), mix with SM neutrinos. The future high-energy muon colliders, which combine high energy and low background, could play a critical role in our searches for HNL.

We can parametrize them in terms of the mass m_N and the mixing angle U_l where l refers to the lepton flavor. In this work, we focus on the regime m_N > O(100) GeV and study the projected sensitivity on the |U_l|² - m_N plane on the future 3 TeV and 10 TeV muon collider. We select the channel which requires heavy neutrino decaying into a W boson and a charged lepton. The projected reach in |U_l|² leads to the best sensitivities in the 100 GeV to 10 TeV realm.