future muon colliders, with special emphasis on the proposed μTRISTAN
experiment. If neutrinos are Majorana particles, they may lead to
charged lepton flavor violating processes that would constitute clear
evidence for New Physics in the lepton sector. Using an effective field
theory framework, we then compare the potential discovery reach with
existing limits from rare lepton decays, finding that muon colliders are
the most sensitive probe of charged lepton flavor violation for a range
of parameters. As a model example, we then consider the type-ii seesaw
model, and show how a muon collider could be used to constrain the CP-
violating phases of the corresponding PMNS matrix.