Heavy Neutral Lepton Search and μ → eγ Constraints in Case of Type-I Seesaw

Within the type-I seesaw mechanism, it is possible to have large (order one) light–heavy neutrino mixing even in the case of low right-handed neutrino mass scale (of the order of GeV). This implies large lepton flavor violation. As an example, we consider the process μ→eγ that can have a branching of up to 10−8 within type-I seesaw (in contrast with the tiny value 10−54 expected). Such an enhancement of lepton flavor violation can be used to constraint the parameter space of long-lived particle experiments.