Heavy neutral lepton search and $μ\to e γ$ constraints in case of type-I seesaw

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