$μ- e$ Conversion With Four Generations

We study $μ- e$ conversion with sequential four generations. A large mass for the fourth generation neutrino can enhance the conversion rate by orders of magnitude. We compare constraints obtained from $μ- e$ conversion using experimental bounds on various nuclei with those from $μ\to e γ$ and $μ\to e\bar e e$. We find that the current bound from $μ- e$ conversion with Au puts the most stringent constraint in this model. The relevant flavor changing parameter $λ_{μe} = V^*_{μ4}V_{e4}^{}$ is constrained to be less than $1.6\times 10^{-5}$ for the fourth generation neutrino mass larger than 100 GeV. Implications for future $μ-e$ conversion, $μ\to eγ$ and $μ\to e\bar e e$ experiments are discussed.