$mu - e$ Conversion With Four Generations

We study $mu - e$ 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 $mu - e$ conversion using experimental bounds on various nuclei with those from $mu to e gamma$ and $mu to ebar e e$. We find that the current bound from $mu - e$ conversion with Au puts the most stringent constraint in this model. The relevant flavor changing parameter $lambda_{mu e} = V^*_{mu 4}V_{e4}^{}$ is constrained to be less than $1.6times 10^{-5}$ for the fourth generation neutrino mass larger than 100 GeV. Implications for future $mu -e$ conversion, $mu to egamma$ and $mu to ebar e e$ experiments are discussed.