Lepton Number Violation and μ, τ Systems

In this chapter we consider lepton flavor violation (LFV) in the charged lepton sector, which is something we have never observed yet. If they exist, the source has to lie outside of the SM. Before discussing the relative new field of LFV \(\tau \) decay search at the B factories and the future, we briefly discuss \(\mu \rightarrow e\) transitions, such as \(\mu \rightarrow e\gamma \) and \(\mu \rightarrow e\) conversion on nuclei, the search of which has a history as long as particle physics itself. Inevitably, we then also touch upon muon \(g-2\) and electric dipole moments.