Charged Lepton-Flavor Violation in Beyond-Standard Models

It is found from discovery of the neutrino oscillation that the lepton-flavor symmetries are not exact in nature, while the charged lepton-flavor violation (cLFV) has not been observed yet. The cLFV is suppressed even if the tiny neutrino masses observed in the experiments are introduced in the standard model (SM). The cLFV processes are proportional to the fourth power of the neutrino masses due to the GIM mechanism, similar to the quark sector. The predicted branching ratio of μ → eγ is smaller than 10. However, the lepton-flavor symmetries are accidental in the SM, and it is a big mystery in particle physics why the cLFV processes are not discovered, when considering beyond-standard models (BSMs) [1]. Searches for cLFV processes have long history. Search for μ → eγ was performed soon after muons were discovered, and it was found that muons are not an excited state of electrons. On 60’s, the twoneutrino hypothesis, in which the lepton flavor conservations were introduced, was proposed to suppress μ → eγ. The tau-lepton flavor conservation has been also tested after tau lepton was discovered on 75’. However, on 98’, the neutrino oscillation was discovered at SuperKamiokande experiments. Now the bounds on the cLFV processes are significantly improved by efforts of experimentalists. The current experimental bounds on the representative cLFV processes of muon are given as follows,