Muonic and nuclear probes for the lepton flavor structure*

The lepton number violation (LNV) and lepton flavor violation (LFV) are important possible ingredients of the lepton physics.

For instance, the neutrinoless double beta decay and the transition of the muonium into antimuonuim are related to those violations.

The former can give us an essential part of fundamental physics, and there are plenty of experimental attempts to observe the process.

The latter has also been one of the attractive phenomena, and the experimental bound will be updated in planned experiments at new high-intensity muon beamlines.

In models with a doubly charged scalar, not only can those two processes be induced, but also the active neutrino masses can be induced radiatively.

The flavor violating decays of the charged leptons constrain the flavor parameters of the models.

In this talk, I will review the current status of LNV and LFV searches and introduce our study about the neutrinoless double beta decay and the muonium-to-antimuonium transition.

I will show that the updated bound of the muonium-to-antimuonium transition rate will be useful to distinguish the models to generate the neutrinoless double beta decay via the doubly charged scalar.