  Dedicated experiments searching for lepton flavour violation can be performed
very sensitively using K-decays and $\mu$-decays as well as neutrinoless double
$\beta$-decay and muonium to antimuonium conversion. Although there is no
confirmed signal reported yet, stringent limits for parameters in speculative
extensions to the standard model can be set. Some models could recently be
ruled out.

High Energy Physics - Experiment