Experiments searching for lepton number violation

Most sensitive searches for lepton number or lepton flavour violation can be carried out at present in kaon decays and in muon decays. Important information can be extracted from searches for neutrinoless;double beta -decay and muonium to antimuonium conversion. Although there is no confirmed signal reported yet from any of these systems, stringent limits for parameters in speculative extensions to the standard model can be set. Some of these theoretical approaches could recently be strongly disfavoured.