Direct Ξ decay and muonium-antimuonium transitions

It now seems certain that the mass difference between K(1)0 and K(2)0 is quite small ( | δm | < 10^(-5) ev with 95% confidence (1)). Okun' and Pontecorvo (2) discuss a connection between δm and the existence of ΔS = 2 weak interactions. They show how such an interaction, if as strong as the ΔS = 1 weak interactions, might lead to a mass splitting of the order of electron volts.