Observation of time-dependent KS and KL interference in the π+π− decay channel from an initial K0 state

Abstract The phase ϕ +− between the amplitudes K L → π + π − and K S → π + π − has been measured. The result is 46 0 ± ± 15 0 for a K L - K S mass difference Δm/ h = (0.526±0.20) × 10 10 s −1 , in good agreement with the “superweak model” of Wolfenstein which predicts ϕ +− SW = 42.6 0 ± 1.0 0 .