Measurement of the branching ratio of the KL→π+π− decay with the KLOE detector

Abstract We present a measurement of the branching ratio of the CP violating decay K L → π + π − performed by the KLOE experiment at the ϕ factory DAΦNE. We use 328 pb −1 of data collected in 2001 and 2002, corresponding to ∼150 million tagged K L mesons. We find BR ( K L → π + π − ) = ( 1.963 ± 0.012 stat . ± 0.017 syst . ) × 10 −3 . This branching ratio measurement is fully inclusive of final-state radiation. Using the above result, we determine the modulus of the amplitude ratio | η + − | to be ( 2.219 ± 0.013 ) × 10 −3 and | ϵ | to be ( 2.216 ± 0.013 ) × 10 −3 .