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

We present a measurement of the branching ratio of the CP violating decay KL→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 KL mesons. We find BR(KL→π+π−)=(1.963±0.012stat.±0.017syst.)×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.