Measurement of the branching fraction for the decay KS→πeν ☆

Abstract We present a measurement of the branching ratio BR (K S →π ± e ∓ ν (ν)) performed using the KLOE detector. K S -mesons are produced in the reaction e + e − → φ → K S K L at the DA Φ NE collider. In a sample of ∼5×10 6 K S -tagged events we find 624±30 semileptonic K S decays. Normalizing to the K S → π + π − count in the same data sample, we obtain BR( K S → πeν )=(6.91±0.37)×10 −4 , in agreement with the Standard Model expectation.