We present a measurement of the branching ratio BR(K → 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.
A. et al. (Mon,) studied this question.