Measurement of the branching fraction for the decay

We present a measurement of the branching ratio BR(K → π±e∓ν̄(ν)) performed using the KLOE detector. KS-mesons are produced in the reaction e +e− → φ → KSKL at the DAΦNE collider. In a sample of ∼5 × 10 6 KS-tagged events we find 624± 30 semileptonic KS decays. Normalizing to the KS→π +π− count in the same data sample, we obtain BR(KS → πeν)= (6.91 ± 0.37) × 10 −4, in agreement with the Standard Model expectation. In the Standard Model, there are only ∆S = ∆Q transitions at tree level. ∆S=−∆Q transitions exist at higher order, but are suppressed by a factor of ∼10–10 [1,2]. Under this assumption, together with TCP -invariance [3,4], it follows that Γ(KS → πeν) = Γ(KL → πeν) to very high accuracy. Using the values of BR(KL → πeν) and τS/τL from ref [5], one obtains BR(KS → πeν) = (6.70± 0.07) × 10. Precise measurements of the KS semileptonic decay rate are important for checking the above assumptions. Until recently, no experimental information was available about the semileptonic decay of the KS-meson. This is in large part due to the smallness of the branching ratio and to the difficulty of isolating πeν̄(ν) decays from ππ decays. Both decays are observed as two charged tracks, but the latter are ∼ 1000 times more abundant. Good particle identification is necessary for isolating a pure sample of πeν̄(ν) events. The CMD-2 collaboration at VEPP-2M operating at the φ