Study of the branching ratio and charge asymmetry for the decay KS → πeν with the KLOE detector

Abstract Among some 400 million K S K L pairs produced in e + e − annihilations at DAΦNE, ∼6500 each of K S → π + e − ν ¯ and K S → π − e + ν decays have been observed with the KLOE detector. From these, the ratio Γ ( K S → π e ν ) / Γ ( K S → π + π − ) = ( 10.19 ± 0.13 ) × 10 −4 is obtained, improving the accuracy on BR ( K S → π e ν ) by a factor of four and providing the most precise test of the Δ S = Δ Q rule. From the partial width Γ ( K S → π e ν ) , a value for f + K 0 ( 0 ) × V u s is obtained that is in agreement with unitarity of the quark-mixing matrix. The lepton charge asymmetry A S = ( 1.5 ± 9.6 stat ± 2.9 syst ) × 10 −3 is compatible with the requirements of CPT invariance. The form-factor slope agrees with recent results from semileptonic K L and K + decays. These are the first measurements of the charge asymmetry and form-factor slope for semileptonic K S decays.