Measurement of the branching fraction for the decay KS → πμν with the KLOE detector

Abstract Based on a sample of 300 million K S mesons produced in ϕ → K L K S decays recorded by the KLOE experiment at the DAΦNE e + e − collider we have measured the branching fraction for the decay K S → π μ ν . The K S mesons are identified by the interaction of K L mesons in the detector. The K S → π μ ν decays are selected by a boosted decision tree built with kinematic variables and by a time-of-flight measurement. Signal efficiencies are evaluated with data control samples of K L → π μ ν decays. A fit to the reconstructed muon mass distribution finds 7223 ± 180 signal events. Normalising to the K S → π + π − decay events the result for the branching fraction is B ( K S → π μ ν ) = ( 4.56 ± 0.11 stat ± 0.17 syst ) × 10 − 4 . It is the first measurement of this decay mode and the result allows an independent determination of | V u s | and a test of the lepton-flavour universality.