A new $K_S \to \pi e \nu$ branching fraction measurement from KLOE-2

Abstract

A new measurement of the branching fraction for the decay $K_S \to \pi e \nu$ is presented, based on a sample of 300 million $K_S$ mesons recorded by the KLOE experiment at the DA$\Phi$NE $e^+e^-$ collider. A two-step signal selection strategy is used, exploiting first kinematic variables and then time-of-flight measurements. Data control samples of $K_L \to \pi e \nu$ decays are used to evaluate signal selection efficiencies. Normalizing the selected sample to the $K_S \to \pi^+ \pi^-$ decay events the result for the branching fraction is $B(K_S \to \pi e \nu) = (7.211 \pm 0.046_{stat} \pm 0.052_{syst}) × 10^{−4}$. The combination with our previous measurement gives $B(K_S\to \pi e \nu) = (7.153 \pm 0.037_{stat} \pm 0.043_{syst} ) x 10^{-4} = (7.153 \pm 0.057) x 10^{-4}$. From this value we derive $f_+ (0)|V_{us} |=0.2170 \pm 0.0009$.