The new $K_S \rightarrow \pi e \nu$ branching fraction measurement at KLOE

Abstract

A new measurement of the branching fraction for the decay $K_S \rightarrow \pi e \nu$ 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 $K_S \rightarrow \pi e \nu$ decays are used to evaluate signal selection efficiencies. Normalizing the selected sample to the number of $K_S \rightarrow \pi^+ \pi^-$ decay events the result for the branching fraction is $B(K_S \rightarrow \pi e \nu)=(7.211 \pm 0.046 \pm 0.052)\times 10^-4$. The combination with our previous measurement gives $B(K_S \rightarrow \pi e \nu)=(7.153 \pm 0.037 \pm 0.043)\times 10^-4 =(7.153 \pm 0.057)\times 10^-4$. From this value we derive $f^+(0)|Vus|=0.2170 \pm 0.0009$.