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

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