The new KS → π e ν branching fraction measurement at KLOE

A new measurement of the branching fraction for the decay KS → π e ν is presented, based on a sample of 300 million KS mesons recorded by the KLOE experiment at the DAΦNE 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 KS → π e ν decays are used to evaluate signal selection efficiencies. Normalizing the selected sample to the number of KS → π⁺ π⁻ decay events the result for the branching fraction is B(KS → π e ν)=(7.211 ± 0.046 ± 0.052)× 10⁻4. The combination with our previous measurement gives B(KS → π e ν)=(7.153 ± 0.037 ± 0.043)× 10⁻4 =(7.153 ± 0.057)× 10⁻4. From this value we derive f⁺(0)|Vus|=0.2170 ± 0.0009.