Measurement of the branching fraction for the decay $K_S\to \pi \mu \nu$ with the KLOE detector

Abstract

Based on a sample of 300 million $K_S$ mesons produced in $\varphi \to K_L{K}_S$ decays recorded by the KLOE experiment at the DA$\Phi$NE $e^{+}{e}^{-}$ collider we have measured the branching fraction for the decay $K_S\to \pi \mu \nu$. The $K_S$ mesons are identified by the interaction of $K_L$ mesons in the detector. The $K_S\to \pi \mu \nu$ 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 $K_L\to \pi \mu \nu$ decays. A fit to the reconstructed muon mass distribution finds $7223\pm 180$ signal events. Normalising to the $K_S\to {\pi }^{+}{\pi }^{-}$ decay events the result for the branching fraction is $\mathcal{B}(K_S\to \pi \mu \nu )=(4.56\pm 0.11_{\mathrm{stat}}\pm 0.17_{\mathrm{syst}})\times 10^{-4}$.

Comments: 16 pages, 12 figures, Accepted in PLB