New Measurement of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>K</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo>→</mml:mo><mml:msup><mml:mi>π</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mi>ν</mml:mi><mml:mover accent="true"><mml:mi>ν</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math>Branching Ratio

Abstract

Three events for the decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ have been observed in the pion momentum region below the ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{0}$ peak, $140&lt;{P}_{\ensuremath{\pi}}&lt;199\text{ }\text{ }\mathrm{MeV}/c$, with an estimated background of $0.93\ifmmode\pm\else\textpm\fi{}0.17(\mathrm{stat}.{)}_{\ensuremath{-}0.24}^{+0.32}(\mathrm{syst}.)$ events. Combining this observation with previously reported results yields a branching ratio of $\mathcal{B}({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}})=({1.73}_{\ensuremath{-}1.05}^{+1.15})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ consistent with the standard model prediction.