Further Evidence for the Decay<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">K</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mi/><mml:mo>→</mml:mo><mml:mrow><mml:msup><mml:mrow><mml:mi>π</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mi>ν</mml:mi><mml:mrow><mml:mrow><mml:mover><mml:mrow><mml:mi>ν</mml:mi></mml:mrow><mml:mrow><mml:mi>¯</mml:mi></mml:mrow></mml:mover></mml:mrow></mml:mrow></mml:math>

Abstract

Additional evidence for the rare kaon decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ has been found in a new data set with comparable sensitivity to the previously reported result. One new event was observed in the pion momentum region examined, $211&lt;P&lt;229\mathrm{MeV}/c$, bringing the total for the combined data set to two. Including all data taken, the backgrounds were estimated to contribute $0.15\ifmmode\pm\else\textpm\fi{}0.05$ events. The branching ratio is $B({K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}){\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.57}_{\ensuremath{-}0.82}^{+1.75}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$.