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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:mspace /><mml:mo>→</mml:mo><mml:mspace /><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:…
An event consistent with the signature expected for the rare kaon decay ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ has been observed. In the pion momentum region examined, $211<P<230\mathrm{MeV}/c$, the backgrounds are estimated to contribute $0.08\ifmmode\pm\else\textpm\fi{}0.03$ events. If the event is due to ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$, the branching ratio is ${4.2}_{\ensuremath{-}3.5}^{+9.7}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$.