Search for Short-Lived Neutral Particles Emitted in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>K</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>Decay

We have searched for the decay process ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{A}^{0}$, where ${A}^{0}$ is any particle of mass less than 100 MeV/${\mathit{c}}^{2}$ decaying into ${e}^{+}{e}^{\ensuremath{-}}$. Upper limits for the branching ratio are given as a function of mass and lifetime of the ${A}^{0}$. For lifetimes shorter than ${10}^{\ensuremath{-}13}$ sec, a limit of 4.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ at the 90% confidence level is obtained.