Rare kaon decays with ‘‘missing energy’’

Rare kaon decays due to the loop-induced standard model operator sL{{γ}}_{{{μ}}}dL{ν}L{{γ}}^{{{μ}}}νL are examined. Isospin-violating mass effects and electroweak radiative corrections are shown to reduce B(K⁺{→}π⁺{ν}{ν}{}) and B(KL{→}π⁰{ν}{ν}{}) relative to B(K⁺{→}π⁰{e}⁺νₑ) by 10% and 5.6%, respectively. Predicted branching ratios for (KL{→}{ν}{ν}{}{γ}) and (KL{→}{ν}{ν}{}) (if neutrinos have mass) are given. The sensitivity of ``missing energy'' rare K decays to new interactions or the emission of light weakly interacting neutral particles, other than neutrinos, is also briefly discussed. {} 1995 The American Physical Society.