Probing new physics with the kaon decays $K\to\pi\pi\!\not\!\!E$

Abstract

The latest search for the rare kaon decay $K^+\to\pi^+\nu\bar\nu$ by the NA62 experiment has produced evidence for it with a branching fraction consistent with the prediction of the standard model. The new result implies that in this decay, with the $\nu\bar\nu$ pair appearing as missing energy ($\not\!\!E$), the room for possible new physics is no longer sizable and that therefore its contributions to underlying four-particle $s\to d\!\not\!\!E$ operators with parity-even $ds$ quark bilinears have become significantly constrained. Nevertheless, we point out that appreciable manifestations from beyond the standard model induced by the corresponding operators with mainly parity-odd $ds$ quark bilinears could still occur in $K\to\pi\pi\!\not\!\!E$ modes, on which there are only minimal empirical details at present. We find in particular that new physics of this kind may enhance the branching fraction of $K_L\to\pi^0\pi^0\!\not\!\!E$ to values reaching its current experimental upper limit and the branching fractions of $K^+\to\pi^+\pi^0\!\not\!\!E$ and $K_L\to\pi^+\pi^-\!\not\!\!E$ to the levels of $10^{-7}$ and $10^{-6}$, respectively. Thus, quests for these decays in existing kaon facilities such as KOTO and NA62 or future ones could provide valuable information complementary to that gained from $K\to\pi\!\not\!\!E$.