The Kaon disintegrates into a Pion, a neutrino and an antineutrino. This process is rare because you need two virtual Z or W particles in the intermediate steps of the decay.
Ok they haven't found anything yet..
In some sense, many of these measurements are ways to provide more points in some kind of line-fitting we have going on. "Sure, your new idea waves away dark matter buuuuuut there's this little matter of its predicted K+ decays. They would be way off. Either come up with a way to explain that or try again."