Observation of the decay $K^+\to \pi^+ \nu \bar\nu$

Abstract

We have observed 1 event consistent with the signature expected of the rare decay of a positive kaon to a positive pion and a neutrino anti-neutrino pair. In the examined momentum region of 211 to 230 MeV/c in the center of mass of the kaon we estimated the backgrounds to be about $0.08 \pm 0.03$ events. From this observation we estimate the branching ratio to be $4.2^{+9.7}_{-3.5} \times 10^{-10}$. In this presentation I will explain the experiment, and the analysis techniques. I will also discuss the expected improvements in the near future from the analysis of new data sets.