Further Evidence for the Decay K+ to pi+ neutrino-antineutrino

Abstract

Additional evidence for the rare kaon decay K+ to pi+ neutrino-antineutrino has been found in a new data set with comparable sensitivity to the previously reported result. One new event was observed in the pion momentum region examined, 211<P<229 MeV/c, bringing the total for the combined data set to two. Including all data taken, the backgrounds were estimated to contribute 0.15 pm 0.05 events. The branching ratio is B=1.57^{+1.75}_{-0.82} 10^{-10}.

Figures (2)

• Figure 1: Display of candidate Event C. On the top left is the end view of the detector showing the track in the target, the drift chamber, and the range stack. On the top right is a blow-up of the track in the target, where the hatched squares represent target fibers hit by the $K^+$ and the open squares indicate those hit by the $\pi^+$; a trigger scintillator that was hit is also shown. The lower right hand box shows the digitized signal in the target fiber where the kaon stopped indicating no additional activity. The pulse was sampled every 2 ns (crosses) and the solid line is a fit. The lower left hand box shows the digitized $\pi\rightarrow\mu$ decay signal in the scintillator where the pion stopped. The curves are fits for the first, second and combined pulses.
• Figure 2: Range vs. energy plot of the final sample. The circles are for the 1998 data and the triangles are for the 1995-97 data set. The group of events around $E=108$ MeV is due to the $K_{\pi 2}$ background. The simulated distribution of expected events from $K^+ \! \rightarrow \! \pi^+ \nu \overline{\nu}$ is indicated by dots.