Rare Kaon Decay From E949 At BNL: K^+ -->pi^+ nunubar

Abstract

In the first year of physics run, the E949 experiment at Brookhaven National Laboratory has already collected $1.8\times 10^{12}$ kaons stopping in the target. Additional evidence for the rare charged kaon decay $K^+\to\pi^+\nu\bar{\nu}$ has been observed. Combined with previous results from the E787 experiment, the branching ratio is measured to be Br($K^+\to\pi^+\nu\bar{\nu}$)=$(1.47^{+1.30}_{-0.89})\times10^{-10}$.