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

TL;DR

The E949 experiment at BNL observed evidence for the rare decay $K^+ o \pi^+ u ar{ u}$, measuring its branching ratio and contributing to understanding rare kaon processes.

Contribution

This work provides new experimental evidence and an updated measurement of the branching ratio for the rare decay $K^+ o \\pi^+ u ar{ u}$, combining data from E949 and E787.

Findings

Measured branching ratio: $(1.47^{+1.30}_{-0.89}) imes 10^{-10}$

Collected $1.8 imes 10^{12}$ kaons in the first year

Additional evidence for the rare decay observed

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($K^+\to\pi^+\nu\bar{\nu}$)=$(1.47^{+1.30}_{-0.89})\times10^{-10}$.