Measurement of the $K^+\to\pi^+\nu\bar{\nu}$ Branching Ratio

TL;DR

This paper reports a measurement of the rare decay $K^+ o \pi^+ u ar{ u}$, combining new experimental data with previous results to estimate its branching ratio, which aligns with Standard Model predictions.

Contribution

First detailed measurement of the $K^+ o \\pi^+ u ar{ u}$ decay branching ratio using the E949 experiment data, improving precision over previous results.

Findings

Observed one candidate event with low background.

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

Result consistent with Standard Model prediction.

Abstract

Experiment E949 at Brookhaven National Laboratory studied the rare decay \$K^+\to\pi^+\nu\bar{\nu}$ and other processes with an exposure of $1.77\times 10^{12}$ $K^+$'s. The data were analyzed using a blind analysis technique yielding one candidate event with an estimated background of $0.30\pm0.03$ events. Combining this result with the observation of two candidate events by the predecessor experiment E787 gave the branching ratio ${\calB}(K^+\to\pi^+\nu\bar{\nu})=(1.47^{+1.30}_{-0.89})\times 10^{-10}$, consistent with the Standard Model prediction of $(0.74\pm0.20)\times 10^{-10}$. This is a more detailed report of results previously published in Physical Review Letters.