Evidence for the Decay K+ --> pi+ nu anti-nu

TL;DR

This paper reports the observation of a rare kaon decay event consistent with K+ --> pi+ nu anti-nu, providing evidence for this decay mode with a measured branching ratio, and discusses background estimation and implications.

Contribution

First experimental evidence for the rare decay K+ --> pi+ nu anti-nu with measured branching ratio and background analysis.

Findings

Observed one event consistent with K+ --> pi+ nu anti-nu

Estimated background contribution is 0.08 +/- 0.03 events

Branching ratio measured as (4.2 +9.7/-3.5) x 10^-10

Abstract

An event consistent with the signature expected for the rare kaon decay K+ --> pi+ nu anti-nu has been observed. In the pion momentum region examined, 211<P<230 MeV/c, the backgrounds are estimated to contribute 0.08 +/- 0.03 events. If the event is due to K+ --> pi+ nu anti-nu, the branching ratio is (4.2 +9.7/-3.5) x E-10.