First Observation of the Rare Decay Mode K-long -> e+ e-

TL;DR

This paper reports the first observation of the extremely rare decay mode K-long -> e+ e-, measuring a branching fraction consistent with theoretical predictions and setting a new record for the smallest measured branching fraction in particle physics.

Contribution

First experimental observation of the K-long -> e+ e- decay, providing a precise measurement of its branching fraction and confirming theoretical models.

Findings

Observed four decay events with minimal background

Measured branching fraction of approximately 8.7 x 10^{-12}

Results are consistent with theoretical predictions

Abstract

In an experiment designed to search for and study very rare two-body decay modes of the K-long, we have observed four examples of the decay K-long -> e+ e-, where the expected background is 0.17+-0.10 events. This observation translates into a branching fraction of 8.7^{+5.7}_{-4.1} X 10^{-12}, consistent with recent theoretical predictions. This result represents by far the smallest branching fraction yet measured in particle physics.