First Observation of the Rare Decay Mode K-long -> e+ e-
  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. 
BNL E871 Collaboration: D. Ambrose, C. Arroyo, M. Bachman, P. de   Cecco, D. Connor, M. Eckhause, K. M. Ecklund, S. Graessle...