First Observation of the Rare Decay Mode <span class="aps-inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msubsup><mrow><mi mathvariant="italic">K</mi></mrow><mrow><mi mathvariant="italic">L</mi></mrow><mrow><mn>0</mn></mrow></msubsup></mrow><mi></mi><mo>→</mo><mi></mi><mrow><msup><mrow><mi mathvariant="italic">e</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow><mrow><msup><mrow><mi mathvariant="italic">e</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math></span>

doi:10.1103/PhysRevLett.81.4309

First Observation of the Rare Decay Mode $K_{L}^{0} \to e^{+} e^{-}$

D. Ambrose et al. (BNL E871 Collaboration)

Phys. Rev. Lett. 81, 4309 – Published 16 November 1998

Abstract

In an experiment designed to search for and study very rare two-body decay modes of the $K_{L}^{0}$ , we have observed four examples of the decay $K_{L}^{0} \to e^{+} e^{-}$ , where the expected background is $0.17 \pm 0.10$ events. This observation translates into a branching fraction of ${8.7}_{- 4.1}^{+ 5.7} \times 10^{- 12}$ , consistent with recent theoretical predictions. This result represents by far the smallest branching fraction yet measured in particle physics.