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First Observation of the Rare Decay Mode<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msubsup><mml:mrow><mml:mi mathvariant="italic">K</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="italic">L</mml:mi></mml:mrow><mml:mrow><mml:mn>0</mml:mn></mml:mrow></mml:msubsup></mml:mrow><mml:mi /><mml:mo>→</mml:mo><mml:mi /><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">e</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow…
In an experiment designed to search for and study very rare two-body decay modes of the KL0, we have observed four examples of the decay KL0→e+e−, where the expected background is 0.17±0.10 events. This observation translates into a branching fraction of 8.7−4.1+5.7×10−12, consistent with recent theoretical predictions. This result represents …