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

BNL E871 Collaboration: D. Ambrose , C. Arroyo , M. Bachman , P. de Cecco , D. Connor , M. Eckhause , K. M. Ecklund , S. Graessle

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A. D. Hancock K. Hartman M. Hebert C. H. Hoff G. W. Hoffmann G. M. Irwin J. R. Kane N. Kanematsu Y. Kuang K. Lang R. Lee R. D. Martin J. McDonough A. Milder W. R. Molzon M. Pommot-Maia P. J. Riley J. L. Ritchie P. D. Rubin V. I. Vassilakopoulos C. B. Ware R. E. Welsh S. G. Wojcicki E. Wolin S. Worm

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classification ✦ hep-ex

keywords decayk-longbranchingfractionobservationrarebackgroundconsistent

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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.

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

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