Long‐lived kaon shows no 2‐muon decay

The mysterious long‐lived K meson has produced another surprise. An experiment to look for $KL0→μ+μ−$ has failed to find any such decays and has established an upper limit to the branching ratio (for two‐muon decay compared to all decays) of $1.82×10−9,$ with a 90% confidence level. Straightforward theoretical estimates had predicted a branching ratio of about $10−8,$ but in any case a fairly plausible lower bound of $6×10−9,$ more than three times larger than the experimental upper limit. The experiment was done at the Berkeley Bevatron by Alan Clark, Tom Elioff, Clive Field, Henry Frisch, Rolland Johnson, Leroy Kerth and William Wenzel, all of Lawrence Radiation Laboratory, Berkeley (Phys. Rev. Lett., 28 June).