THE SMALLEST NONZERO BRANCHING RATIO EVER MEASURED. Most known particles are highly fragile. Not only do they decay quickly but through a myriad of ways. Calculating and then also measuring the relative likelihoods (branching ratios) of the different forms of particle mortality are important diagnostics for understanding how matter behaves at the most fundamental level. A particularly rare form of decay (a very tiny tributary to the Amazon of expiring particles, as it were) is the decay of the K⁺ meson into a pi⁺ meson, a neutrino, and an anti-neutrino, a process sometimes involving the momentary creation of both a charged W boson (the carrier of the weak nuclear force) and its neutral cousin, the Z boson (which itself instantly decays into the two neutrinos), rather than the more common exchange of a single W or Z. A large collaboration at Brookhaven (contact Douglas Bryman, doug@triumf.ca, 604- 222-7338) has examined more than a trillion K decays and, after years of painstaking vigilance, they have finally found one event with the telltale signature. The expected background for this process, for their amount of data, would be 0.08 events. The researchers are therefore confident that they have measured a true K decay branching ratio, with a value of 4.2 x 10^-10 (with an uncertainty of +9.7 or -3.5 x 10^-10). In the next year or so additional data should settle the issue of whether the observed rare K decay conforms to the standard model of particle physics (in which case the measured decay rate will provide information about related processes, such as the decay of top quarks) or represents evidence of "new physics" outside the current theory. (S. Adler et al., Physical Review Letters, 22 September 1997; see also a figure at Physics News Graphics)