An elementary particle such as a meson (a quark-antiquark pair) can decay in many different ways. For each possible decay path, physicists can measure the "branching ratio," a number that describes the relative likelihood for that particular decay path to occur against all other possible modes of decay.
The above diagrams show three different decay paths for the K+ meson. In each case, the K+ meson produces three end products: a + meson, a neutrino (denoted by e) and an anti-neutrino (denoted by a e with a horizontal bar on top to indicate that it is an anti-particle).
The decay depicted in the top panel, recently observed for the first time, represents the smallest branching ratio (4.2 x 10-10) for a particle decay ever measured. In the top figure one of the K's constituents, an anti-strange quark (s bar), first converts into a W boson and an anti-top quark (t bar), which then produces an anti-down (d bar) quark and a second W. (The V parameters designate the strength of the interaction at that particular vertex.) The two lower figures depict other ways in which the K decays into a +, a neutrino, and an antineutrino. (Figure courtesy of the researchers.)
This research is described by S. Adler et al. in Physical Review Letters, v. 79, 2204 (1997). For more information, contact Douglas Bryman, TRIUMF, email@example.com)
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