The K meson, or by its more preferred name, the kaon, is one of the family of mesons ( a class of particles heavier than leptons but lighter than baryons). The k meson has zero spin, a nonzero strangeness quantum number, and a mass of approximately 494 MeV. The k meson is the lightest hadron that contains a strange quark. The mean lifetime of the k meson is 1.24 x 10-8 s.

In the 1930s physicists were trying to describe the strong nuclear force. Because the strong force acts only within the range of the nucleus, it acts only over a very short range of distance. Because the strong force acts on both protons and neutrons in the same way, the strong force must be independent of electric charge. In 1935 Japanese physicist Hideki Yukawa proposed that the nuclear force between protons was mediated by a massive particle, now called...