meson

Any member of a family of subatomic particles composed of a quark and an antiquark (see antimatter). Mesons are sensitive to the strong force, have integral spin, and vary widely in mass. Though unstable, many mesons last a few billionths of a second, long enough to be observed with particle detectors. They are readily produced in the collisions of high-energy subatomic particles (e.g., in cosmic rays).