Mesons are particles composed of an even number of quarks and antiquarks, usually one quark and one antiquark, bound together by the exchange of gluons. Since they interact using the strong nuclear force, mesons are classified as hadrons, along with the proton and neutron. About 100 different mesons are known, varying from one-fifth the mass of a proton to about 10 times the proton's mass.

Mesons were first discovered in cosmic rays, as the products of the collisions of high-energy particles, mostly fast moving protons, with the nucleons of atoms in the atmosphere. Japanese physicist Hideki Yukawa predicted their existence in 1935 as the carriers of the strong force that binds the nucleons of an atom together. Since very few mesons reach the surface of Earth before interacting with particles in the atmosphere, they were not observed until 1946. These first mesons to be identified were called pi mesons, or pions. Although...