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Mesons are a strongly interacting particle that is made up of a quark/antiquark pair and has an even integer spin. All mesons are hadrons, and every meson has a corresponding antimeson. Mesons were first considered by Yukawa Eric Weisstein's World of Biography, who in 1935 proposed the first significant theory of the strong force. His theory required the existence of a particle having a mass about 300 times that of the electron, but 1/6 that of the proton in order to mediate the force of attraction between a proton and neutron in a nucleus. This hypothetical intermediate-mass particle was termed a "meson" (meaning "middle-weight"). Such particles were subsequently sought in cosmic rays, and were apparently found in 1937 by two separate groups. However, the particles seemed to have the wrong lifetime and their masses could not be determined reproducibly. It was subsequently discovered that cosmic ray particles contained two intermediate-mass particles, which came to be named the muon and the pion. The muon turned out to be a heavy counterpart to the electron and not a meson at all (although it is still sometimes called a mu meson for historical reasons), but the pion was indeed a true meson of the kind predicted by Yukawa.