In 1935 a Japanese particle physicist Hideki Yukawa (1907-1981) was interested in the nature of the strong nuclear force that binds protons and neutrons together in the nucleus. Using quantum field theory, he considered that neutrons and protons could be regarded as fields in which virtual particles were exchanged between them. But what kind of virtual particle was it?
Since the strong nuclear force operates over a very short range (about the diameter of a proton), the lifetime of these particles had to be very short or otherwise they would extend beyond the nuclear radius and the strong force would operate outside the boundaries of the nucleus. Yukawa calculated that the mass of these particles must be about 200 times that of the electron and a tenth of that of the proton. In other words they were between the electron and proton in mass. These particles were called mesons from the Greek word for middle. Yukawa postulated that the strong nuclear force was carried by a virtual particle now called the pi-meson or pion. The pion came in three varieties in order to accommodate the possible interactions between nucleons during a nuclear reaction. Yukawa showed that a proton could change into a neutron by emitting a pion with a positive charge or equivalently, by absorbing a negatively-charged pion. Alternatively, the interaction between nucleons could leave them unchanged, which could only be explained by the existence of an uncharged pion. The three charge states of the pion were therefore positive, negative and neutral.
In 1937, five years after he discovered the positron, Anderson found another particle created in cosmic rays. The particle had both positive and negative charge states and a mass 200 times that of the electron. Anderson thought, at first, that he had discovered Yukawa's pion. However this particle did not seem to interact with atomic nuclei as Yukawa's pion was expected to do. Also the short lifetime of the pion meant that it was not expected to live long enough to be detected at ground level. In addition, Anderson could find no trace of any neutral pion. Particle physicists eventually came to the conclusion that this particle was not the pion and called it the mu-meson or muon. It is now known, however, that the muon is not a meson and is not the same as Yukawa's pion.
Yukawa's meson was finally found in 1947 by Cecil Frank Powell (1903-1969), and Guiseppe P.S. Occhialini (1907-1993) from the University of Bristol who exposed nuclear emulsions to cosmic rays at high altitudes. The + and the - were the first to be discovered with a mass of 0.150 u (140MeV/c2). The neutral pion 0 was discovered later in accelerator experiments. There is a plethora of mesons which have now been discovered, including the kaon, lambda, xi and sigma particles. It is now known that the mesons are not fundamental particles but are made up of quarks.