Pi Mesons
By 1932, scientists had identified the basic structure of an atom: a nucleus consisting of protons and neutrons, surrounded by one or more electrons. This model, however, presented a difficult puzzle. How was it that the nucleus, whose only charged particles (protons) all carried the same charge, could stay together? It would appear that the force of electrostatic repulsion among the protons would tend to drive a body with this composition apart.
A number of physicists attempted to solve this problem. Werner Heisenberg suggested in 1932, for example, that protons and neutrons might exchange their identities very rapidly. Two protons might not have time to repel each other, he said, if one or both instantaneously became a neutron.
Another solution was suggested by the Japanese physicist Hideki Yukawa in 1935. Yukawa recognized that any force holding two like-charged particles together must be a very strong force. He suggested that this force could be understood by analogy with another well known and well understood force, electromagnetism.
Scientists say that the electromagnetic force is transmitted between two bodies by the exchange of a mediating particle, the photon. Perhaps, Yukawa said, the nuclear force is transmitted by another kind of mediating particle. Yukawa calculated the characteristics this particle would have to have to play this role, and he predicted that its mass would be about 200-300 times that of the electron, or about one-ninth that of the proton. Since such a mass is about midway between that of the electron and proton, it was eventually given the name meson, or "middle."
Only a year later, a particle apparently fitting Yukawa's description was found. Carl Anderson, at the California Institute of Technology, identified the tracks of a new particle among those produced during a cosmic ray shower. He calculated the mass of the new particle to be 206.77 times that of the electron, in just the range predicted by Yukawa. He gave the name mesotron to the particle, a name later shortened to meson. Anderson found both positively and negatively charged forms of the meson. Both forms are unstable and decay to form an electron or positron and neutrinos or antineutrinos.
Unfortunately, it soon became obvious that Anderson's particle was not the one Yukawa had predicted. The particle did not interact with protons and neutrons and, therefore, could not transmit the nuclear force between these nucleons. The true character of Anderson's meson was not really recognized until about 1960, by which time it was called the mu meson , or muon. Then physicists recognized that the muon is identical to the electron in every way except for its mass. It is really a "heavy electron," a member of a class of particles known as the leptons. As such, the muon can no longer be regarded as a true meson.
The recognition that muons do not interact with nucleons appeared to be a serious problem for Yukawa's theory. The theory was salvaged, however, by the discovery in 1947 of a second type of meson, the pi meson, or pion. Like the muon, the pion was also discovered among the tracks produced during a cosmic ray shower. Its discoverer, the British physicist Cecil Powell, found that the pion is slightly heavier than the muon, about 273 times as massive as the electron. Unlike the muon, however, the pion was found to interact strongly with nucleons, matching the predictions made by Yukawa a decade earlier. For their research on the mesons, Yukawa and Powell both won Nobel Prizes for physics in 1949 and 1950, respectively.
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