MUON, DISCOVERY OF

The discovery of the muon, the first and lightest unstable subatomic particle, involved many experiments over a period of fourteen years. The effort to understand the nature of this particle did much to create the framework in which the science of particle physics developed.

The discovery grew out of studies of "cosmic rays" in the 1930s by Carl D. Anderson of the California Institute of Technology. Our planet is continually bombarded by high-energy radiation that originates outside the solar system, most of which consists of atomic nuclei. These nuclei collide with the nuclei of atoms in the upper atmosphere. By conversion of kinetic energy to particle mass, a cascade of particles is produced. Most of these are highly unstable and do not survive to reach the surface of the Earth, where one finds instead the products of their disintegration.

Anderson used a device called a "cloud chamber" to identify the particles that did reach the surface. In a cloud chamber, a moving electrically charged particle leaves a trail of water droplets along its path, which can be recorded in a photograph. If the chamber is in a magnetic field, the tracks will be curved, and the curvature can be used to measure the momentum of the particles. The spacing between droplets on the track gives a rough measure of the velocity of the particle. If one knows momentum and velocity, the mass of a particle can be estimated.

By 1934 Anderson had determined that a major fraction of the particles he was observing had a mass much more than that of an electron but less than that of a proton. A more exact measure of the mass was not yet possible but Anderson could clearly rule out any known particle. These particles could carry either positive or negative electric charge.

In 1935, the Japanese theorist Hideki Yukawa proposed that a particle two hundred to three hundred times heavier than an electron could transmit a new force that acts only on the scale of the nucleus, explaining why a nucleus holds together despite the mutual repulsion of its protons. By 1938, most physicists in the field believed that Anderson's mysterious particle was the one proposed by Yukawa. The name "mesotron" was given to this particle, later shortened to "meson."

In 1940 it was discovered that the meson was un-stable, breaking up into an electron and some un-seen neutral particles in an average lifetime of two microseconds. As short as this time may seem, it is long enough to allow some of the particles to reach the Earth's surface. This is possible because of an effect from the theory of relativity; the "internal clock" of a particle slows down when it is moving close to the speed of light.

In a series of experiments that began in 1944 under difficult wartime conditions three Italian physicists at the University of Rome, Marcello Conversi, Ettore Pancini, and Oreste Piccioni, allowed mesons to stop in a variety of materials after separating the positive ones from the negative by means of a magnet. It was expected that positive mesons would survive for their normal lifetime, while negative ones would be attracted to nuclei where they would be quickly absorbed. But in 1946 they found that in light elements the negatives managed to survive. This showed that the mesons were not affected by the nuclear force and thus could not be Yukawa's particle.

A Mystery Resolved

The puzzle was resolved in 1947 by Cesar Lattes, Giuseppe Occhialini, and Cecil Frank Powell, working at Britain's Bristol University. They studied cosmic rays at mountaintop altitudes, where it is possible to observe some of the particles directly produced in cosmic ray collisions. They used detectors called "nuclear emulsions," sheets of light-sensitive material similar to that used in ordinary photographic film. While the thickness of the sensitive layer in camera film is usually no more than a few hundredths of a millimeter, in nuclear emulsions it can be as much as 2 millimeters. When the emulsion is developed, the path of a particle appears as a trail of tiny grains of metallic silver, which can be followed in three dimensions with the aid of a medium-power microscope.

Lattes, Occhialini, and Powell studied what happened when particles came to a stop in their emulsions. They discovered that in some cases a particle that appeared to be a meson would stop and then emit another particle of somewhat lower mass. The second particle would also stop within a fraction of a millimeter and decay into an electron and neutral particles, just like a meson. They called the first particle a "pi meson" because it was the primary particle in the two-step process, and pi is the Greek equivalent of the letter p . They hoped that the pi meson would prove to be Yukawa's particle. They assumed that the second particle was the familiar meson, which they called a "mu meson" (the Greek letter mu had by then become the accepted symbol for a meson). This was later shortened to "muon" when physicists decided to reserve the name "meson" for the members of a large family of particles that interact strongly with nuclei.

After these discoveries, the origin of cosmic ray muons became clear. The pi meson has a lifetime eighty times shorter than that of the muon. Thus a major share of them decay into muons near the collision that produced them, high in the atmosphere. The muons, having longer lifetimes, can reach the surface.

In the following year, Lattes took a stack of emulsions to the University of California in Berkeley, where he observed pi mesons produced in the collision of helium nuclei with larger nuclei in a cyclotron. This showed that the pi meson interacted strongly with nuclei, finally establishing it as Yukawa's particle. In the same year, Edward Hincks and Bruno Pontecorvo, at Canada's Chalk River Laboratory, demonstrated that the breakup of muons released two neutral particles in addition to the electron. Neither of the neutrals was a gamma ray, and they interacted weakly with matter. This made it likely that they were neutrinos, neutral counterparts of the electron.

Who Ordered That?

Like the electron, neutrinos spin with an angular momentum one-half of the fundamental quantum unit. Because the muon breaks up into an odd number of particles with half-integer spin, it too must be a half-integer spin particle, called "fermions" after the Italian-American physicist Enrico Fermi. Thus the muon is simply a heavier version of the electron. In response to this discovery the celebrated American physicist Isidor Rabi is reported to have remarked: "Who ordered that ?"

Today the muon is classified as one of six members of a family of particles called "leptons." Three of these, the electron, muon, and tau, are electrically charged and come in both positive and negative forms. The other three are neutrinos, designated the electron, muon, and tau neutrino because each is associated with one of the charged leptons. When a neutrino collides with a nucleus, it can be transformed into its companion charged lepton. All leptons interact weakly with nuclei, so such collisions are very rare.

See also:Anderson, Carl D.; Yukawa, Hideki

Bibliography

Piccioni, O. "The Discovery of the Muon" in History of Original Ideas and Basic Discoveries in Particle Physics, edited by Harvey B. Newman and Thomas Ypsilantis (Plenum Press, New York, 1996).

Galison, P. "The Discovery of the Muon and the Failed Revolution Against Quantum Electrodynamics." Centaurus26 , 262–316 (1983).

Robert H. March