Finding Order among the Particles

views updated

Finding Order among the Particles

Overview

In 1961 the physicist Murray Gell-Mann (1929- ) developed a method of organizing the dozens of subatomic particles that had been discovered to date. This classification system led him to realize that many of these particles were made up of even smaller particles. Gell-Mann named these smaller particles quarks. Eventually, physicists used Gell-Mann's work to formulate the standard model, a way of describing the known types of matter in the universe.

Background

Atoms are composed of particles called protons, neutrons, and electrons. These are called subatomic particles because they are smaller than atoms. During the 1950s and 1960s, physicists discovered many new types of subatomic particles, which were given names such as positrons, pions, and muons. These particles do not generally exist as parts of atoms, but are observed in cosmic rays—high-energy radiation that reaches Earth from space—or created in machines called particle accelerators. Accelerators are used to create high-speed collisions among particles. Such collisions often result in the production of new types of particles.

Physicists began to look for a way to organize the many kinds of subatomic particles that would show how the different types were related. In other words, they were searching for a physics version of the periodic table of the chemical elements. They first divided subatomic particles into two broad groups based on whether the particles are affected by the strong nuclear force. The strong nuclear force is what holds the nucleus of an atom together; it acts over very short distances (10^-15 meters) and allows similarly charged protons to exist close to each other. It is one of the four basic forces; the other three are gravity, electromagnetism, and the weak nuclear force. Particles affected by the strong nuclear force are called hadrons. Examples of hadrons are protons and neutrons. Particles that are not affected by the strong nuclear force are called leptons. There are six types of leptons, one of which is the electron. Leptons are said to be fundamental particles because they do not seem to be made up of simpler structures.

Unlike leptons, there are hundreds of types of hadrons. These particles can be subdivided into two groups called mesons and baryons. These groups are based on the particles' spin. Subatomic particles behave as though they are spinning balls carrying an electric charge. Spin is a measure of both the speed at which the particle is spinning and the mass of the particle. Some particles have a greater spin than others. For instance, protons are assigned a spin of ¹/₂, kaons—particles discovered in cosmic rays—have a spin of zero. The spin of mesons is equal to zero or whole number integers. The spin of baryons is equal to half-integers, such as ¹/₂. Kaons therefore belong to the group of mesons, and protons belong to the group of baryons.

In 1961 Murray Gell-Mann, a physicist at the California Institute of Technology, introduced a method of further organizing hadrons, which he called the Eightfold Way. He grouped similar particles into families. Each family contains 1, 8, 10, or 27 particles. (Most families contain eight particles, as indicated by the name Eightfold Way.) Particles with the same spin belong to the same group. For each family of hadrons, Gell-Mann made a graph in which he plotted the particles' electric charge on one axis and their strangeness on the other. The strangeness of a particle is related to the speed with which it decays (if it decays at all). The data points of these graphs formed geometrical patterns. For instance, families of eight mesons formed hexagonal patterns (six data points arranged in a hexagon with two data points at the center). Families of ten baryons formed triangular patterns (a row of four points, a row of three points, a row of two points, and a row of one point). The relationships revealed by these graphs could be used to predict the mass, magnetic properties, and interactions of the particles. The Israeli physicist Yuval Ne'eman (1925- ) suggested a similar organization system at about the same time.

Impact

Gell-Mann was able to use the Eightfold Way to predict the existence of particles that had not yet been discovered. For instance, he predicted the discovery of a baryon that he called the omega-minus particle, which belongs to a 10-particle family. Gell-Mann had made a graph of this family, and the data points formed a triangular shape. At the time, however, one of the data points was missing. He anticipated that the particle corresponding to this data point would soon be found. In 1964 the omega-minus particle was discovered with the aid of particle accelerator at the Brookhaven National Laboratory in Upton, New York. The properties of this particle corresponded to those that Gell-Mann had predicted, offering proof of the Eightfold Way.

However, the Eightfold Way did not explain why there are so many hadrons. In fact, it suggested to physicists that hadrons were composed of even smaller particles. A similar situation occurred after the creation of the periodic table. In the table, the chemical elements are organized into families with similar properties. Scientists eventually discovered that the patterns exhibited by the chemical families are based on the subatomic particles—the protons, neutrons, and electrons—that made up each element. Similarly, in 1964, Gell-Mann and the physicist George Zweig (1937- ) suggested that hadrons are not fundamental particles—that they are in fact made up of even smaller particles that are responsible for the patterns seen in the families of the Eightfold Way. Gell-Mann called these particles quarks.

Gell-Mann proposed the existence of three types of quarks. Each type is called a flavor, and he named the three flavors up, down, and strange. Unlike all previously observed particles, quarks are thought to have fractional electric charges. Up quarks have a charge of +⅔ and down and strange quarks each have a charge of -⅓. Each flavor of quark also has an antiquark with an opposite charge. (An antiquark is a type of antimatter. Antimatter is identical to matter except that its particles have charges that are opposite to those of matter. Every particle of matter has a corresponding particle of antimatter. For instance, there are antiprotons and antineutrons.) Gell-Mann proposed that all baryons are composed of three quarks and that all mesons are composed of a quark and an antiquark.

A proton, which is a baryon, consists of two up quarks and one down quark. The charges on these three quarks add up to +1 [+⅔[+⅔ + (-⅓) = +1], which is the charge on a proton. A neutron, which is also a baryon, consists of one up quark and two down quarks. The charges on these quarks add up to zero [+⅔ + (-⅓) + (-⅓)=0], which is the charge on a neutron. Gell-Mann's rules could account for every hadron that had been discovered up to that time.

However, some scientists argued against Gell-Mann's ideas because no one had been able to observe quarks individually. It seemed that it was impossible to knock quarks loose from the hadrons of which they were a part. Therefore, a team of physicists led by Jerome I. Friedman (1930- ), Henry W. Kendall (1926-1999), and Richard E. Taylor (1929- ) set out to observe quarks indirectly. They did so by firing high-speed electrons at protons with the aid of a particle accelerator. Most of the electrons passed straight through the protons, indicating that protons are mostly empty space. However, a few of the electrons were reflected from the protons at sharp angles. The pattern formed by the scattering of these electrons indicated that the proton contained three point-like structures. It is believed that these structures are the quarks proposed by Gell-Mann.

In 1965 the physicists Oscar Greenberg (1932- ) and Yoichiro Nambu (1921- ) described a property of quarks called color. In this case, color does not refer to an actual color. Instead, is similar to an electrical charge. Each flavor of quark comes in the colors red, green, and blue. Every baryon must contain one red quark, one blue quark, and one green quark. Every meson must contain one quark and one antiquark of the same color. These color rules support the idea that individual quarks cannot exist in isolation and cannot be observed directly.

More evidence of the existence of quarks occurred when Samuel Chao Chung Ting (1936- ) and Burton Richter (1931- ) discovered the psi meson in 1974. To explain the properties of this particle, the physicists realized that a fourth flavor of quark was needed, which they called the charm quark. Additional particles containing charm quarks were soon found. Fifth and sixth flavors were eventually discovered as well—the bottom quark in 1997 and the top quark in 1994.

Scientists now believe that all matter is composed of three fundamental types of particles: leptons, quarks, and force carriers. Force carriers can be thought of as the "glue" that binds the other particles together. For instance, the force carriers that bind quarks together are called gluons. This description of matter is called the standard model.

The quarks and leptons of the standard model can be organized into three generations, or groups. Each generation consists of a pair of quarks and a pair of leptons. The greater the number of the generation, the heavier the particles. The first generation contains up and down quarks, electrons, and electron neutrinos. (Electron neutrinos have no charge and are believed to have very little or no mass. They play a role in the radioactive decay of some elements.) The particles of the first generation make up the ordinary matter found on Earth. The second generation contains strange and charmed quarks, and leptons called muons and muon neutrinos. The third generation contains bottom and top quarks, and leptons called tau particles and tau neutrinos. The particles of the second and third generations are seen in cosmic rays or are produced in particle accelerators.

Physicists can see patterns in the three generations of particles, but presently there is no explanation as to why these patterns exist. Just as subatomic particles accounted for the patterns seen in the periodic table and quarks accounted for the patterns seen in the Eightfold Way, physicists hope to someday discover what causes the patterns seen in the generations of fundamental particles.

STACEY R. MURRAY

THE LITERATURE OF PARTICLE PHYSICS

It may seem that particle physicists have a fondness for rather bizarre (and even silly) language. Some of their terms are taken directly from everyday speech and given new meanings. Examples include color, flavor, and strangeness. Other terms have their roots in literature. For instance, Gell-Mann took the word quark from a line in James Joyce's 1939 novel Finnegan's Wake. The line, which reads, "Three quarks for Muster Mark!," is part of a dream description involving a character who has passed out drunk on the floor. Joyce invented much of the language in this experimental novel, including the word quark.

The term Eightfold Way derives from Buddhist teaching on the way to nirvana (a state of perfect peace or bliss): "Now this, O monks, is noble truth that leads to the cessation of pain; this is the noble Eightfold Way: namely, right views, right intention, right speech, right action, right living, right effort, right mindfulness, right concentration."

A third example of the blending of physics and literature involves top and bottom quarks, which were initially called truth and >beauty quarks. These names refer to the final lines of the poem "Ode on a Grecian Urn," written by John Keats in 1819: "Beauty is truth, truth beauty,—that is all / Ye know on earth, and all ye need to know."

Science and Its Times: Understanding the Social Significance of Scientific Discovery