smallest units of matter, with diameters in the range of approximately 10^–13 to 10^–16 cm (approximately 4 x 10^–14 to 4 x 10^–17 in). Elementary particle physics—the study of elementary particles and their interactions—is also called high-energy physics, because the energy involved in such dimensions is very high, as the UNCERTAINTY PRINCIPLE, (q.v.) dictates (see QUANTUM THEORY,). The term elementary particle was originally ascribed to the constituents of matter in these extremely small spatial dimensions because they were thought to be indivisible. Most of them are now known to be highly complex, but the name elementary particle is still applied to them.

The Rise of Particle Physics.

The origin of particle physics resulted from the study of smaller and smaller building blocks of matter. Before the 20th century, physicists studied the properties of bulk, or macroscopic, matter. In the late 19th century, however, the physics of atoms and molecules captured the attention of physicists. Atoms and molecules have diameters of about 10^–8 cm (about 4 x 10^–9 in), and the study of their structures resulted in the great achievements of quantum theory between 1925 and 1930. In the early 1930s physicists began investigating the structure of atomic nuclei, which have diameters of 10^–13 to 10^–12 cm (4 x 10^–14 to 4 x 10^–13 in). Enough was learned of nuclear structure to make practical use of atomic nuclei, as in nuclear power generators and in fission and fusion bombs (see ATOM AND ATOMIC THEORY.; NUCLEAR ENERGY,; NUCLEAR WEAPONS,). In the years after World War II, however, physicists came to realize the necessity of studying the structure of elementary particles in order to understand the fundamental structure of atomic nuclei.

Classification.

Several hundred elementary particles are now known experimentally through the various properties by which physicists identify them. They are divided into four classes—the photons, the leptons, the baryons, and the mesons.

Protons and neutrons are the basic constituents of atomic nuclei, which, combined with electrons, form atoms. Photons are the fundamental units of ELECTROMAGNETIC RADIATION, (q.v.), which includes radio waves, visible light, and X rays. The neutron is unstable as an isolated particle, disintegrating by the process

n → p + e + ν[dcl031]e

with an average life of 917 seconds. When combined with protons, however, to form certain atomic nuclei, such as oxygen-16 or iron-56, the neutrons are stabilized. Most of the elementary particles other than the electron, photon, proton, and neutron have been discovered since 1945, some in cosmic rays, the remainder in experiments using high-energy accelerators (see PARTICLE ACCELERATORS,). The existence of other particles has been predicted, but they have not yet been observed—such as the graviton, thought to transmit the gravitational force.

In 1928 the British physicist Paul A. M. Dirac predicted on theoretical grounds that, for every type of elementary particle, there is another type called its antiparticle. The antiparticle of the electron was found in 1932 by the American physicist Carl D. Anderson, who called it the positron. The antiproton was detected in 1955 by the American physicist Owen Chamberlain and the Italian-American nuclear physicist Emilio G. Segrè, a discovery for which they received the 1959 Nobel Prize in physics. It is now known that Dirac's prediction is valid for all elementary particles. Some elementary particles, such as the photon, are their own antiparticles. Physicists generally use a bar to denote an antiparticle; thus ν[dcl031]e is the antiparticle of νe.

Particles may also be classified in terms of their spin, or angular momentum, as bosons or fermions (see FERMION,). Bosons have a spin that is a whole-integer multiple of a certain constant, h; fermions have a spin that is a half-integer multiple of that constant.

Interactions.

Elementary particles exert forces on each other, and they are constantly created and annihilated. Creation, annihilation, and force are, in fact, related phenomena and are collectively called interactions. Four types of interactions of matter are known (although more have been postulated):

Nuclear interactions are the strongest and are responsible for the binding of protons and neutrons to form nuclei. Next in strength are the electromagnetic interactions that are responsible for binding electrons to nuclei in atoms and molecules. From the practical viewpoint, this binding is of great importance because all chemical reactions represent transformations of such electromagnetic binding of electrons to nuclei (see CHEMICAL REACTION,). Much weaker are the so-called weak interactions that govern the radioactive decay of atomic nuclei, first observed (1896–98) by the French physicists and chemists Antoine H. Becquerel, Pierre Curie, and Marie Curie. The gravitational interaction of matter is important on a large scale, although it is the weakest of the elementary particle interactions.

Conservation Laws.

The dynamics of elementary particle interactions are governed by equations of motion that are generalizations of Newton's three fundamental laws of dynamics (see MECHANICS,). In Newtonian dynamics, energy, momentum, and angular momentum are neither created nor destroyed; rather, they are conserved. Energy exists in many forms that can be transformed into each other, but the total energy is conserved and does not change. For elementary particle interactions these conservation laws remain in effect, but additional conservation laws have been discovered that play important roles in the structure and interactions of nuclei and elementary particles.

Symmetry and quantum numbers.

In physics, symmetry principles were applied almost exclusively to problems in fluid mechanics and crystallography until the beginning of the 20th century. After 1925, with the increasing success of quantum theory in describing the atom and atomic processes, physicists discovered that symmetry considerations led to quantum numbers (which describe atomic states) and to selection rules (which govern transitions between atomic states). Because quantum numbers and selection rules are necessary to descriptions of atomic and subatomic phenomena, symmetry considerations are central to the physics of elementary particles.

Parity (P).

Most symmetry principles state that a particular phenomenon is invariant (unchanged) when certain spatial coordinates are transformed, or changed in a certain way. The principle of space reflection symmetry, or parity (P) conservation, states that the laws of nature are invariant when the three spatial coordinates, x, y, and z, of all particles are reflected (that is, when their signs are changed). A reaction (collision, or interaction) between two particles A and B, for example, having vector momenta pA and pB may have a certain probability of yielding two other particles C and D with their own characteristic momenta pC and pD. Let this reaction

A + B → C + D (R)

be called R. If particles A and B with momenta –pA and –pB produce particles C and D with momenta –pC and –pD at the same rate as R, then the reaction is invariable under parity (P).

Charge conjugation symmetry (C).

The symmetry principle of charge conjugation can be illustrated by referring to the reaction R. If the particles A, B, C, and D are replaced by their antiparticles Ā, B[dcl032], C[dcl033], and D[dcl034], then

Ā + B[dcl032] → C[dcl033] + D[dcl034] C(R)

Let this hypothetical reaction be termed C(R). It is the conjugate reaction of R. If (R) and C(R) proceed at the same rate, then the reaction is invariant under charge conjugation (C).

Time inversion symmetry (T).

The symmetry principle of time inversion, or time reversal, has a similar definition. The principle states that if a reaction (R) is invariant under (T), then the rate of the reverse reaction

C + D → A + B T(R)

is in a definite proportion to the rate of (R).

Symmetry and strengths of interactions.

The kinds of symmetry observed by the four different types of interactions have been found to be quite different. Before 1957 it was believed that space reflection symmetry (or parity conservation) is observed in all interactions. In 1956 the Chinese-American physicists Tsung Dao Lee and Chen Ning Yang pointed out that parity conservation had, in fact, not been tested for weak interactions and suggested several experiments to examine it. One of these was performed the following year by the Chinese-American physicist Chien-Shiung Wu (1912–97) and her collaborators, who found that, indeed, space reflection symmetry is not observed in weak interactions (the forces that govern radioactive decay). A consequence was the discovery that the particles emitted in weak interactions tend to spiral along the direction of their motion. In particular, the neutrinos νe and νµ, which are involved only in weak and gravitational interactions, always spin in a left-handed manner. The American physicists James W. Cronin and Val L. Fitch and their collaborators also discovered, in 1964, that time reversal symmetry is not observed in weak interactions. Experiments by Fermilab and CERN in 1998 confirmed that time-reversal symmetry is violated as predicted by Cronin and Fitch.

Symmetry and quarks.

The classification of elementary particles was based on their quantum numbers and thus went hand in hand with ideas about symmetry. Working with such considerations, the American physicists Murray Gell-Mann and George Zweig (1937–    ) independently proposed in 1963 that baryons and mesons are formed from smaller constituents that Gell-Mann called quarks and Zweig called aces. They suggested three kinds of quarks, each having an antiquark. Very good indirect evidence for the quark model of baryons and mesons has been accumulating, especially since the discovery in 1974 of J/ψ particles by the American physicists Samuel C. C. Ting (1936–    ) and Burton Richter (1931–    ). The first traces of quarks were provided by the American physicists Jerome I. Friedman and Henry W. Kendall and the Canadian physicist Richard E. Taylor in a series of experiments conducted between 1967 and 1973 at the Stanford Linear Accelerator Center (SLAC). For this work and for proving the existence of the GLUON, (q.v.) they were granted the 1990 Nobel Prize in physics. It is generally accepted today that six kinds of quarks exist; they are named “up,” “down,” “strange,” “charm,” “bottom,” and “top”. In the late 1990s astronomers found proof that neutrinos, the particles making up the hidden mass within the galaxies and known as “dark matter” because they do not emit light, are not zero mass, as presumed; however, the amount of mass they bear is so small that they remain unaffected by gravitational forces.

Field Theory of Interactions.

Before the mid-19th century, interaction, or force, was commonly believed to act at a distance. The English scientist Michael Faraday initiated the idea that interaction is transmitted from one body to another through a field. The Scottish physicist James Clerk Maxwell put Faraday's ideas into mathematical form, resulting in the first field theory, commonly called Maxwell's equations for electromagnetic interactions. In 1916 Albert Einstein published his theory of gravitational interactions, and that became the second field theory. It is now universally believed that the other two interactions, strong and weak, can also be described by field theories.

With the development of quantum mechanics, certain early difficulties with field theories were encountered in the 1930s and '40s. The difficulties were related to the very strong fields that must exist in the immediate neighborhood of a particle and were called divergence difficulties. To remove part of the difficulty a method called renormalization was developed in the years 1947–49 by the Japanese physicist Shin'ichiro Tomonaga (1906–79), the American physicists Julian Schwinger (1918–    ) and Richard Feynman, and the Anglo-American physicist Freeman Dyson (1923–    ). Renormalization methods showed that the divergence difficulties can be systematically isolated and removed. The program achieved great practical successes, but the foundation of field theory remains unsatisfactory.

Unification of field theories.

The four types of interactions are vastly different from one another. The effort to unify them into a single conceptual whole was started by Albert Einstein before 1920. The American physicists Sheldon Glashow and Steven Weinberg and the Pakistani physicist Abdus Salam in 1979 shared the Nobel Prize in physics for their work on a successful model unifying the theories of electromagnetic and weak interactions. This was done by putting together ideas of gauge symmetry developed by the German mathematician Hermann Weyl, Yang, and the American physicist Robert Laurence Mills (1927–    ) and of broken symmetry developed by the Japanese-American physicist Yoichiro Nambu (1921–    ), the British physicist Peter W. Higgs (1929–    ), and others. A very important contribution to these developments was made by the Dutch physicists Martinus J. G. Veltman and Gerardus 't Hooft, who pushed through the renormalization program for these theories. See SYMMETRY,.

Evaluation.

It is now recognized that the properties of all interactions are dictated by various forms of gauge symmetry. In retrospect, the first use of this idea was Einstein's search for a gravitational theory that is symmetrical with respect to coordinate transformations, which culminated in the general theory of relativity in 1916 (see GRAVITATION,; RELATIVITY,). Exploitation of such ideas will certainly be a principal theme of elementary particle physics during the coming years. Qualitative extension of the concept of gauge symmetry to facilitate, possibly, an eventual unification not only of all interactions, but also of all interactions with all constituent particles, has already been attempted in the ideas of supersymmetry and supergravity. Such developments will no doubt be pursued.

The final goal is an understanding of the fundamental structure of matter through unified symmetry principles. Unfortunately, this goal is not likely to be reached in the near future. There are difficulties in both the theoretical and experimental aspects of the endeavor. On the theoretical side, the mathematical complexities of quantum gauge theory are great. On the experimental side, the study of elementary particle structures at smaller and smaller dimensions requires larger and larger accelerators and detectors (see PARTICLE DETECTORS,).        C.N.Y., CHEN NING YANG, Ph.D.

For further information on this topic, see the Bibliography, sections 390. Quantum physics, 402. Atom and atomic theory, 405. Particle physics.