I.

Introduction

Elementary Particles, originally units of matter believed or provisionally assumed to be fundamental; now, subatomic particles in general. Elementary-particle physics—the study of elementary particles and their interactions—is also called high-energy physics, because the energy involved in probing extremely small distances is very high, as the uncertainty principle dictates. The term “elementary particle” was originally ascribed to these constituents of matter 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.

II.

The Rise of Particle Physics

Particle physics is the latest stage in 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 their attention. Atoms and molecules have diameters of about 10^-8 cm (about 4 × 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 × 10^-14 to 4 × 10^-13 in). Enough was learned of nuclear structure to make practical use of nuclear energy, as in nuclear power generators and in 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.

III.

Classification

Several hundred elementary particles are now known experimentally. They can be divided into several broad classes. Hadrons and leptons are defined according to the types of force that they are subject to (see below). The forces are transmitted by further types of particles, called exchange, or messenger, particles. Examples are listed in the accompanying table.

Protons and neutrons are the basic constituents of atomic nuclei, which, combined with electrons, form atoms. Photons are the fundamental units of electromagnetic radiation, which includes radio waves, visible light, and X-rays. The neutron is unstable as an isolated particle, disintegrating into a proton, an electron, and a type of antineutrino called an electron-antineutrino. This process is symbolized thus:

n → p + e + e

This process should not be thought of as the separation of three particles that were originally all present together in the neutron. The neutron ceases to exist, while the proton, electron, and electron-antineutrino are created.

The neutron has 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 known elementary particles have been discovered since 1945, some in cosmic rays, the remainder in experiments using high-energy accelerators (see Particle Accelerators). The existence of a variety of other particles has been proposed, such as the graviton, thought to transmit the gravitational force.

In 1930 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 found in 1955 by the American physicists Owen Chamberlain and Emilio Segrè. It is now known that Dirac’s prediction is valid for all elementary particles, though some elementary particles, such as the photon, are their own antiparticles. Physicists generally use a bar to denote an antiparticle; thus _e (the electron-antineutrino) is the antiparticle of v_u (the electron-neutrino).

Particles may also be classified in terms of their spin, or intrinsic angular momentum, as bosons or fermions. Bosons have a spin that is a whole-number multiple of h/2p, where h is Planck’s constant; fermions have a spin that is an odd-half-integer multiple of h/2p, such as ” (h/2p). Fermions obey the Pauli exclusion principle.

IV.

Interactions

Elementary particles exert forces on each other, and they are constantly created and annihilated. Forces and processes of creation and annihilation, are, in fact, related phenomena and are collectively called interactions. Four types of interaction, or fundamental forces, are known:

Nuclear, or strong, 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. Much weaker are the so-called weak interactions that govern the radioactive decay of atomic nuclei, first observed (1896-1898) by the French physicists and chemists Antoine H. Becquerel, Pierre Curie, and Marie Curie. The gravitational interaction is important on a large scale, although it is the weakest of the elementary particle interactions.

V.

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.

A.

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.

B.

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). For example, a reaction (a collision or interaction) between two particles A and B having momenta p_A and p_B may have a certain probability of yielding two other particles C and D with their own characteristic momenta p_C and p_D. Let this reaction

A + B → C + D       (R)

be called R. If particles A and B with momenta -p_A and -p_B produce particles C and D with momenta -p_C and -p_D at the same rate as R, then the reaction is invariant under parity (P).

C.

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, , , and , then R becomes this reaction (which may or may not actually occur):

Ā +  →  +        C(R)

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

D.

Time-Reversal 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 equal to the rate of (R).

E.

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 and her colleagues, who found that, indeed, space-reflection symmetry is not observed in weak interactions. A consequence was the discovery that the particles emitted in weak interactions tend to show “handedness”, a fixed relationship between their spins and directions of motion. In particular, neutrinos, which are involved only in weak and gravitational interactions, always spin in a left-handed manner—that is, in relation to its direction of motion, the particle’s spin is in the opposite sense to that of an ordinary corkscrew. The American physicists James W. Cronin and Val L. Fitch and their colleagues also discovered, in 1964, that time-reversal symmetry is not observed in weak interactions. See also CPT Invariance.

F.

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 independently proposed in 1963 that baryons and mesons are formed from smaller constituents that Gell-Mann called quarks. They suggested three kinds of quark, each having an antiquark. The three quarks were named up, down, and strange, and together they accounted for all the baryons and mesons known at the time. Although the idea was mathematically very elegant, there was no experimental evidence for the quarks, so it was not widely accepted. However, the situation slowly changed as evidence began to accumulate. At the Stanford Linear Accelerator Center (SLAC), in California, physicists fired a beam of high-energy electrons at a target of protons. They found that a few of the electrons were scattered through very large angles. Richard Feynman and James Bjorken interpreted this as evidence for point charges inside the protons—the quarks. The 1990 Nobel Prize for Physics was awarded to Jerome Friedman, Henry Kendall, and Richard Taylor for their work on this experiment. The experiment was analogous to a classic particle-scattering experiment of Ernest Rutherford, which in 1911 revealed the existence of the atomic nucleus—itself also a concentration of charge within a larger entity, the atom.

In November 1974 two independent teams announced the discovery of a new type of meson, the J/Ψ. Theoreticians were able to explain its properties by introducing a fourth quark, named the charm quark, c. The J/Ψ is a C, a combination of a charm and an anticharm. Acceptance of the quark idea rapidly grew from this point. The 1976 Nobel Prize went to Samuel Ting and Burton Richter for their joint discovery. However, 1977 brought the discovery of the upsilon meson, a combination of a new kind of quark, the b or bottom quark, with its antiparticle, B. At this point it seemed clear on theoretical grounds that a sixth quark would eventually be discovered. The top quark, t, was finally announced in 1995 after a long experimental run at Fermilab, in Batavia, Illinois. In the process physicists had to sift through 6 trillion reactions to find 17 clear examples of top quark events. Top turns out to be a very heavy quark (about 180 times the mass of a proton) and the delay in its discovery was due to the need for improvements in technology to create a sufficiently powerful accelerator.

VI.

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, comprising Maxwell’s equations for electromagnetic interactions. In 1916 Albert Einstein published his theory of gravitational interactions, and that became the second field theory. 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 1940s. The difficulties were related to the very strong fields that must exist in the immediate neighbourhood of a particle and were called divergence difficulties. To remove part of the difficulty a method called renormalization was developed in the years 1947-1949 by the Japanese physicist Shin’ichirō Tomonaga, the American physicists Julian Schwinger and Richard Feynman, and the Anglo-American physicist Freeman Dyson. Renormalization methods showed that the divergence difficulties can be systematically isolated and removed. The programme achieved great practical successes, but the foundation of field theory remains unsatisfactory.

A.

Unification of Field Theories

The four types of interaction are vastly different from one another. The effort to unify them into a single conceptual whole was started by Albert Einstein before 1920. In 1979 the American physicists Sheldon Glashow and Steven Weinberg and the Pakistani physicist Abdus Salam shared the Nobel Prize for 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, by Yang, and by the American physicist Robert Laurence Mills, and of broken symmetry developed by the Japanese-American physicist Yoichiro Nambu, the British physicist Peter W. Higgs, and others (see Higgs Particle). A very important contribution to these developments was made by the Dutch physicist Gerardus ‘t Hooft, who pushed through the renormalization programme for these theories (‘t Hooft co-won the 1999 Nobel Prize for Physics for his research in electroweak interactions). The picture that has emerged from these efforts is called the Standard Model. In the Standard Model, hadrons consist of mesons (quark-antiquark pairs) or baryons (triplets of quarks), and interact by the exchange of strong force messenger particles called gluons. Leptons are a distinct family of particles that include electrons and neutrinos, and interact through the weak nuclear force, carried by so-called W and Z bosons. There are two types of W bosons with electric charges +1 and -1; each is the antiparticle of the other. The Z boson has no electric charge, nor other distinguishable quantum number, so Z is the antiparticle of itself. The W and Z bosons were discovered in 1983 by two experiments at CERN (the European Laboratory for Particle Physics, near Geneva, Switzerland), using its Super Proton Synchrotron. The discovery was so important that Carlo Rubbia and Simon van der Meer, the two key scientists behind the experiments, received the Nobel Prize for Physics one year later.

B.

Prospects for the Future

It is now recognized that the properties of all interactions are dictated by various forms of gauge symmetry (see Symmetry). In retrospect, the first use of this idea was Einstein’s search for a theory of gravitation that is symmetrical with respect to coordinate transformations, which culminated in the general theory of relativity in 1916. 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 of all interactions has already been attempted in the ideas of supersymmetry and supergravity.

The final goal is an understanding of the fundamental structure of matter through unified symmetry principles. However, there are difficulties in both the theoretical and experimental aspects of the endeavour. 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 particle detectors. Scientists make constant progress in both the theoretical and experimental aspects. The Large Hadron Collider, the world’s largest and most powerful particle accelerator, will become operational at CERN during 2008. It will smash protons moving at almost the speed of light, achieving for the first time the conditions of our universe a fraction of a second after the big bang (see Universe, Origin of the). By recreating in the laboratory the extreme conditions of the early universe, physicists expect to be able to probe deeper into the mysteries of matter.