The fundamental constituents of all the matter in the universe. By the beginning of the 20th century, the electron had been discovered. Subsequently, the proton was discovered. It was not until 1932 that the existence of the neutron was definitely established. Since 1932, it had been known that atomic nuclei consist of both protons and neutrons (except hydrogen, whose nucleus consists of a lone proton). Between 1900 and 1930, quantum mechanics was also making progress in the understanding of physics on the atomic scale. Nonrelativistic quantum theory was completed in an astonishingly brief period (1925–27), but it was the relativistic version that made the greatest impact on our understanding of elementary particles. Dirac’s discovery in 1928 of the equation that bears his name led to the discovery of the positive electron or positron. The mass of the positron is equal to that of the negative electron while its charge is equal in magnitude but opposite in sign. Pairs of particles related to each other in this way are said to be antiparticles of each other. Positrons have only a transitory existence; that is, they do not form part of ordinary matter. Positrons and electrons are produced simultaneously in high-energy collisions of charged particles or gamma rays with matter in a process called pair production.

The union of relativity and quantum mechanics therefore led to speculation as early as 1932 that there might also be antiprotons and antineutrons, bearing a similar relationship to their respective ordinary particles as the positron does to the electron. However, it was not until 1955 that particle beams were made sufficiently energetic to enable these antimatter particles to be observed. It is now understood that all known particles have antimatter equivalents, which are predicted by relativistic quantum equations.

By the mid-1930s the list of known and theoretically postulated particles was still small but steadily growing. At this time the Japanese physicist Hideki Yukawa (1907–81) was studying the possible fundamental interactions that could hold the nucleus together. Since the nucleus is a closely packed collection of positively charged protons and neutral neutrons, clearly it could not be held together by an electromagnetic force; there had to be a different and very large force capable of holding proton charges together at such close proximity. This force would necessarily be restricted to the short range of nuclear dimensions, because evidence of its existence only arose after the discovery of the constituents of the atomic nucleus. Guided by the properties required of this new force, Yukawa proposed the existence of a particle called the meson, which was responsible for transmitting nuclear forces. He suggested that protons and neutrons in the nucleus could interact by emitting and absorbing mesons. For this reason this new type of force was called an exchange force. Yukawa was even able to predict the mass of his meson (meaning ‘middle weight’), which turned out to be intermediate between the proton and the electron.

Only a year after Yukawa had made this suggestion, a particle of intermediate mass was discovered in cosmic radiation. This particle was named the μ‎-meson or muon. The μ‎⁻ has a charge equal to the electron, and its antiparticle μ‎⁺ has a positive charge of equal magnitude. However, physicists soon discovered that muons do not interact with nuclear particles sufficiently strongly to be Yukawa’s meson. It was not until 1947 that a family of mesons with the appropriate properties was discovered. These were the π‎-mesons or pions, which occur in three types: positive, negative, and neutral. Pions, which interact strongly with nuclei, have in fact turned out to be the particles predicted by Yukawa in the 1930s. The nuclear force between protons and neutrons was given the name ‘strong interaction’ (see fundamental interactions) and until the 1960s it was thought to be an exchange force as proposed by Yukawa.

A theory of the weak interaction was also in its infancy in the 1930s. The weak interaction is responsible for beta decay, in which a radioactive nucleus is transformed into a slightly lighter nucleus with the emission of an electron. However, beta decays posed a problem because they appeared not to conserve energy and momentum. In 1930 Pauli proposed the existence of a neutral particle that might be able to carry off the missing energy and momentum in a beta decay and escape undetected. Three years later, Fermi included Pauli’s particle in a comprehensive theory of beta decay, which seemed to explain many experimentally observed results. Fermi called this new particle the neutrino, the existence of which was finally established in the 1950s.

A plethora of experiments involving the neutrino revealed some remarkable properties for this new particle. The neutrino was found to have an intimate connection with the electron and muon, and indeed never appeared without the simultaneous appearance of one or other of these particles. A conservation law was postulated to explain this observation. Numbers were assigned to the electron, muon, and neutrino, so that during interactions these numbers were conserved; i.e. their algebraic sums before and after these interactions were equal. Since these particles were among the lightest known at the time, these assigned numbers became known as lepton numbers (lepton: ‘light ones’). In order to make the assignments of lepton number agree with experiment, it is necessary to postulate the existence of two types of neutrino. Each of these types is associated with either the electron or the muon; there are thus muon neutrinos and electron neutrinos. In 1975 the tau particle or tauon was discovered and was added to the list of particles with assigned lepton numbers. The conservation of lepton number in the various interactions involving the tau requires the existence of an equivalent tau neutrino. The tau neutrino was discovered in 2000. The six particles with assigned lepton numbers are now known as leptons.

Neutrinos have zero charge and were originally thought to have zero rest mass, but there has been some indirect experimental evidence to the contrary, beginning in the last twenty years of the 20th century. In 1985 a Soviet team reported a measurement, for the first time, of a nonzero neutrino mass. The mass measured was extremely small (10 000 times less than the mass of the electron), but subsequent attempts independently to reproduce these results did not succeed. More recently (1998–99), Japanese and US groups have put forward theories and corroborating experimental evidence to suggest, indirectly, that neutrinos do have mass. In these experiments neutrinos are found to apparently ‘disappear’. Since it is unlikely that momentum and energy are actually vanishing from the universe, a more plausible explanation is that the types of neutrinos detected are changing into types that cannot be detected. Most detectors can detect only one type of neutrino at a time. Present theoretical considerations imply that the masses of neutrinos involved cannot be equal to one another, and therefore they cannot all be zero. This speculative work has not yet yielded estimates of the neutrino masses, which is indicated by the use of asterisks in the accompanying table.

In the 1960s, the development of high-energy accelerators and more sophisticated detection systems led to the discovery of many new and exotic particles. They were all unstable and existed for only small fractions of a second; nevertheless they set into motion a search for a theoretical description that could account for them all. The large number of these apparently fundamental particles suggested strongly that they do not, in fact, represent the most fundamental level of the structure of matter. Physicists found themselves in a position similar to Mendeleev when the periodic table was being developed. Mendeleev realized that there had to be a level of structure below the elements themselves, which explained the chemical properties and the interrelations between elements.

Murray Gell-Mann and his collaborators proposed the particle-physics equivalent of the periodic table between 1961 and 1964. In this structure, leptons were indeed regarded as fundamental particles, but the short-lived particles discovered in the 1960s were not. These particles were found to undergo strong interactions, which did not seem to affect the leptons. Gell-Mann called these strongly interacting particles the hadrons and proposed that they occurred in two different types: baryons and mesons. These two different types corresponded to the two different ways of constructing hadrons from constituent particles, which Gell-Mann called quarks. These quarks came in three flavours, up (u), down (d), and strange (s). These three quarks were thought to be the fundamental constituents of hadrons, i.e. matter that undergoes strong interactions: baryons are composed of three quarks (u, d, or s) or three antiquarks (ū, d̄ or s̄); mesons are composed of (u, d, or s) quark–antiquark pairs.

No other combinations seemed to be necessary to describe the full variation of the observed hadrons. This scheme even led to the prediction of other particles that were not known to exist in 1961. For example, in 1961 Gell-Mann not only predicted the Ω‎⁻ (omega-minus) particle, but more importantly told experimentalists exactly how to produce it. The Ω‎⁻ particle was finally discovered in 1964.

Gell-Mann called his scheme ‘the eight-fold way’, after the similarly named Buddhist principle. The scheme requires that quarks have properties not previously allowed for fundamental particles. For example, quarks have fractional electric charges, i.e. charges of 1/3 and 2/3 of the electron charge. Quarks also have a strong affinity for each other through a new kind of charge known as colour charge. Colour charge is therefore responsible for strong interactions, and the force is known as the colour force. This is a revision of Yukawa’s proposal in 1930. Yukawa’s strong force was mediated by π‎-mesons. The strong force is now thought to be mediated by exchange of particles carrying colour charge known as gluons. The theory governing these colour charge combinations is modelled on quantum electrodynamics and is known as quantum chromodynamics.

In November 1974 the discovery of the ψ‎ (psi) particle initiated what later came to be known as ‘the November revolution’. Up to that time, any known hadron could be described as some combination of u, d, or s quarks. These hadrons were very short-lived with lifetimes of about 10⁻²³ s. The ψ‎ particle, however, had a lifetime of 10⁻²⁰ s; i.e. a thousand times longer. This suggested a completely different species of particle. It is now universally accepted that the ψ‎ represents a meson-bound state of a new fourth quark, the charm (c) quark and its antiquark. In 1977 the list of quarks once again increased with the discovery of a new even heavier meson, called the Υ‎ (upsilon) meson. This meson was found to have an even longer lifetime than the ψ‎, and was quickly identified as the carrier of a fifth quark, bottom (b).

Thus, by the end of 1977, five flavours of quark (u, d, s, c, b) were known to exist together with six flavours of lepton (e, μ, τ, ν_e, τ_μ, ν_τ‎). The tau neutrino was also strongly suspected to exist at that time. Assuming that quarks and leptons are the fundamental constituents of matter, many of the strong and weak interactions of hadrons and the weak interactions of leptons can be explained. However, anticipating a symmetry in nature’s building blocks, it was expected that a sixth quark would eventually reveal itself. This quark, labelled top (t), would be the 2/3 electronic charge partner to the b quark (see the accompanying table of quarks). In 1995 the top quark was found at Fermilab, near Chicago, and the symmetry of six quarks with six leptons was finally verified.

By the mid-1970s the standard model had been established as the definitive theory of the fundamental constituents of matter. In the current view, all matter consists of three kinds of particle: leptons, quarks, and mediators. The mediators are the particles by which the four fundamental interactions are mediated. In the standard model, each of these interactions has a particle mediator. For the electromagnetic reaction it is the photon.

For weak interactions the force is mediated by three particles called W⁺, W⁻, and Z⁰ bosons; for the strong force it is the gluon. Current theories of quantum gravity propose the graviton as the mediator for the gravitational interaction, but this work is highly speculative and the graviton has never been detected.

One of the most fundamental problems in the theory of elementary particles is to explain why there are three ‘families’ of particles and why there is such a spread in their masses.