SUBATOMIC PARTICLE

also called elementary particle, any of various self-contained units of matter or energy. More than 200 subatomic particles have been detected, and each appears to have an antiparticle, an antimatter counterpart with the identical mass but opposite electric charge, magnetic moment, or spin. Subatomic particles are, literally, particles that are smaller than atoms. The physical study of such particles became possible only during the 20th century, with the development of increasingly sophisticated apparatus to probe matter at scales of 10-15 metre and less; yet the basic philosophy of the subject now known as subatomic particle physics dates to at least 500 BC when the Greek philosopher Leucippus and his pupil Democritus put forward the notion that matter consists of invisibly small, indivisible particles, which they called atoms. For more than 2,000 years, however, the idea of atoms lay largely neglected, while the opposing view that matter consists of four elementsearth, fire, air, and waterheld sway. By the beginning of the 19th century, the atomic theory of matter had returned to favour, strengthened in particular by the work of John Dalton, an English chemist, whose studies suggested that each chemical element consists of its own unique kind of atom. As such, Dalton's atoms are still the atoms of modern physics. By the close of the century, however, the first indications began to emerge that atoms are not indivisible, as Leucippus and Democritus had imagined, but that they instead contain smaller subatomic particles. In 1896 the French physicist Henri Becquerel discovered radioactivity, and in the following year J.J. Thomson, a professor of physics at Cambridge University in England, demonstrated the existence of tiny particles much smaller in mass than hydrogen, the lightest atom. Thomson had discovered the first subatomic particle, the electron. Six years later, Ernest Rutherford and Frederick Soddy, working at McGill University in Montreal, found that radioactivity occurs when atoms of one type transmute into those of another kind. The idea of atoms as immutable, indivisible objects had become completely untenable. The basic structure of the atom became apparent in 1911, when Rutherford showed that most of the mass of an atom lies concentrated at its centre, in a tiny nucleus. Rutherford postulated that the atom resembled a miniature solar system, with light, negatively charged electrons orbiting around the dense, positively charged nucleus, just as the planets orbit around the Sun. The Danish theorist Niels Bohr refined this model in 1913 by incorporating the new ideas of quantization that had been developed by the German physicist Max Planck at the turn of the century. Planck had theorized that electromagnetic radiation, such as light, occurs in discrete bundles, or quanta, of energy known as photons. Bohr postulated that the electrons circled the nucleus in orbits of fixed size and energy and that an electron could jump from one orbit to another only by emitting or absorbing specific quanta of energy. By thus incorporating quantization into his theory of the atom, Bohr included one of the basic elements of modern subatomic particle theory and prompted wider acceptance of quantization to explain atomic and subatomic phenomena. This article discusses the further development of subatomic particle theory, as well as the various classes of subatomic particles and current areas of research. See also the articles atom; mechanics; and radiation for additional information on the interactions of subatomic particles and on their role in the structure of matter. For details on the detection and measurement of subatomic particles, see particle accelerator. also called elementary particle, any of various self-contained units of matter or energy. More than 200 subatomic particles have been detected so far, and each appears to have an antiparticle, an antimatter counterpart with the identical mass but opposite electric charge, magnetic moment, or spin. For centuries atoms were commonly regarded as the basic constituents of matter. The discovery of the electron (1897) and of the atomic nucleus (1911) finally established that an atom was actually a composite system composed of a cloud of electrons surrounding a tiny, heavy core. In the early 1930s it was determined that the nucleus itself was composed of smaller particles, namely protons and neutrons. By the early 1970s intensive research revealed that these particles too were made up of even more basic unitsquarks. In the late 20th century most physicists believed that quarks, together with another class of subatomic particles known as leptons, constitute the fundamental building-blocks of all matter. The quarks (and antiquarks) are massive particles that have half-integral spin and carry a fractional electric charge. They come in six different varieties. Only two of them, known as up and down, occur in the protons and neutrons of ordinary matter. Four other types of quarks, called strange, charmed, top, and bottom, exist solely in unstable particles that spontaneously decay (break down) in a few hundred-millionths of a second or less. Particles made up of quarks are known collectively as hadrons. Those such as the proton and neutron that consist of three quarks are called baryons; others formed from a quark with an antiquark are called mesons. The leptons also are divided into several types, the most familiar of which is the electron. Other, more massive leptons are the muon and the tau. Like the electron, both of these particles have a negative electric charge and half-integral spin. Each of the leptons has an associated neutrinoi.e., electron-, muon-, and tau-neutrinothat possesses no electric charge and possibly no mass. In contrast to quarks, which take part in strong interactions, leptons respond only to electromagnetic, weak, and gravitational forces. A third major group of subatomic particles consists of bosons. Unlike quarks and leptons, bosons are not building-blocks of matter; rather they transmit the forces of the universe. For example, the electro-weak force, the electromagnetic and weak forces in unified form, is maintained through the emission and absorption of photons and of massive particles known as W and Z bosons by interacting leptons and their associated neutrinos. Other bosons include the gluons, which transmit the strong force that binds quarks in hadrons. Gluons are massless, have spin, and travel at the speed of light. Moreover, they can multiply as they travel from one quark to another, thereby increasing the strength of the force they transmit. The larger the number of gluons exchanged among quarks, the stronger the binding force becomes. Additional reading Nonspecialist introductions to particle physics that give a broad outline of the subject include Frank Close, Michael Marten, and Christine Sutton, The Particle Explosion (1987); Yuval Ne'eman and Yoram Kirsh, The Particle Hunters, 2nd ed. (1996; originally published in Hebrew, 1983); Peter Watkins, Story of the W and Z (1986); Y. Nambu, Quarks: Frontiers in Elementary Particle Physics (1985); Christine Sutton, The Particle Connection: The Most Exciting Scientific Chase Since DNA and the Double Helix (1984); Paul Davies, Superforce: The Search for a Grand Unified Theory of Nature (1984); Frank Close, The Cosmic Onion: Quarks and the Nature of the Universe (1983); Harald Fritzsch, Quarks: The Stuff of Matter (1983; originally published in German, 1981); and J.H. Mulvey (ed.), The Nature of Matter (1981). More detailed historical accounts can be found in Abraham Pais, Inward Bound: Of Matter and Forces in the Physical World (1986); Laurie M. Brown and Lillian Hoddeson (eds.), The Birth of Particle Physics (1983); Steven Weinberg, The Discovery of Subatomic Particles (1983, reissued 1990); Barry Parker, Search for a Supertheory: From Atoms to Superstrings (1987); Leon M. Lederman and David N. Schramm, From Quarks to the Cosmos: Tools of Discovery (1989); Gordon Fraser, Egil Lillestl, and Inge Sellevg, The Search for Infinity: Solving the Mysteries of the Universe (1994); and Gordon Kane, The Particle Garden: Our Universe as Understood by Particle Physicists (1995). More technical introductory texts are Donald H. Perkins, Introduction to High Energy Physics, 3rd ed. (1987); B.G. Duff, Fundamental Particles: An Introduction to Quarks and Leptons (1986); L.B. Okun, Particle Physics: The Quest for the Substance of Substance, trans. from Russian (1985); Ian J.R. Aitchison and Anthony J.G. Hey, Gauge Theories in Particle Physics: A Practical Introduction, 2nd ed. (1989); and Graham G. Ross, Grand Unified Theories (1984). An interesting collection of important papers on electroweak theory is contained in C.H. Lai (ed.), Selected Papers on Gauge Theory of Weak and Electromagnetic Interactions (1981). More recent developments, as well as useful past references, are available in journal articlesfor example, Simon Anthony, Superstrings: A Theory of Everything?, New Scientist, 107(1471):3436 (Aug. 29, 1985); P.Q. Hung and C. Quigg, Intermediate Bosons: Weak Interaction Couriers, Science, 210(4475):120511 (Dec. 12, 1980); an editorial article, Anomaly Cancellation Launches Superstring Bandwagon, Physics Today, 38(7):1720 (July 1985); John Ellis, Waiting for Scornucopia, Physics World, 7(7):3135 (July 1994); Paul Townsend, Unity from Duality, Physics World, 8(9):4146 (September 1995); Edward Witten, Reflections on the Fate of Spacetime, Physics Today, 49(4):2430 (April 1996); and a number of articles in Scientific American: Gerard 'T Hooft, Gauge Theories of the Forces Between Elementary Particles, 242(6):104138 (June 1980); Howard E. Haber and Gordon Kane, Is Nature Supersymmetric?, 254(6):5260 (June 1986); Michael B. Green, Superstrings, 255(3):4860 (September 1986); Gary J. Feldman and Jack Steinberger, The Number of Families of Matter, 264(2):7072A, 72F75 (February 1991); Henry J. Crawford and Carsten H. Greiner, The Search for Strange Matter, 270(1):7277 (January 1994); and Donald H. Weingarten, Quarks by Computer, 274(2):116120 (February 1996). Many of the pertinent articles published in New Scientist before 1985 are collected in Christine Sutton (ed.), Building the Universe (1985). Christine Sutton The development of modern theory Quantum electrodynamics The year of the birth of particle physics is often cited as 1932. Near the beginning of that year James Chadwick, working in England at the Cavendish Laboratory in Cambridge, discovered the existence of the neutron. This discovery seemed to complete the picture of atomic structure that had begun with Rutherford's work in 1911. The elementary particles seemed firmly established as the proton, neutron, and electron. By the end of 1932, however, Carl Anderson in the United States had discovered the first antiparticle: the positron, or antielectron. Moreover, Patrick Blackett and Giuseppi Occhialini, working, like Chadwick, at the Cavendish Laboratory, had revealed how positrons and electrons are created in pairs when cosmic rays pass through dense matter. It was becoming apparent that the simple pictures provided by electrons, protons, and neutrons were incomplete and that a new theory was needed to fully explain the phenomena of subatomic particles. Dirac had provided the foundations for such a theory in 1927 with his quantum theory of the electromagnetic field. Dirac's theory treated the electromagnetic field as a gas of photons (the quanta of light), and it yielded a correct description of the absorption and emission of radiation by electrons in atoms. It was the first quantum field theory. A year later Dirac published his relativistic electron theory, which took correct account of Einstein's special theory of relativity. Dirac's theory showed that the electron must have a spin quantum number of 1/2 and a magnetic moment. It also predicted the existence of the positron, although Dirac did not at first realize this and puzzled over what seemed like extra solutions to his equations. Only with Anderson's discovery of the positron did the picture become clear: radiationa photoncan produce electrons and positrons in pairs, provided the energy of the photon is greater than about 1 MeV, the total mass-energy of the two particles. Dirac's quantum field theory was a beginning, but it explained only one aspect of the electromagnetic interactions between radiation and matter. During the following years other theorists began to extend Dirac's ideas to form a comprehensive theory of quantum electrodynamics (QED) that accounts fully for the interactions of charged particles not only with radiation but also with one another. One important step was to describe the electrons in terms of fields, in analogy to the electromagnetic field of the photons. This enabled theorists to describe everything in terms of quantum field theory. It also helped to cast light on Dirac's positrons. According to QED, the vacuum is filled with electron-positron fields. Real electron-positron pairs are created when energetic photons, represented by the electromagnetic field, interact with these fields. Virtual electron-positron pairs, however, can also exist for minute durations, as dictated by Heisenberg's uncertainty principle, and this at first led to fundamental difficulties with QED. During the 1930s it became clear that, as it stood, QED gave the wrong answers for quite simple problems. For example, the theory said that the emission and reabsorption of the same photon would occur with an infinite probability. This led in turn to infinities occurring in many situations; even the mass of a single electron was infinite according to QED because, on the time scales of the uncertainty principle, the electron could continuously emit and absorb virtual photons. It was not until the late 1940s that a number of theorists working independently resolved the problems with QED. Julian Schwinger and Richard Feynman in the United States and Tomonaga Shin'ichiro in Japan proved that they could rid the theory of its embarrassing infinities by a process known as renormalization. Basically, renormalization acknowledges all possible infinities and then allows the positive infinities to cancel the negative ones; the mass and charge of the electron, which are infinite in theory, are then defined to be their measured values. Once these steps are taken, QED works beautifully. It is the most accurate quantum field theory scientists have at their disposal. In recognition of their achievement, Feynman, Schwinger, and Tomonaga were awarded the Nobel Prize for Physics for 1965; Dirac had been similarly honoured in 1933. Quantum chromodynamics The nuclear binding force As early as 1920, when Rutherford named the proton and accepted it as a fundamental particle, it was clear that the electromagnetic force was not the only force at work within the atom. Something stronger had to be responsible for binding the positively charged protons together, thereby overcoming their natural electrical repulsion. The discovery in 1932 of the neutron showed that there are (at least) two kinds of particles subject to the same force. Later in the same year, Heisenberg made one of the first attempts to develop a quantum field theory that was analogous to QED but appropriate to the nuclear binding force. According to quantum field theory, particles can be held together by a charge-exchange force, which is carried by charged intermediary particles. Heisenberg's application of this theory gave birth to the idea that the proton and neutron were charged and neutral versions of the same particlean idea that seemed to be supported by the fact that the two particles have almost equal masses. Heisenberg proposed that a proton, for example, could emit a positively charged particle that was then absorbed by a neutron; the proton thus became a neutron, and vice versa. The nucleus was no longer viewed as a collection of two kinds of immutable billiard balls but rather as a continuously changing collection of protons and neutrons that were bound together by the exchange particles flitting between them. Heisenberg believed that the exchange particle involved was an electron (he did not have many particles from which to choose). This electron had to have some rather odd characteristics, however, such as no spin and no magnetic moment, and this made Heisenberg's theory ultimately unacceptable. Quantum field theory did not seem applicable to the nuclear binding force. Then, in 1935, a Japanese theorist, Yukawa Hideki, took a bold step: he invented a new particle as the carrier of the nuclear binding force. The size of a nucleus shows that the binding force must be short-ranged, confining protons and neutrons within distances of about 10-14 metre. Yukawa argued that to give this limited range the force must involve the exchange of particles with mass, unlike the massless photons of QED. According to the uncertainty principle, exchanging a particle with mass sets a limit on the time allowed for the exchange and therefore restricts the range of the resulting force. Yukawa calculated a mass of about 200 times the electron's mass, or 100 MeV, for the new intermediary. Because the predicted mass of the new particle was between those of the electron and the proton, the particle was named the mesotron, later shortened to meson. Yukawa's work was little known outside Japan until 1937, when Carl Anderson and his colleague Seth Neddermeyer announced that, five years after Anderson's discovery of the positron, they had found a second new particle in the cosmic radiation. The new particle seemed to have exactly the mass Yukawa had prescribed and thus was seen as confirmation of Yukawa's theory by the Americans J. Robert Oppenheimer and Robert Serber, who made Yukawa's work more widely known in the West. In the following years, however, it became clear that there were difficulties in reconciling the properties expected for Yukawa's intermediary particle with those of the new cosmic ray particle. In particular, as a group of Italian physicists succeeded in demonstrating (while hiding from the occupying German forces during World War II), the cosmic ray particles penetrate matter far too easily to be related to the nuclear binding force. To resolve this apparent paradox, theorists both in Japan and in the United States had begun to think that there might be two mesons. The two-meson theory proposed that Yukawa's nuclear meson decays into the penetrating meson observed in the cosmic rays. In 1947, scientists at Bristol University in England found the first experimental evidence of two mesons in cosmic rays high on the Pic du Midi in France. Using detectors equipped with special photographic emulsion that can record the tracks of charged particles, the physicists at Bristol found the decay of a heavier meson into a lighter one. They called the heavier particle p (or pi), and it has since become known as the p-meson or pion. The lighter particle was dubbed m (or mu) and is now known simply as the muon. (According to the modern definition of a meson as a particle consisting of a quark bound with an antiquark, the muon is not actually a meson. It is classified as a leptona relation of the electron.) Studies of pions produced in cosmic radiation and in the first particle accelerators showed that the pion behaves precisely as expected for Yukawa's particle. Moreover, experiments confirmed that positive, negative, and neutral varieties of pions exist, as predicted by Nicholas Kemmer in England in 1938. Kemmer regarded the nuclear binding force as symmetric with respect to the charge of the particles involved. He proposed that the nuclear force between protons and protons, or neutrons and neutrons, is the same as the one between protons and neutrons. This symmetry required the existence of a neutral intermediary that did not figure in Yukawa's original theory. It also established the concept of a new internal property of subatomic particlesisospin. Kemmer's work followed to some extent the trail Heisenberg had begun in 1932. Close similarities between nuclei containing the same total number of protons and neutrons, but in different combinations, suggest that protons can be exchanged for neutrons and vice versa without altering the net effect of the nuclear binding force. In other words, the force recognizes no difference between protons and neutrons; it is symmetrical under the interchange of protons and neutrons, rather as a square is symmetrical under rotations through 90, 180, and so on. To introduce this symmetry into the theory of the nuclear force, it proved useful to adopt the mathematics describing the spin of particles. In this respect, the proton and neutron are seen as different states of a single basic nucleon. These states are differentiated by an internal property that can have two values, +1/2 and -1/2, in analogy with the spin of a particle like the electron. This new property is called isotopic spin, or isospin for short, and the nuclear binding force is said to exhibit isospin symmetry. Symmetries are important in physics because they simplify the theories needed to describe a range of observations. For example, as far as physicists can tell, all physical laws exhibit translational symmetry. This means that the results of an experiment performed at one location in space and time can be used to predict correctly the outcome of the same experiment in another part of space and time. This symmetry is reflected in the conservation of momentumthe fact that the total momentum of a system remains constant, unless it is acted upon by an external force. Isospin symmetry is an important symmetry in particle physics, although it occurs only in the action of the nuclear binding forceor, in modern terminology, the strong force. The symmetry leads to the conservation of isospin in nuclear interactions that occur via the strong force, thereby determining which reactions can occur. Electroweak theory