The Britannica Guide to Particle Physics

Britannica Educational Publishing, Erik Gregersen

• Publisher: Britannica Educational Publishing
• Year: 2010
• ISBN:
  9781615303823
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

While the atom is universally acknowledged as the basis for most branches of physics, the study of its constituent particles has illuminated significant new areas of research. The behavior of subatomic particles provides crucial information on the structure and nature of atomic nuclei, which in turn reveal much about energy, matter, and often the origins of the universe. Complete with color diagrams and photographs, this volume elucidates the intricacies of this rapidly developing and always compelling field.

CHAPTER 1
BASIC CONCEPTS OF PARTICLE PHYSICS

Subatomic, or elementary, particles are various self-contained units of matter or energy that are the fundamental constituents of all matter. They include electrons, the negatively charged, almost massless particles that nevertheless account for most of the size of the atom, and they include the heavier building blocks of the small but very dense nucleus of the atom, the positively charged protons and the electrically neutral neutrons. But these basic atomic components are by no means the only known subatomic particles. Protons and neutrons, for instance, are themselves made up of elementary particles called quarks, and the electron is only one member of a class of elementary particles that also includes the muon and the neutrino. More unusual subatomic particles—such as the positron, the antimatter counterpart of the electron—have been detected and characterized in cosmic-ray interactions in the Earth’s atmosphere. The field of subatomic particles has expanded dramatically with the construction of powerful particle accelerators to study high-energy collisions of electrons, protons, and other particles with matter. As particles collide at high energy, the collision energy becomes available for the creation of subatomic particles such as mesons and hyperons. Finally, completing the revolution that began in the early 20th century with theories of the equivalence of matter and energy, the study of subatomic particles has been transformed by the discovery that the actions of forces are due to the exchange of “force” particles such as photons and gluons. More than 200 subatomic particles have been detected—most of them highly unstable, existing for less than a millionth of a second—as a result of collisions produced in cosmic-ray reactions or particle-accelerator experiments. Theoretical and experimental research in particle physics, the study of subatomic particles and their properties, has given scientists a clearer understanding of the nature of matter and energy and of the origin of the universe.

The current understanding of the state of particle physics is integrated within a conceptual framework known as the Standard Model. The Standard Model provides a classification scheme for all the known subatomic particles based on theoretical descriptions of the basic forces of matter.

THE DIVISIBLE ATOM

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The physical study of subatomic particles became possible only during the 20th century, with the development of increasingly sophisticated apparatuses to probe matter at scales of 10^-15 metre and less (that is, at distances comparable to the diameter of the proton or neutron). Yet the basic philosophy of the subject now known as particle physics dates to at least 500 BCE, 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 the idea of atoms lay largely neglected, while the opposing view that matter consists of four elements—earth, fire, air, and water—held sway. But 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 particles.

In 1896 the French physicist Henri Becquerel discovered radioactivity, and in the following year J.J. Thomson, a professor of physics at the University of Cambridge 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 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 the dense, positively charged nucleus, just as the planets orbit 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 now known as photons. Bohr postulated that 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 introduced one of the basic elements of modern particle physics and prompted wider acceptance of quantization to explain atomic and subatomic phenomena.

SIZE

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Subatomic particles play two vital roles in the structure of matter. They are both the basic building blocks of the universe and the mortar that binds the blocks. Although the particles that fulfill these different roles are of two distinct types, they do share some common characteristics, foremost of which is size.

The small size of subatomic particles is perhaps most convincingly expressed not by stating their absolute units of measure but by comparing them with the complex particles of which they are a part. An atom, for instance, is typically 10^-10 metre across, yet almost all of the size of the atom is unoccupied “empty” space available to the point-charge electrons surrounding the nucleus. The distance across an atomic nucleus of average size is roughly 10^-14 metre—only

the diameter of the atom. The nucleus, in turn, is made up of positively charged protons and electrically neutral neutrons, collectively referred to as nucleons, and a single nucleon has a diameter of about 10^-15 metre—that is, about

that of the nucleus and

that of the atom. (The distance across the nucleon, 10^-15 metre, is known as a fermi, in honour of the Italian-born physicist Enrico Fermi, who did much experimental and theoretical work on the nature of the nucleus and its contents.)

The sizes of atoms, nuclei, and nucleons are measured by firing a beam of electrons at an appropriate target. The higher the energy of the electrons, the farther they penetrate before being deflected by the electric charges within the atom. For example, a beam with an energy of a few hundred electron volts (eV) scatters from the electrons in a target atom. The way in which the beam is scattered (electron scattering) can then be studied to determine the general distribution of the atomic electrons.

At energies of a few hundred megaelectron volts (MeV; 10⁶ eV), electrons in the beam are little affected by atomic electrons. Instead, they penetrate the atom and are scattered by the positive nucleus. Therefore, if such a beam is fired at liquid hydrogen, whose atoms contain only single protons in their nuclei, the pattern of scattered electrons reveals the size of the proton. At energies greater than a gigaelectron volt (GeV; 10⁹ eV), the electrons penetrate within the protons and neutrons, and their scattering patterns reveal an inner structure. Thus, protons and neutrons are no more indivisible than atoms are. Indeed, they contain still smaller particles, which are called quarks.

Quarks are as small as or smaller than physicists can measure. In experiments at very high energies, equivalent to probing protons in a target with electrons accelerated to nearly 50,000 GeV, quarks appear to behave as points in space, with no measurable size. They must therefore be smaller than 10^-18 metre, or less than

the size of the individual nucleons they form. Similar experiments show that electrons too are smaller than it is possible to measure.

ELEMENTARY PARTICLES

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Electrons and quarks contain no discernible structure. They cannot be reduced or separated into smaller components. It is therefore reasonable to call them “elementary” particles, a name that in the past was mistakenly given to particles such as the proton, which is in fact a complex particle that contains quarks. The term subatomic particle refers both to the true elementary particles, such as quarks and electrons, and to the larger particles that quarks form.

Although both are elementary particles, electrons and quarks differ in several respects. Whereas quarks together form nucleons within the atomic nucleus, the electrons generally circulate toward the periphery of atoms. Indeed, electrons are regarded as distinct from quarks and are classified in a separate group of elementary particles called leptons. There are several types of lepton, just as there are several types of quark. Only two types of quark are needed to form protons and neutrons, however, and these, together with the electron and one other elementary particle, are all the building blocks that are necessary to build the everyday world. The last particle required is an electrically neutral particle called the neutrino.

Neutrinos do not exist within atoms in the sense that electrons do, but they play a crucial role in certain types of radioactive decay. In a basic process of one type of radioactivity, known as beta decay, a neutron changes into a proton. In making this change, the neutron acquires one unit of positive charge. To keep the overall charge in the beta-decay process constant and thereby conform to the fundamental physical law of charge conservation, the neutron must emit a negatively charged electron. In addition, the neutron also emits a neutrino (strictly speaking, an antineutrino), which has little or no mass and no electric charge. Beta decays are important in the transitions that occur when unstable atomic nuclei change to become more stable, and for this reason neutrinos are a necessary component in establishing the nature of matter.

The neutrino, like the electron, is classified as a lepton. Thus, it seems at first sight that only four kinds of elementary particles—two quarks and two leptons—should exist. In the 1930s, however, long before the concept of quarks was established, it became clear that matter is more complicated.

SPIN

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The concept of quantization led during the 1920s to the development of quantum mechanics, which appeared to provide physicists with the correct method of calculating the structure of the atom. In his model, Niels Bohr had postulated that the electrons in the atom move only in orbits in which the angular momentum (angular velocity multiplied by mass) has certain fixed values. Each of these allowed values is characterized by a quantum number that can have only integer values. In the full quantum mechanical treatment of the structure of the atom, developed in the 1920s, three quantum numbers relating to angular momentum arise because there are three independent variable parameters in the equation describing the motion of atomic electrons.

In 1925, however, two Dutch physicists, Samuel Goudsmit and George Uhlenbeck, realized that, to explain fully the spectra of light emitted by the atoms of alkali metals, such as sodium, which have one outer valence electron beyond the main core, there must be a fourth quantum number that can take only two values, -½ and +½. Goudsmit and Uhlenbeck proposed that this quantum number refers to an internal angular momentum, or spin, that the electrons possess. This implies that the electrons, in effect, behave like spinning electric charges. Each therefore creates a magnetic field and has its own magnetic moment. The internal magnet of an atomic electron orients itself in one of two directions with respect to the magnetic field created by the rest of the atom. It is either parallel or antiparallel. Hence, there are two quantized states—and two possible values of the associated spin quantum number.

The concept of spin is now recognized as an intrinsic property of all subatomic particles. Indeed, spin is one of the key criteria used to classify particles into two main groups: fermions, with half-integer values of spin (½,

, …), and bosons, with integer values of spin (0, 1, 2, …). In the Standard Model all of the “matter” particles (quarks and leptons) are fermions, whereas “force” particles such as photons are bosons. These two classes of particles have different symmetry properties that affect their behaviour.

ANTIPARTICLES

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Two years after the work of Goudsmit and Uhlenbeck, the English theorist P.A.M. Dirac provided a sound theoretical background for the concept of electron spin. To describe the behaviour of an electron in an electromagnetic field, Dirac introduced the German-born physicist Albert Einstein’s theory of special relativity into quantum mechanics. Dirac’s relativistic theory showed that the electron must have spin and a magnetic moment, but it also made what seemed a strange prediction. The basic equation describing the allowed energies for an electron would admit two solutions, one positive and one negative. The positive solution apparently described normal electrons. The negative solution was more of a mystery. It seemed to describe electrons with positive rather than negative charge.

The mystery was resolved in 1932, when Carl Anderson, an American physicist, discovered the particle called the positron. Positrons are very much like electrons: they have the same mass and the same spin, but they have opposite electric charge. Positrons, then, are the particles predicted by Dirac’s theory, and they were the first of the so-called antiparticles to be discovered. Dirac’s theory, in fact, applies to any subatomic particle with spin ½. Therefore, all spin-½ particles should have corresponding antiparticles. Matter cannot be built from both particles and antiparticles, however. When a particle meets its appropriate antiparticle, the two disappear in an act of mutual destruction known as annihilation. Atoms can exist only because there is an excess of electrons, protons, and neutrons in the everyday world, with no corresponding positrons, antiprotons, and antineutrons.

Positrons do occur naturally, however, which is how Anderson discovered their existence. High-energy subatomic particles in the form of cosmic rays continually rain down on the Earth’s atmosphere from outer space, colliding with atomic nuclei and generating showers of particles that cascade toward the ground. In these showers the enormous energy of the incoming cosmic ray is converted to matter, in accordance with Einstein’s theory of special relativity, which states that E = mc², where E is energy, m is mass, and c is the velocity of light. Among the particles created are pairs of electrons and positrons. The positrons survive for a tiny fraction of a second until they come close enough to electrons to annihilate. The total mass of each electron-positron pair is then converted to energy in the form of gamma-ray photons.

Electrons and positrons produced simultaneously from individual gamma rays curl in opposite directions in the magnetic field of a bubble chamber. Here the gamma ray has lost some energy to an atomic electron, which leaves the long track, curling left. The gamma rays do not leave tracks in the chamber, because they have no electric charge. Courtesy of the Lawrence Berkeley Laboratory

Using particle accelerators, physicists can mimic the action of cosmic rays and create collisions at high energy. In 1955 a team led by the Italian-born scientist Emilio Segrè and the American Owen Chamberlain found the first evidence for the existence of antiprotons in collisions of high-energy protons produced by the Bevatron, an accelerator at what is now the Lawrence Berkeley National Laboratory in California. Shortly afterward, a different team working on the same accelerator discovered the antineutron.

Since the 1960s physicists have discovered that protons and neutrons consist of quarks with spin ½ and that antiprotons and antineutrons consist of antiquarks. Neutrinos too have spin ½ and therefore have corresponding an...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Introduction
• Chapter 1: Basic Concepts of Particle Physics
• Chapter 2: The Development of Modern Particle Theory
• Chapter 3: Particle Accelerators
• Chapter 4: Biographies
• Appendix
• Glossary
• Bibliography
• Index