Elementary Particles

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.