When a particle and its antiparticle collide, both can be annihilated and other particles such as photons or pions produced. In some cases this represents the total conversion of mass into energy. For example, the collision between an electron and its antiparticle, a positron, results in the conversion of their combined masses into the energy of two photons. The reverse process, pair production, is the simultaneous creation of a particle and its antiparticle from the particles that result from their mutual annihilation.
The existence of antiparticles for electrons was predicted in 1928 by P. A. M. Dirac's relativistic quantum theory of the electron. According to the theory both positive and negative values are possible for the total relativistic energy of a free electron. In 1932, Carl D. Anderson, while studying cosmic rays, discovered the predicted positron, the first known antiparticle. About 23 years passed before the discovery of the next antiparticles—the antiproton was discovered by Owen Chamberlain and Emilio Segrè in 1955 at the Univ. of California, and the antineutron was discovered the following year—but the existence of antiparticles for all known particles was by then firmly established in theory.
The existence of antiparticles makes possible the creation of antimatter, composed of atoms made up of antiprotons and antineutrons in a nucleus surrounded by positrons. A very simple type of "atom" incorporating antiparticles is positronium, a brief pairing of a positron and an electron that may occur before their annihilation; it was first created and identified in the laboratory in 1951. Di-positronium, a molecule consisting of two positronium, was created in 2007. A few simple nuclei of antimatter have been created in the laboratory, such as the antideuteron (see deuterium). In 1995 nine atoms of antihydrogen (a single positively charged positron orbiting a single negatively charged antiproton) were created at CERN (near Geneva, Switzerland) by an Italian-German team headed by Walter Oelert.
Any antimatter in our part of the universe is necessarily very short-lived (the antihydrogen atoms, for example, survived for only 40 billionths of a second) because of the overwhelming preponderance of ordinary matter, by which the antimatter is quickly annihilated. Although scientists for a time considered the possibility that entire galaxies of antimatter could have evolved in a part of the universe far removed from our own, observations now indicate that this is not the case. The experimental work of Val L. Fitch and James W. Cronin in 1964 demonstrated an asymmetry in matter/antimatter reactions that may explain why the universe is composed mostly of matter. For their discovery, they shared the 1980 Nobel Prize in Physics.
The laws of nature are very nearly symmetrical with respect to particles and antiparticles. For example, an antiproton and a positron can form an antihydrogen atom, which has almost exactly the same properties as a hydrogen atom. A physicist whose body was made of antimatter, doing experiments in a laboratory also made of antimatter, using chemicals and substances comprised of antiparticles, would find almost exactly the same results in all experiments. This leads to the question of why the formation of matter after the Big Bang resulted in a universe consisting almost entirely of matter, rather than being a half-and-half mixture of matter and antimatter. The discovery of CP violation helped to shed light on this problem by showing that this symmetry, originally thought to be perfect, was only approximate.
Particle-antiparticle pairs can annihilate each other, producing photons; since the charges of the particle and antiparticle are opposite, charge is conserved. For example, the antielectrons produced in natural radioactive decay quickly annihilate themselves with electrons, producing pairs of gamma rays.
Antiparticles are produced naturally in beta decay, and in the interaction of cosmic rays in the Earth's atmosphere. Because charge is conserved, it is not possible to create an antiparticle without either destroying a particle of the same charge (as in beta decay), or creating a particle of the opposite charge. The latter is seen in many processes in which both a particle and its antiparticle are created simultaneously, as in particle accelerators. This is the inverse of the particle-antiparticle annihilation process.
Although particles and their antiparticles have opposite charges, electrically neutral particles need not be identical to their antiparticles. The neutron, for example, is made out of quarks, the antineutron from antiquarks, and they are distinguishable from one another because an antineutron, unlike a neutron, will rapidly annihilate itself by colliding with neutrons in ordinary matter. However, it is speculated that some neutral particles (such as some proposed types of WIMPs) are their own antiparticles, and can therefore annihilate with themselves. Some particles have no antiparticles; these include the photon, the hypothetical graviton, and any other hypothetical massless gauge bosons.
In 1932, soon after the prediction of positrons by Paul Dirac, Carl D. Anderson found that cosmic-ray collisions produced these particles in a cloud chamber— a particle detector in which moving electrons (or positrons) leave behind trails as they move through the gas. The electric charge-to-mass ratio of a particle can be measured by observing the curling of its cloud-chamber track in a magnetic field. Originally, positrons, because of the direction that their paths curled, were mistaken for electrons travelling in the opposite direction.
The antiproton and antineutron were found by Emilio Segrè and Owen Chamberlain in 1955 at the University of California, Berkeley. Since then the antiparticles of many other subatomic particles have been created in particle accelerator experiments. In recent years, complete atoms of antimatter have been assembled out of antiprotons and positrons, collected in electromagnetic traps.
... the development of quantum field theory made the interpretation of antiparticles as holes unnecessary, even though it lingers on in many textbooks. — Steven Weinberg in The quantum theory of fields, Vol I, p 14, ISBN 0-521-55001-7
Solutions of the Dirac equation contained negative energy quantum states. As a result, an electron could always radiate energy and fall into a negative energy state. Even worse, it could keep radiating infinite amount of energy because there were infinitely many negative energy states available. To prevent this unphysical situation from happening, Dirac proposed that a "sea" of negative-energy electrons fills the universe, already occupying all of the lower energy states so that, due to the Pauli exclusion principle no other electron could fall into them. Sometimes, however, one of these negative energy particles could be lifted out of this Dirac sea to become a positive energy particle. But when lifted out, it would leave behind a hole in the sea which would act exactly like a positive energy electron with a reversed charge. These he interpreted as the proton, and called his paper of 1930 A theory of electrons and protons.
Dirac was aware of the problem that his picture implied an infinite negative charge for the universe. Dirac tried to argue that we would perceive this as the normal state of zero charge. Another difficulty was the difference in masses of the electron and the proton. Dirac tried to argue that this was due to the electromagnetic interactions with the sea, until Hermann Weyl proved that hole theory was completely symmetric between negative and positive charges. Dirac also predicted a reaction + → +, where an electron and a proton annihilate to give two photons. Robert Oppenheimer and Igor Tamm proved that this would cause ordinary matter to disappear too fast. A year later, in 1931, Dirac modified his theory and postulated the positron, a new particle of the same mass as the electron. The discovery of this particle the next year removed the last two objections to his theory.
However, the problem of infinite charge of the universe remains. Also, as we now know, bosons also have antiparticles, but since they do not obey the Pauli exclusion principle, hole theory doesn't work for them. A unified interpretation of antiparticles is now available in quantum field theory, which solves both these problems.
If a particle and antiparticle are in the appropriate quantum states, then they can annihilate each other and produce other particles. Reactions such as + → + (the two-photon annihilation of an electron-positron pair) is an example. The single-photon annihilation of an electron-positron pair, + → cannot occur because it is impossible to conserve energy and momentum together in this process. The reverse reaction is also impossible for this reason. However, in quantum field theory this process is allowed as an intermediate quantum state for times short enough that the violation of energy conservation can be accommodated by the uncertainty principle. This opens the way for virtual pair production or annihilation in which a one particle quantum state may fluctuate into a two particle state and back. These processes are important in the vacuum state and renormalization of a quantum field theory. It also opens the way for neutral particle mixing through processes such as the one pictured here: which is a complicated example of mass renormalization.
Quantum states of a particle and an antiparticle can be interchanged by applying the charge conjugation (C), parity (P), and time reversal (T) operators. If |p,σ,n> denotes the quantum state of a particle (n) with momentum p, spin J whose component in the z-direction is σ, then one has
One may try to quantize an electron field without mixing the annihilation and creation operators by writing
where we use the symbol k to denote the quantum numbers p and σ of the previous section and the sign of the energy, E(k), and ak denotes the corresponding annihilation operators. Of course, since we are dealing with fermions, we have to have the operators satisfy canonical anti-commutation relations. However, if one now writes down the Hamiltonian
then one sees immediately that the expectation value of H need not be positive. This is because E(k) can have any sign whatsoever, and the combination of creation and annihilation operators has expectation value 1 or 0.
So one has to introduce the charge conjugate antiparticle field, with its own creation and annihilation operators satisfying the relations
where k has the same p, and opposite σ and sign of the energy. Then one can rewrite the field in the form
where the first sum is over positive energy states and the second over those of negative energy. The energy becomes
where E0 is an infinite negative constant. The vacuum state is defined as the state with no particle or antiparticle, i.e., and . Then the energy of the vacuum is exactly E0. Since all energies are measured relative to the vacuum, H is positive definite. Analysis of the properties of ak and bk shows that one is the annihilation operator for particles and the other for antiparticles. This is the case of a fermion.
This approach is due to Vladimir Fock, Wendell Furry and Robert Oppenheimer. If one quantizes a real scalar field, then one finds that there is only one kind of annihilation operator; therefore real scalar fields describe neutral bosons. Since complex scalar fields admit two different kinds of annihilation operators, which are related by conjugation, such fields describe charged bosons.
By considering the propagation of the negative energy modes of the electron field backward in time, Richard Feynman reached a pictorial understanding of the fact that the particle and antiparticle have equal mass m and spin J but opposite charges q. This allowed him to rewrite perturbation theory precisely in the form of diagrams, called Feynman diagrams, of particles propagating back and forth in time. This technique now is the most widespread method of computing amplitudes in quantum field theory.
This picture was independently developed by Ernst Stueckelberg, and has been called the Feynman-Stueckelberg interpretation of antiparticles.