Everything about Antiparticles totally explained
Corresponding to most kinds of
particle, there's an associated
antiparticle with the same
mass and opposite
charge. For example, the antiparticle of the electron is the positively charged antielectron, or
positron, which is produced naturally in certain types of
radioactive decay.
The laws of nature are very nearly symmetrical with respect to particles and antiparticles. For example, an antiproton and an antielectron 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, 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 isn't 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're distinguishable from one another because an antineutron, unlike a neutron, will rapidly annihilate itself by colliding with neutrons in ordinary matter. However, it's 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.
History
Experiment
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.
Hole theory
... 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'd 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 don't 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.
Particle-antiparticle annihilation
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, + → can't occur because it's 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.
Properties of antiparticles
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
» :
where
E0 is an infinite negative constant. The
vacuum state is defined as the state with no particle or antiparticle,
for example,
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's 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.
The Feynman-Stueckelberg interpretation
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.
Further Information
Get more info on 'Antiparticles'.
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