Dictionary Definition
antiparticle n : a particle that has the same
mass as another particle but has opposite values for its other
properties; interaction of a particle and its antiparticle results
in annihilation and the production of radiant energy
User Contributed Dictionary
English
Pronunciation
 /ˈæntiˌpɑɹtɪkəl/
Noun
 A subatomic particle corresponding to another particle with the same mass, spin and mean lifetime but with charge, parity, strangeness and other quantum numbers flipped in sign.
Translations
a subatomic particle
 Catalan: antipartícula
 Finnish: antihiukkanen
 French: antipartícule
 German: Antiteilchen
 Italian: antiparticella
 Japanese: 反粒子 (はんりゅうし, hanryūshi)
 Russian: античастица (antičastíca)
 Spanish: antipartícula
 Swedish: antipartikel
See also
Extensive Definition
Corresponding to most kinds of particle,
there is 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 halfandhalf 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.
Particleantiparticle 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 particleantiparticle
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.
History
Experiment
In 1932, soon after the
prediction of positrons
by Paul
Dirac, Carl D.
Anderson found that cosmicray 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
chargetomass ratio of
a particle can be measured by observing the curling of its
cloudchamber 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
0521550017
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 negativeenergy 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.
Particleantiparticle 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 twophoton annihilation of an
electronpositron pair) is an example. The singlephoton
annihilation of an electronpositron 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.
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
zdirection is σ, then one has

 CPT \ p,\sigma,n>\ =\ (1)^\ p,\sigma,n^c>,

 T\ p,\sigma,n>\ \alpha \ p,\sigma,n>,
 CP\ p,\sigma,n>\ \alpha \ p,\sigma,n^c>,
 C\ p,\sigma,n>\ \alpha \ p,\sigma,n^c>,
 the same mass m
 the same spin state J
 opposite electric charges q and q.
Quantum field theory
This section draws upon the ideas, language and
notation of canonical
quantization of a quantum
field theory.
One may try to quantize an electron field
without mixing the annihilation and creation operators by
writing

 \psi (x)=\sum_u_k (x)a_k e^,\,
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 anticommutation relations. However, if
one now writes down the
Hamiltonian

 H=\sum_ E(k) a^\dagger_k a_k,\,
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

 b_ = a^\dagger_k\ \mathrm\ b^\dagger_=a_k,\,
where k has the same p, and opposite σ and sign
of the energy. Then one can rewrite the field in the form

 \psi(x)=\sum_ u_k (x)a_k e^+\sum_ u_k (x)b^\dagger _k e^,\,
where the first sum is over positive energy
states and the second over those of negative energy. The energy
becomes

 H=\sum_ E_k a^\dagger _k a_k + \sum_ E(k)b^\dagger_k b_k + E_0,\,
where E0 is an infinite negative constant. The
vacuum
state is defined as the state with no particle or antiparticle,
i.e., a_k 0\rangle=0 and b_k 0\rangle=0. 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.
The FeynmanStueckelberg 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 FeynmanStueckelberg
interpretation of antiparticles.
See also
References
 Feynman, Richard P. "The reason for antiparticles", in The 1986 Dirac memorial lectures, R.P. Feynman and S. Weinberg. Cambridge University Press, 1987. ISBN 0521340004.
 Weinberg, Steven. The quantum theory of fields, Volume 1: Foundations. Cambridge University Press, 1995. ISBN 0521550017.
antiparticle in Min Nan: Hoánlia̍pchú
antiparticle in Catalan: Antipartícula
antiparticle in Czech: Antičástice
antiparticle in German: Antiteilchen
antiparticle in Modern Greek (1453):
Αντισωματίδιο
antiparticle in Spanish: Antipartícula
antiparticle in Basque: Antipartikula
antiparticle in French: Antiparticule
antiparticle in Indonesian: Antipartikel
antiparticle in Italian: Antiparticella
antiparticle in Hebrew: אנטיחלקיק
antiparticle in Latvian: Antidaļiņa
antiparticle in Lombard: Antipartisèla
antiparticle in Hungarian: Antirészecske
antiparticle in Malayalam: പ്രതികണം
antiparticle in Japanese: 反粒子
antiparticle in Norwegian: Antipartikkel
antiparticle in Norwegian Nynorsk:
Antipartikkel
antiparticle in Portuguese: Antipartícula
antiparticle in Romanian: Antiparticulă
antiparticle in Russian: Античастицы
antiparticle in Simple English:
Antiparticle
antiparticle in Slovak: Antičastica
antiparticle in Serbian: Античестица
antiparticle in Turkish: Antiparçacıklar
antiparticle in Ukrainian: Античастинка
antiparticle in Urdu: ضد ذرہ
antiparticle in Vietnamese: Phản hạt
antiparticle in Chinese: 反粒子