- Antiparticle
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The laws of nature are very nearly symmetrical with respect to particles and antiparticles. For example, an

antiproton and apositron can form anantihydrogen 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 andantimatter . The discovery ofCP 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 ofcosmic ray s 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 inparticle accelerator s. 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 thephoton , the hypotheticalgraviton , and any other hypothetical massless gaugeboson s.**History****Experiment**In 1932, soon after the prediction of

positron s byPaul Dirac ,Carl D. Anderson found that cosmic-ray collisions produced these particles in acloud chamber — aparticle detector in which movingelectron s (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 amagnetic field . Originally, positrons, because of the direction that their paths curled, were mistaken for electrons travelling in the opposite direction.The

antiproton andantineutron were found byEmilio Segrè andOwen Chamberlain in 1955 at theUniversity of California, Berkeley . Since then the antiparticles of many other subatomic particles have been created in particle accelerator experiments. In recent years, complete atoms ofantimatter have been assembled out of antiprotons and positrons, collected in electromagnetic trapsFact|date=September 2008.**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 0-521-55001-7Solutions 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 thePauli exclusion principle no other electron could fall into them. Sometimes, however, one of these negative energy particles could be lifted out of thisDirac 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 theproton , 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 SubatomicParticle|Electron+SubatomicParticle|Proton+ → SubatomicParticle|Photon+SubatomicParticle|Photon, where an electron and a proton annihilate to give two photons.Robert Oppenheimer andIgor 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 inquantum 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 SubatomicParticle|Electron + SubatomicParticle|Positron → SubatomicParticle|Photon + SubatomicParticle|Photon (the two-photon annihilation of an electron-positron pair) is an example.The single-photon annihilation of an electron-positron pair, SubatomicParticle|Electron + SubatomicParticle|Positron → SubatomicParticle|Photon 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 theuncertainty 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 thevacuum state andrenormalization of a quantum field theory. It also opens the way forneutral particle mixing through processes such as the one pictured here: which is a complicated example ofmass renormalization .**Properties of antiparticles**Quantum state s 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::$CPT\; |p,sigma,n>\; =\; (-1)^\{J-sigma\}\; |p,-sigma,n^c>,$where**n**denotes the charge conjugate state, "i.e.", the antiparticle. This behaviour under^{c}**CPT**is the same as the statement that the particle and its antiparticle lie in the sameirreducible representation of thePoincare group . Properties of antiparticles can be related to those of particles through this. If**T**is a good symmetry of the dynamics, then::$T\; |p,sigma,n>\; propto\; |-p,-sigma,n>,$::$CP\; |p,sigma,n>\; propto\; |-p,sigma,n^c>,$::$C\; |p,sigma,n>\; propto\; |p,sigma,n^c>,$where the proportionality sign indicates that there might be a phase on the right hand side. In other words, particle and antiparticle must have

*the same mass**m**

*the same spin state**J**

*oppositeelectric charge s**q**and**-q**.**Quantum field theory**"This section draws upon the ideas, language and notation of

canonical quantization of aquantum field theory ."One may try to quantize an electron field without mixing the annihilation and creation operators by writing

::$psi\; (x)=sum\_\{k\}u\_k\; (x)a\_k\; e^\{-iE(k)t\},,$

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 "a

_{k}" denotes the corresponding annihilation operators. Of course, since we are dealing withfermion s, we have to have the operators satisfy canonical anti-commutation relations. However, if one now writes down the Hamiltonian::$H=sum\_\{k\}\; E(k)\; a^dagger\_k\; a\_k,,$

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

::$b\_\{kprime\}\; =\; a^dagger\_k\; mathrm\{and\}\; b^dagger\_\{kprime\}=a\_k,,$

where "k" has the same "p", and opposite σ and sign of the energy. Then one can rewrite the field in the form

::$psi(x)=sum\_\{k\_+\}\; u\_k\; (x)a\_k\; e^\{-iE(k)t\}+sum\_\{k\_-\}\; u\_k\; (x)b^dagger\; \_k\; e^\{-iE(k)t\},,$

where the first sum is over positive energy states and the second over those of negative energy. The energy becomes

::$H=sum\_\{k\_+\}\; E\_k\; a^dagger\; \_k\; a\_k\; +\; sum\_\{k\_-\}\; |E(k)|b^dagger\_k\; b\_k\; +\; E\_0,,$

where "E

_{0}" is an infinite negative constant. Thevacuum state is defined as the state with no particle or antiparticle, "i.e.", $a\_k\; |0\; angle=0$ and $b\_k\; |0\; angle=0$. Then the energy of the vacuum is exactly "E_{0}". Since all energies are measured relative to the vacuum,**H**is positive definite. Analysis of the properties of "a_{k}" and "b_{k}" shows that one is the annihilation operator for particles and the other for antiparticles. This is the case of afermion .This approach is due to

Vladimir Fock ,Wendell Furry andRobert 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 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, calledFeynman diagram s, 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.**See also***

Gravitational interaction of antimatter

* Parity,charge conjugation andtime reversal symmetry .

*CP violation s and thebaryon asymmetry of the universe .

*Quantum field theory and thelist of particles

*Baryogenesis **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 0-521-34000-4.

*Weinberg, Steven. "The quantum theory of fields, Volume 1: Foundations". Cambridge University Press, 1995. ISBN 0-521-55001-7.

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