Corresponding to most kinds of particle, there is an associated antiparticle with the same mass and opposite charges. (The exceptions are massless gauge bosons such as the photon.) Even electrically neutral particles, such as the neutron, are not identical to their antiparticle. In the example of the neutron, the 'ordinary' particle is made out of quarks and the antiparticle out of antiquarks. The laws of nature were thought to be symmetric between particles and antiparticles until CP violation experiments found that time-reversal symmetry is violated in nature. This small asymmetry is involved in baryogenesis, the process by which our universe came to consist almost entirely of matter, with almost no free antimatter. For the physics of antimatter, see the article on antiparticles; for other senses of this term, see antimatter (disambiguation). ... For the physics of antimatter, see the article on antiparticles; for other senses of this term, see antimatter (disambiguation). ... Annihilation is defined as total destruction or complete obliteration of an object;[1] having its root in the Latin nihil (nothing). ... For the DC Comics Superhero also called Atom Smasher, see Albert Rothstein. ... Penning traps are devices for the storage of charged particles using a constant magnetic field and a constant electric field. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... The antiproton (aka pbar) is the antiparticle of the proton. ... The antineutron is the antiparticle of the neutron. ... Image of a typical positron emission tomography (PET) facility Positron emission tomography (PET) is a nuclear medicine medical imaging technique which produces a three-dimensional image or map of functional processes in the body. ... Antimatter or contra-terrene matter is matter that is composed of the antiparticles of those that constitute normal matter. ... An antimatter weapon is a hypothetical device using antimatter as a power source, a propellant, or an explosive for a weapon. ... The ALPHA collaboration consists of scientists from a number scientific institutions whose goal it is to trap neutral antimatter in the form of antihydrogen in a magnetic trap and consecutively conduct experiments with the trapped antiatoms. ... Helmeted Athena, of the Velletri type. ... The ATRAP collaboration at CERN developed out of TRAP, a collaboration whose members pioneered cold antiprotons, cold positrons, and first made the ingredients of cold antihydrogen to interact. ... CERN logo The European Organization for Nuclear Research (French: ), commonly known as CERN (see Naming), pronounced (or in French), is the worlds largest particle physics laboratory, situated just northwest of Geneva on the border between France and Switzerland. ... Paul Adrien Maurice Dirac, OM, FRS (IPA: [dɪræk]) (August 8, 1902 – October 20, 1984) was a British theoretical physicist and a founder of the field of quantum physics. ... Carl Anderson at LBNL 1937 Carl David Anderson (3 September 1905 – 11 January 1991) was a U.S. experimental physicist. ... Thousands of particles explode from the collision point of two relativistic (100 GeV per ion) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... This article or section is in need of attention from an expert on the subject. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... Boson (game) Bosons, named after Satyendra Nath Bose, are particles which form totally-symmetric composite quantum states. ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... This article or section does not adequately cite its references or sources. ... For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the Σc++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... The six flavours of quarks and their most likely decay modes. ... In particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics. ... T-symmetry is the symmetry of physical laws under a time-reversal transformation. ... Baryogenesis is the generic designation for the physical processes that generate matter (more specifically, a class of fundamental particle called baryon) from an otherwise matter-empty state (such as it is generally believed to be the state of the Universe at its onset, the so-called Big Bang). ...

Particle-antiparticle pairs can annihilate each other if they are in appropriate quantum states. They can also be produced in various processes. These processes are used in today's particle accelerators to create new particles and to test theories of particle physics. High energy processes in nature can create antiparticles. These are visible in cosmic rays and in certain nuclear reactions. The word antimatter properly refers to (elementary) antiparticles, composite antiparticles made with them (such as antihydrogen) and to larger assemblies of either. Annihilation is defined as total destruction or complete obliteration of an object;[1] having its root in the Latin nihil (nothing). ... A quantum state is any possible state in which a quantum mechanical system can be. ... For the DC Comics Superhero also called Atom Smasher, see Albert Rothstein. ... Thousands of particles explode from the collision point of two relativistic (100 GeV per ion) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... In nuclear physics, a nuclear reaction is a process in which two nuclei or nuclear particles collide to produce products different from the initial particles. ... For the physics of antimatter, see the article on antiparticles; for other senses of this term, see antimatter (disambiguation). ... Antihydrogen is the antimatter counterpart of hydrogen. ...

[edit] History

[edit] 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. Year 1932 (MCMXXXII) was a leap year starting on Friday (the link will display full 1932 calendar) of the Gregorian calendar. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ... Paul Adrien Maurice Dirac, OM, FRS (IPA: [dɪræk]) (August 8, 1902 – October 20, 1984) was a British theoretical physicist and a founder of the field of quantum physics. ... Carl David Anderson (3 September 1905 – 11 January 1991) was a U.S. experimental physicist. ... Discovery of the positron in 1932 by Carl D. Anderson in a cloud chamber The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. ... The Compact Muon Solenoid (CMS) is an example of a large particle detector. ... e- redirects here. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... This article or section is in need of attention from an expert on the subject. ... Magnetic field lines shown by iron filings In physics, a magnetic field is a solenoidal vector field in the space surrounding moving electric charges and magnetic dipoles, such as those in electric currents and magnets. ...

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. Portrait of Dr. Emilio Segre Emilio Gino Segrè (February 1, 1905 - April 22, 1989) was an Italian American physicist who, with Owen Chamberlain, won the 1959 Nobel Prize in Physics for their discovery of the antiproton. ... Owen Chamberlain Owen Chamberlain (July 10, 1920 – February 28, 2006) was a prominent American physicist. ... Year 1955 (MCMLV) was a common year starting on Saturday (link displays the 1955 Gregorian calendar). ... Sather tower (the Campanile) looking out over the San Francisco Bay and Mount Tamalpais. ... For the physics of antimatter, see the article on antiparticles; for other senses of this term, see antimatter (disambiguation). ...

[edit] Hole theory

... the development of quantum field theory made the interpretation of antiparticles as holes unnecessary, even though unfortunately it lingers on in many textbooks.  —  Steven Weinberg in The quantum theory of fields, Vol I, p 14, ISBN 0-521-55001-7 Steven Weinberg (born May 3, 1933) is an American physicist. ...

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. In physics, the Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928 and provides a description of elementary spin-½ particles, such as electrons, consistent with both the principles of quantum mechanics and the theory of special relativity. ... The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925. ... The Dirac sea is a theoretical model of the vacuum as an infinite sea of particles possessing negative energy. ... In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ...

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 e + p → γ + γ, 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. Hermann Klaus Hugo Weyl (November 9, 1885 – December 9, 1955) was a German mathematician. ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... J. Robert Oppenheimer[1] (April 22, 1904 – February 18, 1967) was an American theoretical physicist, best known for his role as the director of the Manhattan Project, the World War II effort to develop the first nuclear weapons, at the secret Los Alamos laboratory in New Mexico. ... Igor Tamm. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ...

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. Boson (game) Bosons, named after Satyendra Nath Bose, are particles which form totally-symmetric composite quantum states. ... Quantum field theory (QFT) is the quantum theory of fields. ...

[edit] Particle-antiparticle annihilation

Main article: Annihilation. Annihilation is defined as total destruction or complete obliteration of an object;[1] having its root in the Latin nihil (nothing). ...

An example of a virtual pion pair which influences the propagation of a kaon causing a neutral kaon to mix with the antikaon. This is an example of renormalization in quantum field theory— the field theory being necessary because the number of particles changes from one to two and back again.

If a particle and antiparticle are in the appropriate quantum states, then they can annihilate each other and produce other particles. Reactions such as e⁺  +  e^-  →  γ  +  γ (the two-photon annihilation of an electron-positron pair) is an example. The single-photon annihilation of an electron-positron pair, e⁺  +  e^-  →  γ 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. Image File history File links K0-Kobar oscillation through a two pion intermediate state File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... In particle physics, pion (short for pi meson) is the collective name for three subatomic particles: Ï€0, Ï€+ and π−. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force. ... In particle physics, Kaons (also called K-mesons and denoted K) are a group of four mesons distinguished by the fact that they carry a quantum number called strangeness. ... Figure 1. ... Quantum field theory (QFT) is the quantum theory of fields. ... Quantum field theory (QFT) is the quantum theory of fields. ... In quantum physics, the Heisenberg uncertainty principle is a mathematical property of a pair of canonical conjugate quantities - usually stated in a form of reciprocity of spans of their spectra. ... In quantum field theory, the vacuum state, usually denoted , is the element of the Hilbert space with the lowest possible energy, and therefore containing no physical particles. ... Figure 1. ... Quantum field theory (QFT) is the quantum theory of fields. ... In quantum field theory, mass renormalization refers to the quantum corrections to the mass of a particle through its self interactions, or through interactions with other particles. ...

[edit] 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). 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 A quantum state is any possible state in which a quantum mechanical system can be. ... C-symmetry means the symmetry of physical laws over a charge-inversion transformation. ... P-symmetry is simply the spatial symmetry exhibited during a reflection. ... T-symmetry is the symmetry of physical laws under a time-reversal transformation— The universe is not symmetric under time reversal, although in restricted contexts one may find this symmetry. ...

CPT |p,σ,n>  =  (-1)^J-σ |p,-σ,n^c>,

where n^c denotes the charge conjugate state, i.e., the antiparticle. This behaviour under CPT is the same as the statement that the particle and its antiparticle lie in the same irreducible representation of the Poincare group. Properties of antiparticles can be related to those of particles through this. If T is a good symmetry of the dynamics, then In mathematics, the term irreducible is used in several ways. ... In physics and mathematics, the Poincaré group is the group of isometries of Minkowski spacetime. ...

T |p,σ,n>  α  |-p,-σ,n>

CP |p,σ,n>  α  |-p,σ,n^c>

C |p,σ,n>  α  |p,σ,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
• opposite electric charges q and -q.

Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ...

[edit] Quantum field theory

This section draws upon the ideas, language and notation of canonical quantization of a quantum field theory. In physics, canonical quantization is one of many procedures for quantizing a classical theory. ... Quantum field theory (QFT) is the quantum theory of fields. ...

One may try to quantize an electron field without mixing the annihilation and creation operators by writing The magnitude of an electric field surrounding two equally charged (repelling) particles. ...

ψ(x)  =  ∑_k u_k(x) a_k e^{-i E(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 with fermions, we have to have the operators satisfy canonical anti-commutation relations. However, if one now writes down the Hamiltonian In particle physics, fermions are particles with half-integer spin, such as protons and electrons. ... The quantum Hamiltonian is the physical state of a system, which may be characterized as a ray in an abstract Hilbert space (or, in the case of ensembles, as a trace class operator with trace 1). ...

H  =  ∑_k E(k) a⁺_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_k'  =  a⁺_k and b⁺_k'  =  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

ψ(x)  =  ∑_k(+) u_k(x) a_k e^{-i E(k)t}  +  ∑_k(-) u_k(x) b⁺_k e^{-i E(k)t},

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

H  =  ∑_k(+) E(k) a⁺_k a_k  +  ∑_k(-) |E(k)| b⁺_k b_k  +  E₀,

where E₀ is an infinite negative constant. The vacuum state is defined as the state with no particle or antiparticle, ie, a_k |0> = 0 and b_k |0> = 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 a_k and b_k shows that one is the annihilation operator for particles and the other for antiparticles. This is the case of a fermion. In quantum field theory, the vacuum state, usually denoted , is the element of the Hilbert space with the lowest possible energy, and therefore containing no physical particles. ... In particle physics, fermions are particles with half-integer spin, such as protons and electrons. ...

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. Vladimir Aleksandrovich Fock (or Fok, Владимир Александрович Фок) (22 December 1898 - December 27, 1974) was a Soviet physicist, who did foundational work on quantum mechanics. ... J. Robert Oppenheimer[1] (April 22, 1904 – February 18, 1967) was an American theoretical physicist, best known for his role as the director of the Manhattan Project, the World War II effort to develop the first nuclear weapons, at the secret Los Alamos laboratory in New Mexico. ... It has been suggested that quartic interaction be merged into this article or section. ... In particle physics, bosons, named after Satyendra Nath Bose, are particles having integer spin. ... In particle physics, bosons, named after Satyendra Nath Bose, are particles having integer spin. ...

[edit] 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. Richard Phillips Feynman (May 11, 1918 – February 15, 1988; IPA: ) was an American physicist known for expanding the theory of quantum electrodynamics, the physics of the superfluidity of supercooled liquid helium, and particle theory. ... In quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one. ... In this Feynman diagram, an electron and positron annihilate and become a quark-antiquark pair. ... Quantum field theory (QFT) is the quantum theory of fields. ...

This picture was independently developed by Ernst Stueckelberg, and has been called the Feynman-Stueckelberg interpretation of antiparticles. Ernst Carl Gerlach Stueckelberg (February 1, 1905, Basel - September 4, 1984, Basel) was a Swiss mathematician and physicist. ...

[edit] See also

• Gravitational interaction of antimatter
• Parity, charge conjugation and time reversal symmetry.
• CP violations and the baryon asymmetry of the universe.
• Quantum field theory and the list of particles
• Baryogenesis

The factual accuracy of this article is disputed. ... In physics, a parity transformation (also called parity inversion) is the simultaneous flip in the sign of all spatial coordinates: A 3×3 matrix representation of P would have determinant equal to –1, and hence cannot reduce to a rotation. ... C-symmetry means the symmetry of physical laws over a charge-inversion transformation. ... T-symmetry is the symmetry of physical laws under a time-reversal transformation. ... In particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics. ... The baryon asymmetry problem in astrophysics refers to the apparent fact that the baryons in the universe which have been observed are overwhelmingly matter as opposed to anti-matter. ... Quantum field theory (QFT) is the quantum theory of fields. ... This is a list of particles in particle physics, including currently known and hypothetical elementary particles, as well as the composite particles that can be built up from them. ... Baryogenesis is the generic designation for the physical processes that generate matter (more specifically, a class of fundamental particle called baryon) from an otherwise matter-empty state (such as it is generally believed to be the state of the Universe at its onset, the so-called Big Bang). ...

[edit] 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.