Corresponding to most kinds of particle, there is an associated antiparticle with the same mass and opposite charge. Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Physics, a charge may refer to one of many different quantities such as the Electric charge in Electromagnetism or the Color charge in 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 positrons or antielectron is the Antiparticle or the Antimatter counterpart of the Electron. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation.
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. Antihydrogen is the Antimatter counterpart of Hydrogen. Whereas the common hydrogen Atom is composed of an Electron and Proton 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. In Physical cosmology, baryogenesis is the generic term for hypothetical physical processes that produced an asymmetry between Baryons and anti-baryons in In Particle physics and Quantum chemistry, antimatter is the extension of the concept of the Antiparticle to Matter, where antimatter is composed 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. In Particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics
Particle-antiparticle pairs can annihilate each other, producing photons; since the charges of the particle and antiparticle are opposite, charge is conserved. Annihilation is defined as "total destruction" or "complete obliteration" of an object having its root in the Latin nihil (nothing For example, the antielectrons produced in natural radioactive decay quickly annihilate themselves with electrons, producing pairs of gamma rays. Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions
Antiparticles are produced naturally in beta decay, and in the interaction of cosmic rays in the Earth's atmosphere. In Nuclear physics, beta decay is a type of Radioactive decay in which a Beta particle (an Electron or a Positron) is emitted For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on 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. In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. 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. In Astrophysics, weakly interacting massive particles or WIMPs are hypothetical particles serving as one possible solution to the Dark matter problem Some particles have no antiparticles; these include the photon, the hypothetical graviton, and any other hypothetical massless gauge bosons. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena In Physics, the graviton is a hypothetical Elementary particle, a Boson to be exact that mediates the force of Gravity in the framework In Particle physics, bosons are particles which obey Bose-Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein
Contents |
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. Year 1932 ( MCMXXXII) was a Leap year starting on Friday of the Gregorian calendar. The positrons or antielectron is the Antiparticle or the Antimatter counterpart of the Electron. Carl David Anderson ( 3 September 1905 &ndash 11 January 1991) was an American Physicist. The cloud chamber, also known as the Wilson chamber, is used for detecting particles of Ionizing radiation. In experimental and applied Particle physics and Nuclear engineering, a particle detector, also known as a radiation detector, is a device used to The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J 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. Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges 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. The antiproton ( pronounced p-bar) is the Antiparticle of the Proton. The antineutron is the Antiparticle of the Neutron. It was discovered (in proton-proton collisions in the Bevatron at Berkeley by Bruce Cork in Emilio Gino Segrè ( February 1, 1905 – April 22, 1989) was an Italian Physicist and Nobel laureate in Owen Chamberlain ( July 10, 1920 &ndash February 28, 2006) was an American Physicist, and Nobel laureate in physics Year 1955 ( MCMLV) was a Common year starting on Saturday (link displays the 1955 Gregorian calendar) The University of California Berkeley (also referred to as Cal, Berkeley and UC Berkeley) is a major research university located in 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. In Particle physics and Quantum chemistry, antimatter is the extension of the concept of the Antiparticle to Matter, where antimatter is composed
Hole theory
. . . the development of quantum field theory made the interpretation of antiparticles as holes unnecessary, even though it lingers on in many textbooks. In quantum field theory (QFT the forces between particles are mediated by other particles — 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. Steven Weinberg (born May 3, 1933) is an American Physicist, and Nobel laureate in Physics for his contributions with Abdus Salam In Physics, the Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928 and provides 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. The Pauli exclusion principle is a quantum mechanical principle formulated by Wolfgang Pauli in 1925 Sometimes, however, one of these negative energy particles could be lifted out of this Dirac sea to become a positive energy particle. The Dirac sea is a theoretical model of the Vacuum as an infinite sea of particles possessing Negative energy. 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. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive
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. Hermann Klaus Hugo Weyl ( 9 November 1885 – 8 December 1955) was a German Mathematician. Dirac also predicted a reaction e−+p+ → γ+γ, where an electron and a proton annihilate to give two photons. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Robert Oppenheimer and Igor Tamm proved that this would cause ordinary matter to disappear too fast. Igor Yevgenyevich Tamm ( Russian И́горь Евге́ньевич Та́мм) ( July 8 1895 &ndash April 12 1971) was A year later, in 1931, Dirac modified his theory and postulated the positron, a new particle of the same mass as the electron. The positrons or antielectron is the Antiparticle or the Antimatter counterpart of 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. In Particle physics, bosons are particles which obey Bose-Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein A unified interpretation of antiparticles is now available in quantum field theory, which solves both these problems. In quantum field theory (QFT the forces between particles are mediated by other particles
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 e− + p+ → γ + γ (the two-photon annihilation of an electron-positron pair) is an example. The single-photon annihilation of an electron-positron pair, e− + p+ → 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. In quantum field theory (QFT the forces between particles are mediated by other particles In Quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the Momentum of the particle uncertain 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. In Quantum field theory, the vacuum state (also called the vacuum) is the Quantum state with the lowest possible Energy. In Quantum field theory, the Statistical mechanics of fields and the theory of self-similar geometric structures renormalization refers to a collection In quantum field theory (QFT the forces between particles are mediated by other particles 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. In Theoretical physics and Quantum field theory a particle's self-energy \Sigma represents the contribution to the particle's Energy, or
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. In Quantum physics, a quantum state is a mathematical object that fully describes a quantum system. In Physics, C-symmetry means the symmetry of physical laws under a charge -conjugation transformation. In Physics, a parity transformation (also called parity inversion) is the flip in the sign of one Spatial Coordinate. T Symmetry is the symmetry of physical laws under a Time reversal transformation &mdash T t \mapsto -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
where nc 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. In the mathematical field of Representation theory, group representations describe abstract groups in terms of Linear transformations of In Physics and Mathematics, the Poincaré group, named after Henri Poincaré, is the group of isometries of Minkowski spacetime Properties of antiparticles can be related to those of particles through this. If T is a good symmetry of the dynamics, then
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.
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. In quantum field theory (QFT the forces between particles are mediated by other particles
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. In Physics, a field is a Physical quantity associated to each point of Spacetime. Of course, since we are dealing with fermions, we have to have the operators satisfy canonical anti-commutation relations. In Particle physics, fermions are particles which obey Fermi-Dirac statistics; they are named after Enrico Fermi. However, if one now writes down the Hamiltonian
then one sees immediately that the expectation value of H need not be positive. In Quantum mechanics, the Hamiltonian H is the Observable corresponding to the Total energy of the system 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. In Quantum field theory, the vacuum state (also called the vacuum) is the Quantum state with the lowest possible Energy. 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. In Particle physics, fermions are particles which obey Fermi-Dirac statistics; they are named after Enrico Fermi.
This approach is due to Vladimir Fock, Wendell Furry and Robert Oppenheimer. Vladimir Aleksandrovich Fock (or Fok, Владимир Александрович Фoк ( December 22 1898 &ndash December 27 1974 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. For the pseudoscientific "scalar field theory" see " Scalar field theory (pseudoscience " In Theoretical physics, In Particle physics, bosons are particles which obey Bose-Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein Since complex scalar fields admit two different kinds of annihilation operators, which are related by conjugation, such fields describe charged bosons. In Particle physics, bosons are particles which obey Bose-Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein
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. Richard Phillips Feynman (ˈfaɪnmən May 11 1918 – February 15 1988 was an American Physicist known for the Path integral formulation of quantum This allowed him to rewrite perturbation theory precisely in the form of diagrams, called Feynman diagrams, of particles propagating back and forth in time. In Quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system Motivation and history When calculating Scattering cross sections in Particle physics, the interaction between particles can be described This technique now is the most widespread method of computing amplitudes in quantum field theory. In quantum field theory (QFT the forces between particles are mediated by other particles
This picture was independently developed by Ernst Stueckelberg, and has been called the Feynman-Stueckelberg interpretation of antiparticles. This article is about the physicist for his grandfather the Swiss artist see Ernst Alfred Stueckelberg Ernst Carl Gerlach Stueckelberg (
See also
- Gravitational interaction of antimatter
- Parity, charge conjugation and time reversal symmetry. The gravitational interaction of antimatter with Matter or Antimatter has not been conclusively observed by physicists In Physics, a parity transformation (also called parity inversion) is the flip in the sign of one Spatial Coordinate. In Physics, C-symmetry means the symmetry of physical laws under a charge -conjugation transformation. T Symmetry is the symmetry of physical laws under a Time reversal transformation &mdash T t \mapsto -t
- CP violations and the baryon asymmetry of the universe. In Particle physics, CP violation is a violation of the postulated CP symmetry of the laws of physics See also Baryogenesis The baryon asymmetry problem in Physics refers to the apparent fact that the Baryons in the universe which have been
- Quantum field theory and the list of particles
- Baryogenesis
References
- Feynman, Richard P. In quantum field theory (QFT the forces between particles are mediated by other particles This is a list of the different types of particles known and hypothesized In Physical cosmology, baryogenesis is the generic term for hypothetical physical processes that produced an asymmetry between Baryons and anti-baryons in "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.
Dictionary
antiparticle
-noun
- (physics) 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.
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