For each kind of particle, there is an associated antiparticle with the same mass but opposite electromagnetic, weak, and strong charges, as well as spin. Some particles, notably photons, have no distinct antiparticle, or, put in another way, are identical to their antiparticle. Such particles are called real neutral particles, in contrast with, say, neutrons or neutral kaons, which are not identical to their antiparticles. Each quantum number of a real neutral particle is identical with its antiparticle's one. Particles explode from the collision point of two relativistic velocity (100 GeV) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... In physics, spin is an intrinsic angular momentum associated with microscopic particles. ... In physics, the photon (from Greek φοτος, meaning light) is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by quantum electrodynamics (QED) which is the oldest part of the Standard Model of particle physics. ... In physics, a real neutral particle is a particle that is its own antiparticle. ... Properties In physics, the neutron is a subatomic particle with no net electric charge and a mass of 940 MeV/c² (1. ... The neutral Kaons are symmetric and antisymmetric mixtures of the quark combinations down-antistrange and antidown-strange. ... A quantum number is a number used to parametrise certain properties of particles or other systems in quantum mechanics. ...

Particle-antiparticle pairs can arise from the interactions of other particles, and can annihilate one another to produce other particles. The annihilation products (and possible means of pair production) depend on the interactions of the particles involved. For example, an electron and positron (or antielectron) will tend to annihilate to photons, the quanta of the electromagnetic field, since their interactions are primarily electromagnetic. Proton-antiproton pairs, interacting through the strong nuclear force, tend to annihilate to collections of mesons and their own antiparticles, mostly various types of pion. In either case, however, increasing the energy of the collision (as in a particle accelerator) can lead to the production of more exotic products as the necessary energy becomes available, and this process is an important tool in particle physics. Properties The electron (sometimes called negatron; commonly represented as e−) is a subatomic particle. ... A positron is the antiparticle of the electron. ... In physics, the photon (from Greek φοτος, meaning light) is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by quantum electrodynamics (QED) which is the oldest part of the Standard Model of particle physics. ... An electromagnetic field is composed of two related vectorial fields, the electric field and the magnetic field. ... Properties In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... The antiproton is the antiparticle of the proton. ... The strong nuclear force or strong interaction (also called color force or colour force) is a fundamental force of nature which affects only quarks and antiquarks, and is mediated by gluons in a similar fashion to how the electromagnetic force is mediated by photons. ... In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ... In particle physics, pion (short for the Greek pi meson = P middle) is the collective name for three subatomic particles discovered in 1947: π0, π+ and π−. Pions are the lightest mesons. ... One of the early particle accelerators responsible for development of the atomic bomb. ... Particles explode from the collision point of two relativistic velocity (100 GeV) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ...

Antiparticles are produced by certain nuclear reactions (most notably positive beta decay), and by sufficiently energetic particle collisions, including naturally occurring cosmic ray collisions. Antimatter is a collection of antiparticles, in particular antiprotons, antineutrons and positrons in a similar composition as matter. In nuclear physics, a nuclear reaction is a process in which two nuclei or nuclear particles collide, to produce products different to the initial products. ... In nuclear physics, beta decay (sometimes called neutron decay) is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. ... The antiproton is the antiparticle of the proton. ... The antineutron is the antiparticle of the neutron. ... A positron is the antiparticle of the electron. ...

The existence of antiparticles was predicted by Dirac a few years before the first one, the antielectron or positron, was found by Carl D. Anderson in a cloud chamber experiment. Dirac's prediction stemmed from the existence of negative energy states, which in a relativistic universe cannot be discarded a priori. To keep all interacting electrons in his theory from eventually falling into negative energy states, Dirac posited that these extra states must all be filled already. In that case, a negative-energy particle could be promoted to a positive energy state, creating a real particle and leaving a hole that would behave exactly the same, but with opposite charge. Paul Adrien Maurice Dirac, (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. ... The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. ...

History

Experiment

In 1932, Carl D. Anderson found positrons created by cosmic-ray collisions in a cloud chamber, 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. 1932 is a leap year starting on a Friday. ... Carl David Anderson (3 September 1905 – 11 January 1991) was a U.S. experimental physicist. ... Electric charge is a fundamental FATTY STASHEconserved property of some subatomic particles, which determines their electromagnetic interactions. ... Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... In physics, a magnetic field is an entity produced by moving electric charges (electric currents) that exerts a force on other moving charges. ...

The antiproton and antineutron were found by Emilio Segrè and Owen Chamberlain in 1955. 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. 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 (b. ... 1955 is a common year starting on Saturday. ... Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. ...

Hole theory

It is tempting to sometimes think of antimatter as consisting of negative energy, or possibly even having negative mass. However, this cannot be the case. When a particle and its antiparticle annihilate, the energy released is the sum of mc² of the two particles (or more accurately, the sum of the of the particles). If antimatter had negative energy, the energy released from the two colliding particles would equal 0, since the positive and negative energies would cancel each other out.

Dirac developed "hole theory" when, among other things he looked at the true form of the equation E = mc², which is actually E² = p²c² + m²c⁴, and realized that the "²" sign means that the equation for energy can have two solutions, a negative energy solution and a positive energy solution. Dirac's own relativistic wave equation for electrons seemed to imply that both negative- and positive-energy electrons really existed. That being the case, since a collection of electrons would tend to radiate energy in their interactions, there seemed to be nothing to stop every electron in the universe from emitting enough energy to fall into a negative energy state. To prevent this from happening in his theory (which would obviously be at odds with the world we see around us), Dirac invoked the Pauli exclusion principle applying to all fermions such as the electron; this prevents any two electrons from occupying exactly the same quantum state. He proposed a "sea" of negative-energy electrons that would fill the universe, already occupying all of the lower energy states so that no other electrons could fall into them. The Dirac equation is a relativistic quantum mechanical wave equation invented by Paul Dirac in 1928. ... The Pauli exclusion principle, commonly referred to simply as the exclusion principle, is a quantum mechanical principle formulated by Wolfgang Pauli in 1925, which states that no two identical fermions may occupy the same quantum state. ... Fermions, named after Enrico Fermi, are particles which form totally-antisymmetric composite quantum states. ...

However, given the right combination of energy and momentum, from, for example, a collision of two photons, one of these particles could be lifted out of the Dirac sea of negative energy to become a positive energy particle. But when lifted out, it would leave behind a hole in the sea of negative energy — the absence of a negative-energy electron, which would itself, according to the mathematics, act exactly like a positive-energy electron, only with a positive charge. If an electron were to hit this hole, a negative-energy state would become available to it, and the electron could emit enough energy in the form of photons to send it into that lower state, disappearing into the sea of negative-energy electrons. The Dirac sea is a theoretical model of the vacuum as an infinite sea of particles possessing negative energy. ...

In the lab, this would appear as a pair of photons colliding and transforming into an electron and positron. The positron then would hit another electron (or possibly, the same one), and energy would be released as the two particles annihilate each other.

This theory of antimatter is completely consistent with what has been observed in the laboratory, and theoretically, the anti-particle should exhibit a "normal" gravitational force. This description is also similar to the behavior of charge carriers in a semiconductor, for which the hole theory is still considered valid. A semiconductor is a material that is an insulator at very low temperature, but which has a sizable electrical conductivity at room temperature. ...

However, nobody (including Dirac) was very satisfied with the idea that the universe was completely filled with a sea of negative-energy electrons. For one thing, it was hard to see how the corresponding infinite sea of negative charge could be made to make sense. For another, bosons also have antiparticles, but since they do not obey the Pauli exclusion principle, hole theory doesn't work for them. Boson (game) Bosons, named after Satyendra Nath Bose, are particles which form totally-symmetric composite quantum states. ...

Antiparticles in quantum field theory

The root of the trouble lay in Dirac's treatment of his wave theory for the electron as a way of reconciling special relativity with the single-particle quantum mechanics represented by the Schrödinger equation. Eventually the Dirac theory would became incorporated into quantum electrodynamics (QED), the first successful quantum field theory; the hole theory was abandoned and the situation became far less paradoxical. Special relativity (SR) or the special theory of relativity is the physical theory published in 1905 by Albert Einstein. ... Fig. ... In physics, the Schrödinger equation, proposed by the Austrian physicist Erwin Schrödinger in 1925, describes the time-dependence of quantum mechanical systems. ... Quantum electrodynamics (QED) is a quantum field theory of electromagnetism. ... Quantum field theory (QFT) is the application of quantum mechanics to fields. ...

In Shin'ichiro Tomonaga's and Julian Schwinger's formulation, the entity described by Dirac's wave equation is not the wave function of a single electron, but, rather, a quantum field constructed of operators that create and destroy both electrons and positrons. Dirac's positive-energy solutions to the wave equation are associated with operators that destroy electrons (by convention) and create positrons; his negative-energy solutions are associated with the creation of electrons and the destruction of positrons. The particles themselves occur only in positive-energy states, though they can seem to have negative energy in the intermediate virtual states used in the bookkeeping of perturbation theory. Julian Seymour Schwinger (February 12, 1918 -- July 16, 1994) was an American theoretical physicist. ... This article is about operators in mathematics, for other kinds of operators see operator (disambiguation). ... In the description of the interaction between elementary particles in quantum field theory, a virtual particle is a temporary elementary particle, used to describe an intermediate stage in the interaction. ... In quantum mechanics, perturbation theory is a set of approximation schemes for describing a complicated quantum system in terms of a simpler one. ...

Richard Feynman's formulation of QED treated positrons in a seemingly very different (and very strange) but actually equivalent manner: he thought of the Dirac wave equation as describing the wave function of a particle, but allowed the particle to travel backwards in time. Feynman then interpreted the negative-energy solutions as positive-energy electrons moving backwards in time, in which case they would act much like forward-in-time particles with the opposite charge—antiparticles. Two particles of the same charge travelling in different directions through time could attract electromagnetically. Richard Phillips Feynman (May 11, 1918–February 15, 1988) (surname pronounced FINE-man; in IPA) was one of the most influential American physicists of the 20th century, expanding greatly the theory of quantum electrodynamics. ... The Stückelberg-Feynman interpretation, named for Ernst Stueckelberg and Richard Feynman, of antimatter asserts that antiparticles can be treated to be normal particles traveling backwards in time. ...

Annihilation and pair production then appear to be nothing but a generalization of scattering, as can be seen in the following example. Say you have an electron, travelling forward through time, and (according to Feynman's picture) it emits a photon with enough energy and in the right direction to send it hurling back in time. It continues along for a while, then emits another photon backward in time, which sends it hurling forward through time once again. In particle physics, scattering is a class of phenomena by which particles are deflected by collisions with other particles. ...

 t5 -----*-------/- t4 ----/------/-- t3 ---/------/--- t2 --/------/---- t1 -/-------*----- 

The Y axis is time and the X axis is position. The "*" are places where photons are emitted, the "/" and "" trace out the path of the particle, from left to right, and the "-" designate a specific point in time, labeled as t1, t2, t3, t4, and t5.

To us, observing this reaction travelling only forward in time, at T1 we see a photon split up into two particles, a positron and an electron. The electron is travelling off to the right while the positron moves to the left, colliding with a regular electron at T5 and releasing energy. The whole reaction appears to be the scattering of an electron and a photon, with an intermediate state consisting of a pair of electrons and a virtual positron. In the description of the interaction between elementary particles in quantum field theory, a virtual particle is a temporary elementary particle, used to describe an intermediate stage in the interaction. ...

Feynman made this picture precise with his formalism of Feynman diagrams, in which particle and antiparticle interactions are visualized as a set of paths in space-time. Many would argue that the paths shouldn't be taken too literally: these diagrams just represent terms in a perturbation theory approximation to quantum field theory. However, Feynman himself seems to have thought of them much more literally as representing particles zigzagging back and forth through space-time in a grand path integral encompassing all possibilities. A Feynman diagram is a bookkeeping device for performing calculations in quantum field theory, invented by American physicist Richard Feynman. ... In special relativity and general relativity, time and three-dimensional space are treated together as a single four-dimensional pseudo-Riemannian manifold called spacetime. ... This article is about a formulation of quantum mechanics. ...

Properties of antiparticles

A particle's wave function can be changed to that of its antiparticle by applying the charge conjugation, parity, and time reversal (which, contrary to the name, involves complex conjugation in addition to replacing t with −t) operations. 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. ... In mathematics, the complex conjugate of a complex number is given by changing the sign of the imaginary part. ...

The charge conjugation operator has no effect on the momentum. The parity operation negates the momentum, since -∂/∂x=∂/∂(-x). The time reversal operator also negates the momentum, because the momentum operator is changed from to . Thus the net effect of the CPT operation leaves the momentum unchanged. In physics, momentum is a physical quantity related to the velocity and mass of an object. ...

The energy is unchanged by the parity and charge conjugation operators. The time reversal operator also leaves the energy unchanged as shown. Complex conjugation negates the energy (see above argument for momentum). Replacing t with −t also negates the energy (see above argument for momentum under parity transformations). Thus the net effect of time reversal leaves momentum unchanged, as was to be shown. Since energy is invariant under the C, P, and T operators, it is invariant under CPT transformations as well.

Because the Hamiltonian (which determines the time evolution of the system) commutes with CPT, (CPT)^-1H(CPT)=H, that is, all of known physics possesses CPT symmetry. Since the momentum has the same magnitude after the CPT operation, this implies that the mass does as well, so a particle and its antiparticle must have the same mass, as will be shown. In physics, Hamiltonian has distinct but closely related meanings. ... For an electrical switch that periodically reverses the current see commutator (electric) In mathematics the commutator of two elements g and h of a group G is the element g −1 h −1 gh, often denoted by [ g, h ]. It is equal to the groups identity if and only... CPT-symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity and time simultaneously. ... Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ...

For the particle, , where A is the potential momentum (such as magnetic potential times electric charge). For the antiparticle we choose the negative energy solution for a particle, ie , then apply CPT. The potential (both A and V) is negated by the charge conjugation, giving . The kinetic momentum is negated by the parity, and the potentials are replaced with their mappings under a parity transformation, giving . The time reversal negates the total momentum and the hamiltonian, giving .

Due to the fact that for a spherically symmetric potential, which is required for this argument (without it parity will have a more complicated form which replaces ψ(x) with for some a(x)), potential momentum is an even function of position and potential energy is an odd function of position, which gives it the same form as that of the particle, in terms of the particles potential. Since the momentum is the same, the mass must be the same as well in order for the hamiltonian to be invariant.

Obviously orbital angular momentum is negated, since r X p transforms into (-r) X (p), where X is the cross product and r instead of x is the position (since we are now considering multiple dimensions). Total angular momentum must also be negated as seen from the commutation relations such as [J_x,J_y] = iJ_z which transforms under complex conjugation to [J_x,J_y] = − iJ_z, etc. Spin is thus also negated(note the spin quantum number is the same as it expresses the magnitude alone). In physics, angular momentum intuitively measures how much the linear momentum is directed around a certain point called the origin; the moment of momentum. ...

Charges, such as electric charge, and color charge are negated because the corresponding potentials are negated. Electric current densities (along with other current densities) may similarly be seen to be negated. In quantum chromodynamics (QCD), color or color charge refers to a certain property of the subatomic particles called quarks. ... In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ...

The intrinsic parity is unchanged, since C, P, and T all commute, thus [P,CPT]=0. In physics, in particular quantum mechanics, the intrinsic parity is a phase factor that arises as an eigenvalue of the parity operation (a reflection about the origin). ...

Since the hamiltonian and the energy are CPT invariant, if the original particle was possible, the result is possible as well. Thus there is no reason applying the CPT operation to a particle will not produce another particle (this is the modern theoretical argument for the existence of antiparticles).

In summary here are the properties of antiparticles (only those that distinguish particle species are considered here):

See also: List of particles List of particles in particle physics. ...