Pair production

Pair production

Overview
Pair production refers to the creation of an elementary particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...

 and its antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric 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...

, usually from a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 (or another neutral boson
Boson
In particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of Satyendra Nath Bose....

). For example an electron and its antiparticle, the positron, may be created. This is allowed, provided there is enough energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 available to create the pair at least the total rest mass energy of the two particles and that the situation allows both energy and momentum to be conserved. Other pairs produced could be a muon and anti-muon or a tau and anti-tau.
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Encyclopedia
Pair production refers to the creation of an elementary particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...

 and its antiparticle
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric 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...

, usually from a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 (or another neutral boson
Boson
In particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of Satyendra Nath Bose....

). For example an electron and its antiparticle, the positron, may be created. This is allowed, provided there is enough energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 available to create the pair at least the total rest mass energy of the two particles and that the situation allows both energy and momentum to be conserved. Other pairs produced could be a muon and anti-muon or a tau and anti-tau. However all other conserved quantum numbers (angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

, electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

, lepton number) of the produced particles must sum to zero thus the created particles shall have opposite values of each. For instance, if one particle has electric charge of +1 the other must have electric charge −1, or if one particle has strangeness +1 then another one must have strangeness −1.

Examples

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In nuclear physics
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...

, this occurs when a high-energy photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 interacts with a nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

. The energy of this photon can be converted into mass through Einstein's equation where is energy, is mass and is the speed of light. The photon must have enough energy to create the mass of an electron plus a positron. The mass of an electron is 9.11 × 10−31 kg, the same as a positron.

If there is more energy in the photon than this bare minimum, the electron and positron will have some kinetic energy – meaning they will be moving. The electron and positron can move in opposite directions (at an angle of 180 degrees) meaning they have a total momentum of zero or they can move at an angle of less than 180 degrees resulting in a net combined momentum. However, if the photon had only just enough energy to create the mass of the electron-positron pair then the electron and positron will be at rest. This could violate the conservation of momentum since the photon has momentum and the two resulting particles have none if they are stationary (since momentum = mass × velocity). This means that the pair production must take place near another photon or the nucleus of an atom since they will be able to absorb the momentum of the original photon. In other words, since the momentum of the initial photon must be absorbed by something, pair production by a single photon cannot occur in empty space; the nucleus (or another particle) is needed to conserve both momentum and energy.

Energy


Photon-nucleus pair production can only occur if the photons have an energy exceeding twice the rest energy of an electron . These interactions were first observed in Patrick Blackett's counter-controlled cloud chamber
Cloud chamber
The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When a charged particle interacts with the mixture, it ionizes it...

, leading to the 1948 Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

. The same conservation laws apply for the generation of other higher energy particles such as the muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...

 and tau.

In semiclassical general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

, pair production is also invoked to explain the Hawking radiation
Hawking radiation
Hawking radiation is a thermal radiation with a black body spectrum predicted to be emitted by black holes due to quantum effects. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974, and sometimes also after the physicist Jacob Bekenstein...

 effect. According to quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

, particle pairs are constantly appearing and disappearing as a quantum foam
Quantum foam
Quantum foam, also referred to as spacetime foam, is a concept in quantum mechanics, devised by John Wheeler in 1955. The foam is supposed to be the foundations of the fabric of the universe. Additionally, it can be used as a qualitative description of subatomic spacetime turbulence at extremely...

. In a region of strong gravitational tidal forces, the two particles in a pair may sometimes be wrenched apart before they have a chance to mutually annihilate
Annihilation
Annihilation is defined as "total destruction" or "complete obliteration" of an object; having its root in the Latin nihil . A literal translation is "to make into nothing"....

. When this happens in the region around a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

, one particle may escape as its antiparticle partner is captured by the black hole.

Pair production is also the hypothesized mechanism behind the pair instability supernova type of stellar explosions, where pair production suddenly lowers pressure inside a supergiant star, leading to a partial implosion, and then explosive thermonuclear burning. Supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 SN 2006gy
SN 2006gy
SN 2006gy was an extremely energetic supernova, sometimes referred to as a hypernova or quark-nova, that was discovered on September 18, 2006. It was first observed by Robert Quimby and P. Mondol, and then studied by several teams of astronomers using facilities that included the Chandra, Lick, and...

 is hypothesized to have been a pair production type supernova.

In 2008 the Titan laser aimed at a 1-millimeter-thick gold target was used to generate positron–electron pairs in large numbers.

See also

  • Electron–positron annihilation
  • Meitner–Hupfeld effect
    Meitner–Hupfeld effect
    The Meitner–Hupfeld effect is an anomalously large scattering of Gamma rays by heavy elements. Later on, the Meitner–Hupfeld effect was explained by a broad theory from which evolved the Standard Model, a theory for explaining the structure of the atomic nucleus...

  • Pair instability supernova
  • Two-photon physics
    Two-photon physics
    Two-photon physics, also called gamma-gamma physics, is a branch of particle physics for the interactions between two photons. If the energy in the center of mass system of the two photons is large enough, matter can be created.-Experiments:...


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