Pair production

Feynman diagram for pair production. A photon decays into an electron-positron pair.

Pair production refers to the creation of an elementary particle and its antiparticle, usually when a photon (or another neutral boson) interacts with a nucleus. For example an electron and its antiparticle, the positron, may be created. This is allowed, provided there is enough energy 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, electric charge, lepton number) of the produced particles must sum to zero – thus the created particles shall have opposite values of each other. For instance, if one particle has electric charge of +1 the other must have electric charge of −1, or if one particle has strangeness of +1 then another one must have strangeness of −1. The probability of pair production in photon-matter interactions increases with increasing photon energy and also increases with atomic number approximately as Z².

Examples

γ
+
γ
 → 
e⁻
 + 
e⁺

In nuclear physics, this occurs when a high-energy photon interacts with a nucleus. The energy of this photon can be converted into mass through Einstein's equation E = m c² where E is energy, m is mass and c is the speed of light. The photon must have enough energy to create the mass of an electron plus a positron. The rest mass of an electron is 9.11 × 10⁻³¹ kg(0.511 MeV), the same as a positron. Without a nucleus to absorb momentum, a photon decaying into electron-positron pair (or other pairs for that matter) can never conserve energy and momentum simultaneously.

Energy

Photon-nucleus pair production can only occur if the photons have an energy exceeding twice the rest energy (m_e c²) of an electron (1.022 MeV). These interactions were first observed in Patrick Blackett's counter-controlled cloud chamber, leading to the 1948 Nobel Prize in Physics. The same conservation laws apply for the generation of other higher energy particles such as the muon and tau.

In semiclassical general relativity, pair production is also invoked to explain the Hawking radiation effect. According to quantum mechanics, particle pairs are constantly appearing and disappearing as a quantum foam. 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. When this happens in the region around a black hole, one particle may escape while its antiparticle partner is captured by the black hole.

Pair production is also the hypothesized mechanism behind the pair instability supernova type of stellar explosion, where pair production suddenly lowers the pressure inside a supergiant star, leading to a partial implosion, and then explosive thermonuclear burning. Supernova SN 2006gy 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
• Pair-instability supernova
• Two-photon physics
• Dirac equation

External links

• Theory of photon-impact bound-free pair production