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| Low energy phenomena | Photoelectric effect | |
| Mid-energy phenomena | Compton scattering | |
| High energy phenomena | Pair production | |
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon (or another neutral boson). 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 (though not necessarily on shell). All other conserved quantum numbers (angular momentum, electric charge) of the produced particles must sum to zero — thus the created particles shall have opposite values of each (for instance, if one particle has strangeness +1 then another one must have strangeness −1).
In nuclear physics, this occurs when a high-energy photon interacts with an atomic nucleus, allowing it to produce an electron and a positron without violating conservation of momentum. Since the momentum of the initial photon must be absorbed by something, pair production cannot occur in empty space out of a single photon; the nucleus is needed to conserve both momentum and energy.
Pair production can only occur if the photon has an energy exceeding the twice the rest mass(me) of an electron (1.022 MeV); the same applies for the generation of other higher energy leptons such as the muon and tau. These interactions were first observed in Patrick Blackett's counter-controlled bubble chamber, leading to the 1948 Nobel Prize in Physics.
In semiclassical general relativity, pair production is also invoked to explain the Hawking radiation effect. According to quantum mechanics, at short scales short-lived particle-pairs are constantly appearing and disappearing (see 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, with its antiparticle being captured by the 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 SN 2006gy is hypothesized to have been a pair production type supernova.
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