Matter creation

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Matter creation is the process inverse to particle annihilation. It is the conversion of massless particles into one or more massive particles. This process is the time reversal of annihilation. Since all known massless particles are bosons and the most familiar massive particles are fermions, usually what is considered is the process which converts two bosons (e.g. photons) into two fermions (e.g., an electron-positron pair).

[edit] Photon pair production

Because of momentum conservation laws, the creation of a pair of fermions out of a single photon cannot occur. However, matter creation is allowed by momentum conservation law when in the presence of another particle (it may be another photon or other boson, or even a fermion) which can share photon's momentum. Thus, matter can be created out of two photons, for example (this is the process inverse to annihilation).

The law of energy conservation sets a minimum photon energy required for creation of a pair of fermions: this threshold energy must be greater than the total rest energy of the fermions created. To create an electron-positron pair the total energy of the photons must be at least 2m_ec² = 2×0.511 MeV = 1.022 MeV (m_e is the mass of one electron and c is the speed of light in vacuum), an energy value that corresponds to soft gamma ray photons. The creation of a much more massive pair, like a proton and antiproton, requires photons with energy of more than 1.88 GeV (hard gamma ray photons).

First calculations of rate of e⁺/e^- pair production in photon-photon collision was done in 1934.^[1] It was predicted that the process of e⁺/e^- pair creation (via collisions of photons) dominates in collision of ultrarelativistic charged particles — because those photons are radiated in narrow cones along the direction of motion of original particle greatly increasing photon flux.

In high-energy particle colliders, matter creation events have yielded a wide variety of exotic heavy particles precipitating out of colliding photon jets.^{[citation needed]} Currently, two-photon physics studies creation of various fermion pairs both theoretically and experimentally (using particle accelerators, air showers, radioactive isotopes, etc).

As shown above, to produce ordinary baryonic matter out of a photon gas, this gas must not only have a very high photon density, but also be very hot – the energy (temperature) of photons must obviously exceed the rest mass energy of the given matter particle pair. The threshold temperature for production of electrons is about 10¹⁰ kelvins, 10¹³ K for protons and neutrons, etc. In the early universe (see Big Bang) photons and massive particles freely interconverted.