Matter Creation - Photon Pair Production

Photon Pair Production

Because of momentum conservation laws, the creation of a pair of fermions (matter particles) out of a single photon cannot occur. However, matter creation is allowed by these laws when in the presence of another particle (another boson, or even a fermion) which can share the primary photon's momentum. Thus, matter can be created out of two photons.

The law of conservation of energy 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 by Lev Landau in 1934. It was predicted that the process of e+–e− pair creation (via collisions of photons) dominates in collision of ultra-relativistic 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 (see two-photon physics). 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 K, 1013 K for protons and neutrons, etc. According to the Big Bang theory, in the early universe, photons and fermions (massive particles of matter) would inter-convert freely. As photon gas expanded and cooled, some fermions would be left over (in extremely small amounts ~10−10) because low energy photons could no longer break them apart. Those left-over fermions would have become the matter we see today in the universe around us.