Breit–Wheeler process
The Breit–Wheeler process or Breit–Wheeler pair production is a proposed physical process in which a positron–electron pair is created from the collision of two photons. It is the simplest mechanism by which pure light can be potentially transformed into matter. The process can take the form γ γ′ → e+ e− where γ and γ′ are two light quanta (for example, gamma photons).
The multiphoton Breit–Wheeler process, also referred to as nonlinear Breit–Wheeler or strong field Breit–Wheeler in the literature, occurs when a high-energy probe photon decays into pairs propagating through a strong electromagnetic field (for example, a laser pulse). In contrast with the linear process, this can take the form of γ + n ω → e+ e−, where n represents the number of photons, and ω represents the coherent laser field.
The inverse process, e+ e− → γ γ′, in which an electron and a positron collide and annihilate to generate a pair of gamma photons, is known as electron–positron annihilation or the Dirac process for the name of the physicist who first described it theoretically and anticipated the Breit–Wheeler process.
This mechanism is theoretically characterized by a very weak probability, so producing a significant number of pairs requires two extremely bright, collimated sources of photons having photon energy close to or above the electron and positron rest mass energy. Manufacturing such a source, for instance, a gamma-ray laser, is still a technological challenge. In many experimental configurations, pure Breit–Wheeler is dominated by other more efficient pair creation processes that screen pairs produced via this mechanism. The Dirac process (pair annihilation) has, on the other hand, been extensively verified. This is also the case for the multi-photon Breit–Wheeler, which was observed at the Stanford Linear Accelerator Center in 1997 by colliding high-energy electrons with a counter-propagating terawatt laser pulse.
Although this mechanism is still one of the most difficult to be observed experimentally on Earth, it is of considerable importance for the absorption of high-energy photons travelling cosmic distances.
The photon–photon and the multiphoton Breit–Wheeler processes are described theoretically by the theory of quantum electrodynamics.