Breit–Wheeler process

The Breit–Wheeler process or Breit–Wheeler pair production is a 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. The Breit–Wheeler process or Breit–Wheeler pair production is a 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. The multiphoton Breit–Wheeler process, also referred to as nonlinear Breit–Wheeler or strong field Breit–Wheeler in the literature, is the extension of the pure photon–photon Breit–Wheeler process when a high-energy probe photon decays into pairs propagating through an electromagnetic field (for example, a laser pulse). In contrast with the previous process, this one can take the form of γ + n ω → e+ e−, where ω represents the coherent photons of the 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. Although the pure photon–photon Breit–Wheeler process was one of the first sources of pairs to be described, its experimental validation has yet to be accomplished. 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 or above the electron and positron rest mass energy. Manufacturing such a source, 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 nonetheless been by far verified experimentally. It is also the case of the multiphoton Breit–Wheeler at the Stanford Linear Accelerator Center in 1997 by colliding a 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 traveling cosmic distances. The photon–photon and the multiphoton Breit–Wheeler processes are described theoretically by the theory of quantum electrodynamics. The photon–photon Breit–Wheeler process was described theoretically by Gregory Breit and John A. Wheeler in 1934 in Physical Review. It followed previous theoretical work of Paul Dirac on antimatter and pair annihilation. In 1928, Paul Dirac's work proposed that electrons could have positive and negative energy states following the framework of relativistic quantum theory but did not explicitly predict the existence of a new particle. Although the process is one of the manifestations of the mass–energy equivalence, as of 2017, the pure Breit–Wheeler has never been observed in practice because of the difficulty in preparing colliding gamma ray beams and the very weak probability of this mechanism. Recently, different teams have proposed novel theoretical studies on possible experimental configurations to finally observe it on Earth. In 2014, physicists at Imperial College London proposed a relatively simple way to physically demonstrate the Breit–Wheeler process. The collider experiment that the physicists proposed involves two key steps. First, they would use an extremely powerful high-intensity laser to accelerate electrons to nearly the speed of light. They would then fire these electrons into a slab of gold to create a beam of photons a billion times more energetic than those of visible light. The next stage of the experiment involves a tiny gold can called a hohlraum (German for 'empty room'). Scientists would fire a high-energy laser at the inner surface of this hohlraum to create a thermal radiation field. They would then direct the photon beam from the first stage of the experiment through the centre of the hohlraum, causing the photons from the two sources to collide and form electrons and positrons. It would then be possible to detect the formation of the electrons and positrons when they exited the can. Monte Carlo simulations suggest that this technique is capable of producing of the order of 105 Breit–Wheeler pairs in a single shot.