Breit–Wheeler process or Breit–Wheeler pair production is a physical process in which a pair of positron–electron is created in 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 of γ γ′ → e+ e− where γ and γ′ are two light quanta .
The multiphoton Breit-Wheeler process, also refers 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 pair annihilation or 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 source of pair to be described, its experimental validation has yet to be performed. Indeed, this mechanism is theoretically characterized by a very weak probability and requires an extremely bright and collimated source of photons of energy close or above the electron and positron rest mass energy (generally gamma photons) to obtain a significant number of pairs. The generation of such sources 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 Centre 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 travelling cosmic distances .
The photon-photon and the multiphoton Breit-Wheeler processes are described theoretically by the quantum electrodynamics theory.
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.
Photon-photon Breit-Wheeler possible experimental configurations
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 speed up electrons to just below 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 gold can, 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 can, 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 .
As of January 2016, Breit Wheeler pair production hasn't yet been achieved in the laboratory .
Multiphoton Breit-Wheeler experients
The multiphoton Breit-Wheeler has however already been observed and studied experimentally. One of the most efficient configuration to maximize the multiphoton Breit-Wheeler pair production consists on colliding head-on a bunch of gamma photon with a counter-propagating (or with a slight collision angle, the co-propagating configuration being the less efficient configuration) ultra-high intensity laser pulse. In order to first create the photons and then have the pair production in an all-in-one setup, the similar configuration can be used by colliding giga-electronvolt (GeV) electrons. Depending on the laser intensity, these electrons will first radiate gamma photons via the so-called nonlinear inverse Compton scattering mechanism when interacting with the laser pulse. Still interacting with the laser, the photons then turn into multiphoton Breit–Wheeler electron-positron pairs.
This method has been considered In 1997 at the Stanford Linear Accelerator Centre. Researchers were able to conduct the multiphoton Breit–Wheeler process using electrons to first create high-energy photons , which then underwent multiple collisions to produce electrons and positrons, all within same chamber. Electrons were accelerated in the linear accelerator to an energy of 46.6 GeV before being sent head-on to a Neodymium (Nd:glass) lineary polarized laser of intensity W/cm2 (maximal electric field amplitude of around V/m), of wavelength 527 nanometers and duration 1.6 picoseconds. In this configurations, it has been estimated that photons of energy up to 29 GeV were generated. This leaded to the yield of 106 ±14 positrons with a broad energy spectrum in the GeV level (peak around 13 GeV).
The latter experiment may be reproduced in the future at SLAC with more powerful present laser technologies. The use of higher laser intensities (1020 W/cm2 is now easily achievable with turnkey short-pulse titane-sapphire laser solutions) would significantly enhance process efficiencies (inverse nonlinear Compton and nonlinear Breit-Wheeler pair creation) leading to higher antimmater production at several orders of magnitude. It could enable to perform high-resolution measurements as well as to characterize additional mass-shift, nonlinear and spin effects .
The extreme intensities expected to be available on the upcoming multi-petawatt laser systems will allow us to contemplate all-optical, laser-electron collision schemes. In an all-optical configuration, the electron beam is generated from the laser interaction with a gas jet in the so-called laser wakefield acceleration regime. The electron bunch is made to interact with a second high-power laser in order to study QED processes. The feasibility of an all-optical multiphoton Breit-Wheeler pair production scheme has been first demonstrated theoretically in . This scheme is therefore restricted to multi-beam short-pulse extreme-intensity laser facilities, as will be the case of the CILEX-Apollon  and ELI systems  (CPA titanium sapphire technology at 0.8 micrometer, duration of 15-30 femtoseconds). The generation of electron beams of few GeV and few nano-Coulomb is possible with a first laser of 1 petawatt combined with the use of tuned and optimized gas-jet density profiles such as two-step profiles. Strong pair generation can be achieved by colliding head-on this electron beam with a second laser of intensity above 1022 W/cm2. In this configuration at this level of intensity, theoretical studies predict that several hundreds of pico-Coulomb of antimatter could be produced . This experimental setup could even be one of the most prolific positron yield factory. This all-optical scenario may be preliminary tested with lower laser intensities of the order of 1021 W/cm2.
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Direct production of electron-positron pairs in two-photon collisions, the Breit-Wheeler process, is one of the basic processes in the universe. However, it has never been directly observed in the laboratory because of the absence of the intense γ-ray sources
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