Two-photon physics

Two-photon physics, also called gamma–gamma physics, is a branch of particle physics that describes the interactions between two photons. Normally, beams of light pass through each other unperturbed. Inside an optical material, and if the intensity of the beams is high enough, the beams may affect each other through a variety of non-linear effects. In pure vacuum, some weak scattering of light by light exists as well. Also, above some threshold of this center-of-mass energy of the system of the two photons, matter can be created. Photon–photon interactions limit the spectrum of observed gamma-ray photons at moderate cosmological distances to a photon energy below around 20 GeV, that is, to a wavelength of greater than approximately 6.2e-11m. This limit reaches up to around 20 TeV at merely intergalactic distances. An analogy would be light traveling through a fog: At near distances a light source is more clearly visible than can at long distances due to the scattering of light by fog particles. Similarly, the further a gamma-ray travels through the universe, the more likely it is to be scattered by an interaction with a low energy photon from the extragalactic background light. At those energies and distances, very high energy gamma-ray photons have a significant probability of a photon-photon interaction with a low energy background photon from the extragalactic background light resulting in either the creation of particle-antiparticle pairs via direct pair production or (less often) by photon-photon scattering events that lower the incident photon energies. This renders the universe effectively opaque to very high energy photons at intergalactic to cosmological distances. Two-photon physics can be studied with high-energy particle accelerators, where the accelerated particles are not the photons themselves but charged particles that will radiate photons. The most significant studies so far were performed at the Large Electron–Positron Collider (LEP) at CERN.