Two-photon physics, also called gamma–gamma physics, is a branch of particle physics that describes the interactions between two photons. If the energy at the center of mass system of the two photons is large enough, matter can be created.
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. If the transverse momentum transfer is large, one or both electrons can be deflected enough to be detected; this is called tagging. The other particles that are created in the interaction are tracked by large detectors to reconstruct the physics of the interaction.
From quantum electrodynamics it can be found that photons cannot couple directly to each other, since they carry no charge, but half wavelength is a positive charge and the next half wavelength is a negative charge. A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion–antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon structure, or what is "inside" the photon.
We distinguish three interaction processes:
- Direct or pointlike: The photon couples directly to a quark inside the target photon. If a lepton–antilepton pair is created, this process involves only quantum electrodynamics (QED), but if a quark–antiquark pair is created, it involves both QED and perturbative quantum chromodynamics (QCD).  The intrinsic quark content of the photon is described by the photon structure function, experimentally analyzed in deep-inelastic electron–photon scattering. 
- Single resolved: The quark pair of the target photon form a vector meson. The probing photon couples to a constituent of this meson.
- Double resolved: Both target and probe photon have formed a vector meson. This results in an interaction between two hadrons.
For the latter two cases, the scale of the interaction is such as the strong coupling constant is large. This is called Vector Meson Dominance (VMD) and has to be modelled in non-perturbative QCD.
- Channelling radiation has been considered as a method to generate polarized high energy photon beams for gamma–gamma colliders.
- Matter creation
- Pair production
- Neutron gamma gamma
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