Two-photon physics

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.

Frequently, photon-photon interactions will be studied via ultraperipheral collisions (UPCs)[2] of heavy ions, such as gold or lead.

The strong interaction between the quarks composing the nuclei is thus greatly suppressed, making the weaker electromagnetic

Light-by-light scattering, as predicted in,[3] can be studied using the strong electromagnetic fields of the hadrons collided at the LHC,[4][5] it has first been seen in 2016 by the ATLAS collaboration[6][7] and was then confirmed by the CMS collaboration.,[8] including at high two-photon energies.

[9] The best previous constraint on the elastic photon–photon scattering cross section was set by PVLAS, which reported an upper limit far above the level predicted by the Standard Model.

[10] Observation of a cross section larger than that predicted by the Standard Model could signify new physics such as axions, the search of which is the primary goal of PVLAS and several similar experiments.