gammagamma

Gamma-gamma, often written as γγ, is a term used in physics to denote processes in which two photons interact or collide. It appears in both high-energy particle physics and astrophysics and is a useful tool for testing quantum electrodynamics and probing for new phenomena.

In particle physics, γγ collisions can occur when two real photons interact. They can be produced in ultraperipheral collisions of charged particles, such as heavy ions, or via photon emission from energetic charged particles in colliders. The resulting final states include fermion-antifermion pairs (e+e−, μ+μ−, hadrons), W+W−, and, via loop processes, Higgs bosons. A particularly rare but informative process is light-by-light scattering, γγ → γγ, which proceeds only through loop diagrams and has been observed in heavy-ion experiments at the LHC. Measurements of γγ processes test quantum electrodynamics at high energies and constrain new charged particles that could appear in the loops. Theoretical treatments often employ the equivalent photon approximation or dedicated loop calculations.

In astrophysics, gamma-gamma interactions describe the attenuation of very high-energy gamma rays as they travel through cosmic photon fields. A high-energy γ-ray can interact with low-energy background photons (for example, infrared or microwave photons) to produce an electron-positron pair if the center-of-mass energy exceeds 2m_e c^2. This γγ absorption shapes the observed spectra of distant blazars and gamma-ray bursts and provides information about the extragalactic background light and intergalactic magnetic fields.

Name origins: γγ is shorthand for a two-photon process and is used in various experimental and theoretical contexts. Ongoing work in γγ physics helps test standard-model predictions and explore hints of new physics, including light particles that couple to photons.