Egamma

Egamma is a term used in particle physics to denote the interaction between an electron and a photon within quantum electrodynamics (QED). In Feynman diagrams, this interaction is represented by the electron–photon vertex, the fundamental source of electromagnetic coupling for charged leptons.

In the standard formulation, the vertex factor is -i e γμ, where e is the elementary electric charge and γμ are Dirac gamma matrices. The strength of the electromagnetic interaction is set by the fine-structure constant α ≈ 1/137, with e^2/(4π) = α. The Egamma interaction is the U(1) gauge interaction and is renormalizable, forming the basis of QED.

Egamma processes include Compton scattering (electron–photon scattering), electron bremsstrahlung (an electron emits a photon in the electromagnetic field of a nucleus), and photon-induced electron–positron pair production. The concept also underpins higher-order radiative corrections to electron scattering and other electromagnetic processes observed in detectors.

In experimental contexts, photons and electrons are reconstructed as separate objects, but their interaction is central to calibration and event interpretation, particularly in electromagnetic calorimetry and tracking systems. The term Egamma may appear in literature as shorthand for the electron–photon vertex or for studies focusing on electromagnetic processes, and should not be confused with similarly named organizations or datasets.