QED processes in two photon reactions

Abstract

I review experimental results on the reactions e⁺e⁻ → e⁺e⁻e⁺e⁻ and e⁺e⁻ → e⁺e⁻ μ⁺μ⁻ from PETRA and PEP. Recent high statistics measurements are compared to QED predictions in the form of diagrammatic leading order (α⁴) calculations and to the equivalent photon approximation. The leading order calculation describes the data well over the full kinematic range covered by experiments (0.1 ≲ Q² ≲ 100 GeV²/c²).The “two photon interaction”graph is found to saturate the observed cross section. Only for small masses of the produced leptonic system, first indications of a bremsstrahlung type background are observed at the percent level. The leptonic structure function F₂ ^γ(x, Q²) is measured and also agrees with the behavior expected from QED. photon approximation. Under most conditions, the “two-photon interaction” graph alone adequately accounts for the data. Only at x = 1, a “bremsstrahlung” background at the percent level is observed. The structure function approach gives an appropriate description of the data on eγ → eμμ. The measured “muonic” structure function <F ^γ₂ (x, Q²)> agrees with the behavior expected from QED.

Invited talk given at the 5th International Workshop on Photon Photon Collisions, Aachen, April 1983.