Measurements of the QED structure of the photon

Abstract.

The structure of both quasi-real and highly virtual photons is investigated using the reaction ${\rm e^+e^-}\rightarrow{\rm e^+e^-}\mu^+\mu^-$, proceeding via the exchange of two photons. The results are based on the complete OPAL dataset taken at ${\rm e^+e^-}$ centre-of-mass energies close to the mass of the Z boson. The QED structure function $F^\gamma _2$ and the differential cross-section ${\rm d}\sigma/{\rm d}x$ for quasi-real photons are obtained as functions of the fractional momentum x from the muon momentum which is carried by the struck muon in the quasi-real photon for values of $Q^2$ ranging from 1.5 to 400 GeV$^2$. The differential cross-section ${\rm d}\sigma/{\rm d}x$ for highly virtual photons is measured for $1.5< Q^2 < 30$ GeV$^2$ and $1.5< P^2 < 20$ GeV$^2$, where $Q^2$ and $P^2$ are the negative values of the four-momentum squared of the two photons such that $Q^2>P^2$. Based on azimuthal correlations the QED structure functions $F^\gamma _{\rm A}$ and $F^\gamma _{\rm B}$ for quasi-real photons are determined for an average $Q^2$ of 5.4 GeV$^2$.