Photon structure function fit with the xFitter framework

The theory of the strong interaction, Quantum Chromodynamics (QCD), is one of the main constituents of the Standard Model of elementary particle physics. Particularly interesting is the strong coupling constant αs, the only fundamental QCD parameter, which can be only determined by experimental in- vestigations. Therefore, the precise knowledge of αs has been subject of many investigations, using several different methods, like for example the Z- and τ- decays at LEP. The current world average value of αs(mZ) = 0.1172 ± 0.0020, where mZ = 91.1876 GeV denotes the mass of the Z-boson [1]. In terms of QCD, the photon offers particular interesting investigation possibilities, which can be shown by Deep Inelastic Scattering (DIS) of electrons and photons. Despite the assumption, that the photon is a point-like particle, hadron production can be observed. This is leading to the introduction of structure functions, as for DIS electron-nucleon scattering and can be explained by quantum fluctuations due to Heisenberg’s uncertainty principle. It allows the photon for a short time period Δt, the violation of the energy conservation by the amount ΔE and ending up in the production of a fermion pair γ −→ f f¯ −→ γ. In the case, one of those fermions (target) interacts via a gauge boson, the parton content of the photon gets resolved. This is displayed in fig. 1 on the left-hand side and schematically compared to electron-nucleon scattering e−N → e−X on the right-hand side.