The Qcd Coupling and Parton Distributions at High Precision

The physics of the strong interactions always has been tightly connected to the study of nucleons at shorter and shorter distances. The measurement of the anomalous magnetic moments of the proton and neutron in 1933 and 1939 made clear that nucleons are no elementary particles. During the 1950ies the Hofstadter experiments revealed the charge distributions inside nucleons at scales Q ≃ 0.5 ·M N . Yet it was unknown how these distributions came about. In 1964 Murray Gell– Mann proposed the quark model, to catalog the plethora of observed baryons and mesons. Independently G. Zweig suggested aces as the building blocks of hadrons. A direct connection to the lepton-nucleon scattering data was not made at that time. Back in 1954 C.N. Yang and R Mills proposed novel bosonic field theories based on gauge invariance with respect to non-abelian groups. This development went unrelated to strong interactions for a long time. With the advent of the Stanford Linear Accelerator in 1968 the nucleon structure could be resolved at much shorter distances by the MIT-SLAC experiments beyond the resonant region W ≥ 2GeV for values Q up to 30 GeV. The remarkable finding by these experiments were that i) the structure function νW2(ν,Q ) which has been expected to depend on both kinematic variables ν and Q independently, turned out to take the same values for fixed values of x = Q/(2MNν) irrespectively of ν and Q 2 at high enough values. This phenomenon is called scaling. ii) The ratio of the longitudinal structure function WL and W2 turned out to be very small. Bjorken 13 had predicted