Mesonic contributions to the spin and flavor structure of the nucleon

Since its initial investigation at SLAC in the late 60's, deep inelastic lepton scattering has provided a wonderful tool to explore perturbative QCD and to test many features of the Standard Model. The crucial theoretical tools needed to understand deep-inelastic scattering, namely the operator product expansion and the renormalization group (which are now used throughout particle physics), were developed in parallel with the data taking. Using these techniques one can often eliminate the need to understand the detailed structure of a target in order to make a rigorous test of QCD. Famous examples include the Adler, Gross Llewellyn Smith and Bjorken sum-rules. With perturbative QCD now well tested and established there are three important frontiers in deep-inelastic scattering. Two of these, namely the behavior at very small values of Bjorken x which is currently being explored at HERA and the higher-twist corrections have traditionally been the domain of high energy physics. The third frontier, which lies at the boundary of nuclear and particle physics is the major concern of this paper. This is the wealth of information that deep-inelastic scattering data contains on the non-perturbative structure of hadrons. The authors concentrated on the non-perturbative structure of the free nucleon and particular its meson cloud