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 ofteneliminate 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 Smithand Bjorken sum-rules. With perturbative QCD now weil tested and established there are three important frontiers in deep-inelastic scattering. Two of these, namely the behaviour at very small values of Bjorken x (defined in Sect. 2 below) which is currently being explored at HERA and the higher-twist corrections (again see Sect. 2), have traqitionally been the domain of high energy physicists. The third frontier, which lies at the boundary of nuclear and particle physics, is our major concern. This is the wealth of information that deep-inelastic scattering data contains on the nonperturbative structure of hadrons (Tho 84a). The idea of using deep-inelastic scattering to probe non-perturbative structure first reached prominence in connection with the discovery of the nuclear EMC effect (Aub+ 83). The fact that the structure function of a nucleus was not simply A times the structure function of a free nucleon, even in the valence region, suggested that the quark structure of the nucleon itself may be changed inside nuclear matter. Then quark degrees of freedom would playavital role in understanding nuclear structure. This is one issue which still needs further study and we refer the interested reader to some recent reviews (GST 95, Arn 94). We shall focus on the non-perturbative structure of the free nucleon and particular its meson cloud. Although the role of the pion cloud of the nucleon in deep inelastic scattering beg an with Drell, Yan and Levy in 1970 (DLY 70) and Sullivan (Sul 72) in 1972, modern interest in it also sprang from the nuclear EMC effect. Realizing that virtual pion exchange plays a crucial role in nuclear binding, Llewellyn Smith (Lle 83) and Ericson and Thomas (ET 84) explored the consequences for [...