The 1/N c -expansion of QCD suggests large flavor asymmetries of the polarized antiquark distributions in the nucleon. This is confirmed by model calculations in the large-N c limit (chiral quark-soliton model), which give sizable results for ∆ū(x)− ∆d(x) and ∆ū(x)+ ∆d(x)− 2∆s(x). We compute the contributions of these flavor asymmetries to the spin asymmetries in hadron production in semi-inclusive deep-inelastic scattering. We show that the large flavor asymmetries predicted by the chiral quark-soliton model are consistent with the recent HERMES data for spin asymmetries in charged hadron production.

We show that the instanton model of the QCD vacuum indicates the parametric smallness of the twist-3 contributions to the polarized structure function h L . This smallness is related to the diluteness of the QCD instanton vacuum.

We investigate the role of the flavor asymmetry of the nucleon's polarized antiquark distributions in Drell-Yan lepton pair production in polarized nucleon-nucleon collisions at HERA (fixed-target) and RHIC energies. It is shown that the large polarized antiquark flavor asymmetry predicted by model calculations in the large-N c limit (chiral quark-soliton model) has a dramatic effect on the double spin asymmetries in high mass lepton pair production, as well as on the single spin asymmetries in lepton pair production through W ± -bosons at M 2 = M 2 W .

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We compute in the instanton vacuum the nucleon matrix elements of the twist-4 QCD operators describing power corrections to the second moments of the unpolarized structure functions, F L and F 2 . Our approach takes into account the leading contribution in the packing fraction of the instanton medium,ρ/R ≪ 1. Parametrically leading are the matrix elements of a twist-4 quark-gluon operator, which are of the order of the inverse instanton size, 1/ρ 2 = (600 MeV) 2 . The matrix elements of the four-fermion (diquark) operators are suppressed by a factor (ρ/R) 4 and numerically small. These results are in agreement with the pattern of phenomenological 1/Q 2 -corrections to R = σ L /σ T and F 2 found in QCD fits to the data. In particular, the rise of R at low Q 2 can be obtained from instanton-type vacuum fluctuations at a low scale.