Single Spin Asymmetries in High Energy Reactions and Nonperturbative QCD Effects

We discuss some experimental and theoretical results on single spin asymmetries (SSA) in high energy lepton-hadron and hadron-hadron reactions. In particular, recent results on meson SSA obtained by HERMES are considered in detail. We also discuss the SSA results obtained recently by COMPASS, as well as those from BRAHMS, PHENIX and STAR. Special attention is paid to a possible nonperturbative QCD mechanism that might be responsible for the observed meson SSA. This mechanism originates from the spin-flip quark-gluon chromomagnetic interaction induced by the complex topological structure of the QCD vacuum. We argue that in semi-inclusive deep-inelastic scattering a large SSA is expected not only for mesons but also for baryons due to strong nonperturbative final state interactions between $ud$-diquark and $u$-quark in the fragmenting proton.