Single-spin asymmetries (SSA) in hadronic reactions have been among the most difficult phenomena to understand from first principles ini QCD. Large SSAs have been observed in hadronic reactions for decades. In general, such single-spin asymmetries require a correlation of a particle spin direction and the orientation of the production or scattering plane. In hadronic processes, such correlations can provide a window to the physics of initial and final state interactions at the parton level. Recently, significant SSAs were reported in semi-inclusive DIS (SIDIS) by the HERMES collaboration at HERA for a longitudinally polarized target, the SMC collaboration at CERN for a transversely polarized target, and the CLAS collaboration at the Thomas Jefferson National Accelerator Facility (JLab) with a polarized beam.
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