Evidence for a Single-Spin Azimuthal Asymmetry in Semi-inclusive Pion Electroproduction

Single-spin asymmetries for semi-inclusive pion production in deep-inelastic scattering have been measured for the first time. A significant target-spin asymmetry of the distribution in the azimuthal angle $\phi$ of the pion relative to the lepton scattering plane was formed for ${\pi }^{+}$ electroproduction on a longitudinally polarized hydrogen target. The corresponding analyzing power in the $\sin \phi$ moment of the cross section is $0.022\pm 0.005\pm 0.003$. This result can be interpreted as the effect of terms in the cross section involving chiral-odd spin distribution functions in combination with a chiral-odd fragmentation function that is sensitive to the transverse polarization of the fragmenting quark.