Single Spin Asymmetry in Inclusive Pion Production
  It is shown how the single spin asymmetry observed in inclusive pion production is related, in the helicity basis, to the imaginary part of the product of two different distribution amplitudes, rather than to the usual quark and gluon distribution functions; there is then no reason why it should be zero even in massless perturbative QCD, provided the quark intrinsic motion is taken into account. A simple model is constructed which reproduces the main features of the data. 
Mauro Anselmino, M.E. Boglione and F. Murgia