Within the QCD-improved parton model and assuming the factorization theorem
to hold in the helicity basis and for higher twist contributions, we show how
non zero single spin asymmetries in hadron-hadron high energy and moderately
large pTβ inclusive processes can be obtained, 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 on the single
spin asymmetry observed in inclusive pion production in pp collisions.Comment: 11 pages + 1 figure, plain LaTeX, figure appended as uuencoded,
compressed postscript file. Revised version, to be published in Phys. Lett.
B; extensive changes in Sections 1 and 2, new References added, results and
conclusions essentially unchange