Single spin asymmetry for p^(uparrow)p to π X in perturbative QCD

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 $p_T$ 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 $p\,p$ collisions.