Single Spin Asymmetries in Inclusive Hadron Production

The general formalism to describe single spin asymmetries in hadron-hadron high energy and large $p_T$ inclusive production within the QCD-improved parton model and assuming the factorization theorem to hold for higher twist contributions is discussed. Non zero values of the single spin asymmetries originate from and reveal non perturbative universal properties of quarks: the quark distribution and fragmentation analysing powers, which need not be zero, 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 asymmetries observed in inclusive pion production in $pp$ collisions.