Parton intrinsic motion: Suppression of the Collins mechanism for transverse single spin asymmetries in p(up arrow)p ->pi X

We consider a general formalism to compute inclusive polarized and unpolarized cross sections within pQCD and the factorization scheme, taking into account parton intrinsic motion in distribution and fragmentation functions, as well as in the elementary dynamics. Surprisingly, the intrinsic partonic motion, with all the correct azimuthal angular dependences, produces a strong suppression of the transverse single spin asymmetry arising from the Collins mechanism. As a consequence, and in contradiction with earlier claims, the Collins mechanism is unable to explain the large asymmetries found in p(up arrow)p-->piX at moderate to large Feynman x(F). The Sivers effect is not suppressed.