arXiv:hep-ph/0408356v1 31 Aug 2004 IC/HEP/04-5 Parton intrinsic motion: suppression of the Collins mechanism for transverse single spin asymmetries in p ↑ p → πX M. Anselmino 1 , M. Boglione 1 , U. D’Alesio 2 , E. Leader 3 , F. Murgia 2 1 Dipartimento di Fisica Teorica, Universit` a di Torino and INFN, Sezione di Torino, Via P. Giuria 1, I-10125 Torino, Italy 2 INFN, Sezione di Cagliari and Dipartimento di Fisica, Universit` a di Cagliari, C.P. 170, I-09042 Monserrato (CA), Italy 3 Imperial College, Prince Consort Road, London, SW7 2BW, England Abstract We consider a general formalism to compute inclusive polarised and un- polarised cross sections within pQCD and the factorisation scheme, taking into account parton intrinsic motion in distribution and fragmentation func- tions, as well as in the elementary dynamics. Surprisingly, the intrinsic par- tonic 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 ear- lier claims, the Collins mechanism is unable to explain the large asymmetries found in p ↑ p → πX at moderate to large Feynman x F . The Sivers effect is not suppressed.