ar X iv : h ep - p h / 02 11 19 8 v 1 1 3 N ov 2 00 2 Sivers effect and transverse single spin asymmetries in Drell - Yan processes ∗

Sivers asymmetry, adopted to explain transverse single spin asymmetries (SSA) observed in inclusive pion production, p↑ p → πX and p̄↑ p → πX, is used here to compute SSA in Drell-Yan processes; in this case, by considering the differential cross section in the lepton-pair invariant mass, rapidity and transverse momentum, other mechanisms which may originate SSA cannot contribute. Estimates for RHIC experiments are given. Single spin asymmetries in high energy inclusive processes are a unique testing ground for QCD; they cannot originate from the simple spin pQCD dynamics – dominated by helicity conservation – but need some non perturbative chiral-symmetry breaking in the large distance physics. Among the best known transverse single spin asymmetries (SSA) let us mention: i) the large polarization of Λ’s and other hyperons produced in pN → Λ↑X; ii) the large asymmetry AN = dσ−dσ dσ↑+dσ↓ observed in p↑ p → π X and p̄↑ p → π X processes; iii) the similar azimuthal asymmetry observed in l p↑ → l π X. Several models [1] to explain the data within QCD dynamics can be found in the literature; here we focus on a phenomenological approach based on the generalization of the factorization theorem with the inclusion of parton intrinsic motion k⊥. The cross section for a generic process AB → C X then reads: