Single Spin Asymmetries in Dis

We consider possible mechanisms for single spin asymmetries in inclusive Deep Inelastic Scattering (DIS) processes with unpolarized leptons and transversely polarized nucleons. Tests for the effects of non-zero intrinsic k⊥, for the properties of spin dependent quark fragmentations and for quark helicity conservation are suggested. Single spin asymmetries in large pT inclusive hadronic reactions are forbidden in leading-twist perturbative QCD, reflecting the fact that single spin asymmetries are zero at the partonic level and that collinear parton configurations inside hadrons do not allow single spin dependences. Similarly, one might expect single spin asymmetries to vanish in large angle and high energy exclusive processes. However, experiments tell us in several cases, both in inclusive [1, 2] and exclusive reactions [3], that single spin asymmetries are large and indeed non negligible. The usual arguments to explain this apparent disagreement between pQCD and experiment invoke the moderate pT values of the data – a few GeV, not quite yet in the true perturbative regime – and the importance of higher-twist effects. Several phenomenological models have recently attempted to explain the large single spin asymmetries observed in pp → πX [4]-[10], as twist-3 effects which might be due to intrinsic partonic k⊥ in the fragmentation and/or distribution functions. Single spin effects in exclusive processes are harder to explain, as one cannot rely on the cross-section factorization theorem, as one does in the inclusive case, but has to deal with helicity amplitudes; in particular one needs quite significant single helicity flip partonic amplitudes which, however, are bound to be of O(αsmq/ √ s) in pQCD, unless one resorts again to intrinsic k⊥ effects. We consider here a process in which one has convincing evidence that partons and perturbative QCD work well and successfully describe the unpolarized and leading-twist spin data, namely Deep Inelastic Scattering (DIS). In particular we shall discuss single spin asymmetries in the inclusive, lN → l+jets and lN → hX, reactions looking at possible origins of such asymmetries and devising strategies to isolate and discriminate among them. According to the QCD hard scattering picture and the factorization theorem [11]-[13] the cross-section for the lN → hX reaction is given by