10 22 8 v 1 2 O ct 1 99 8 SINGLE SPIN ASYMMETRIES IN p ↑ p , p̄ ↑ p INCLUSIVE PROCESSES

Single spin asymmetries in p ↑ (¯ p ↑) p → π X have been measured to be large. They can be understood within perturbative QCD by introducing new k ⊥ and spin dependences in fragmentation and/or distribution functions. We discuss first how these effects in distribution functions alone can account for existing data and predict single spin asymmetries in other processes; then we see how similar effects in fragmentation functions alone can fit the existing data; finally we devise strategies to discriminate between the two contributions. We have recently presented a formalism to evaluate single spin asymmetries (SSA) in inclusive particle production at high energy and moderately large p T 1,2. This formalism is based on the parton model approach and on factorization theorems, generalized to polarized reactions and to the inclusion of transverse momentum effects in the partonic distributions of the initial, polarized proton (the so-called " Sivers " effect). This requires the introduction of a new, soft, nonperturbative distribution function (non-diagonal in the helic-ity basis), ∆ N ˜ f. It has been shown 1 how recent experimental results on SSA for pions in the fragmentation region 3 can be well reproduced by opportunely parametrizing ∆ N ˜ f. This fit to ∆ N ˜ f has been in turn used to give predictions for SSA in several other interesting processes 2. In this contribution we present a complete generalization of our formalism, with the inclusion of partonic transverse momentum effects both in the partonic distributions of the initial polarized hadron (Sivers effect) and in the fragmentation process (the so-called " Collins " effect). Due to space limitations, we only give the final result of our formalism, in the case of p ↑ p → π X processes. A more detailed discussion will be presented elsewhere 4 (see also Ref.s 1,2). At leading order in transverse momentum effects, we have: