Phenomenology of single spin asymmetries in p ↑ p → πX

A general formalism, based on factorization theorem and pQCD hard interactions, is developed for the description of single spin effects in hadron-hadron inclusive processes. The transverse momentum, k⊥, of the quarks inside the hadrons and of the hadrons relatively to the fragmenting quark, is taken into account in distribution and fragmentation functions, and leads to possible non zero single spin asymmetries. The role of k⊥ and spin dependent quark fragmentations – the so-called Collins effect – is investigated in details in pp → πX processes: it is shown how the experimental data can be well described, obtaining an explicit analytical expression for the spin asymmetry of a polarized fragmenting quark. Predictions for other processes, possible further applications and experimental tests are discussed.