Phenomenology of single spin asymmetries inp↑p→πX

A phenomenological description of single transverse spin effects in hadron-hadron inclusive processes is proposed, assuming a generalized factorization scheme and PQCD hard interactions. The transverse momentum ${\mathit{k}}_{\perp }$ of the quarks inside the hadrons and of the hadrons relative to the fragmenting quark is taken into account in distribution and fragmentation functions, and leads to possible nonzero single spin asymmetries. The role of ${\mathit{k}}_{\perp }$ and spin-dependent quark fragmentations—the so-called Collins effect—is investigated in details in $p^{\uparrow }\overrightarrow{p}\pi X$ processes: it is shown how the experimental data could be described, obtaining an explicit expression for the spin asymmetry of a polarized fragmenting quark, on which some comments are made. Predictions for other processes, possible further applications, and experimental tests are discussed.