Single Spin Asymmetry In Inclusive Pion Production ∗

It is shown how the single spin asymmetry observed in inclusive pion production is related, in the helicity basis, to the imaginary part of the product of two different distribution amplitudes, rather than to the usual quark and gluon distribution functions; there is then no reason why it should be zero even in massless perturbative QCD, provided the quark intrinsic motion is taken into account. A simple model is constructed which reproduces the main features of the data. Spin physics in large pT inclusive hadronic processes has unique features; not only it probes the internal structure of hadrons, but, as spin dependent observables involve delicate interference effects among different amplitudes, it tests the theory at a much deeper level than unpolarized processes. We consider here the single spin asymmetry in inclusive pion production in p− p collisions, p+ p → π+X. Let the two protons move along the ẑ-axis in their c.m. frame and x̂-ẑ be the scattering plane. The proton moving in the +ẑ direction is polarized transversely to the scattering plane, i.e. along (↑) or opposite (↓) the ŷ-axis. The single spin asymmetry AN is then defined by: AN(xF , pT ) = dσ − dσ dσ↑ + dσ↓ (1) where dσ is the differential cross section and ↑, ↓ refer to the proton spin directions; we denote by pL and pT the c.m. longitudinal and transverse pion Talk presented by F. Murgia at the XI International Symposium on High Energy Spin Physics, September 15-22, Bloomington, Indiana.