Intrinsic parton motion soft mechanisms and the longitudinal spin asymmetry ALL in high energy pp → πX

Abstract

The longitudinal double spin asymmetry ALL in the reaction pp → πX has been measured at RHIC with extremely interesting consequences. If the gluon polarization in a proton were as big as needed to resolve the famous ‘spin crisis’ then ALL would be large and positive. Latest RHIC results indicate that ALL is small and disfavour large positive values of the gluon polarization. We examine whether the soft mechanisms (Collins, Sivers, Boer–Mulders), essential for generating transverse single spin asymmetries, have any significant influence on ALL, and whether they could alter the conclusion that the gluon polarization is necessarily small. It turns out that the contribution from these effects is essentially negligible.