arXiv:0710.1569·v2·High Energy Physics — Phenomenology
Intrinsic parton motion soft mechanisms and the longitudinal spin asymmetry A_LL in high energy pp -> pi X
M. Anselmino (1)🇮🇹 · M. Boglione (1)🇮🇹 · U. D'Alesio (2,3)🇮🇹 · E. Leader (2,4)🇮🇹 · S. Melis (1,2)🇮🇹 · F. Murgia (3) ((1) University and INFN, Torino, Italy, (2) University of Cagliari, Italy, (3) INFN, Cagliari, Italy, (4) Imperial College, London, U.K.)🇮🇹
Abstract
The longitudinal double spin asymmetry A_LL in the reaction pp --> pi X 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 A_LL would be large and positive. Latest RHIC results indicate that A_LL 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 A_LL, 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.
Comments: RevTeX, 9 pages, 3 eps figures. Revised and shortened version; title slightly modified; figs. 3,4 removed; a new figure for the unpol. cross section added; no changes in results and conclusions; matches the published version