Intrinsic parton motion soft mechanisms and the longitudinal spin asymmetry A_LL in high energy pp -> pi X

TL;DR

This paper investigates whether soft mechanisms like Collins, Sivers, and Boer-Mulders significantly influence the longitudinal spin asymmetry A_LL in high energy proton-proton collisions, concluding their impact is negligible and supporting small gluon polarization.

Contribution

It provides a detailed analysis showing that soft mechanisms do not significantly affect A_LL, reinforcing the evidence for small gluon polarization in the proton.

Findings

Soft mechanisms have negligible influence on A_LL

Results support small gluon polarization in protons

Large positive gluon polarization is disfavored

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.