Reassessment of the Collins Mechanism for Single-spin Asymmetries and the behavior of Delta d(x) at large x

TL;DR

This paper reevaluates the Collins mechanism's explanation for single-spin asymmetries, revealing issues with the transversely polarized d quark density and proposing a bound-respecting parametrization that fits experimental data well.

Contribution

It demonstrates that standard polarized d quark densities violate the Soffer bound under the Collins mechanism, and shows that bound-respecting parametrizations align better with data.

Findings

Standard Delta d(x) violates the Soffer bound.

Imposing the bound with PQCD-consistent parametrizations fits data well.

Implications for neutron asymmetry at large x are significant.

Abstract

It is shown that the Collins mechanism explanation of the transverse single-spin asymmetries in p^{\uparrow} p -> \pi X leads to a transversely polarized d quark density Delta_T d(x) which violates the Soffer bound when one uses several standard forms for the longitudinally polarized d quark density Delta d(x) obtained from polarized deep inelastic scattering. Imposition of the Soffer bound with these Delta d(x) yields results in hopeless disagreement with the data. Remarkably, imposition of the Soffer bound, but using parametrizations of Delta d(x) that respect the PQCD condition Delta q(x) / q(x) -> 1 as x -> 1, leads to an excellent fit to most of the data. The implications for the polarized DIS neutron longitudinal asymmetry A_1^n at large x are dramatic.