Transverse spin structure of the nucleon from lattice QCD simulations

TL;DR

This paper presents the first lattice QCD calculations of the lowest moments of transverse spin densities in the nucleon, revealing significant distortions and predicting a large negative Boer-Mulders function for up and down quarks.

Contribution

It introduces the first lattice QCD computation of transverse spin density moments, highlighting their impact on quark distributions and spin-orbit correlations.

Findings

Significant contributions from helicity flip distributions.

Strongly distorted densities of transversely polarized quarks.

Large negative Boer-Mulders function for up and down quarks.

Abstract

We present the first calculation in lattice QCD of the lowest two moments of transverse spin densities of quarks in the nucleon. They encode correlations between quark spin and orbital angular momentum. Our dynamical simulations are based on two flavors of clover-improved Wilson fermions and Wilson gluons. We find significant contributions from certain quark helicity flip generalized parton distributions, leading to strongly distorted densities of transversely polarized quarks in the nucleon. In particular, based on our results and recent arguments by Burkardt [Phys. Rev. D 72 (2005) 094020], we predict that the Boer-Mulders-function $h_1^\perp$, describing correlations of transverse quark spin and intrinsic transverse momentum of quarks, is large and negative for both up and down quarks.