Nucleon spin densities in a light-front constituent quark model

TL;DR

This paper calculates nucleon spin densities using a light-front constituent quark model, revealing significant distortions and specific predictions for Sivers and Boer-Mulders functions consistent with lattice results.

Contribution

It provides new predictions for spin density distortions and transverse momentum-dependent functions in nucleons within a light-front quark model.

Findings

Significant distortions in transverse spin densities.

Opposite sign Sivers function for up and down quarks.

Large negative Boer-Mulders function for both quark flavors.

Abstract

The first moment of longitudinal and transverse spin densities of quarks in the nucleon are calculated in a light-front constituent quark model for the different cases of quark and nucleon polarization. Significant distortions are found for the transverse spin densities. In particular the Sivers function is predicted with opposite sign for up and down quarks and the Boer-Mulders function is predicted large and negative for both up and down quarks, in agreement with lattice calculations. Quite a different spin distribution is obtained for up and down quarks in the cases of quarks and proton transversely or longitudinally polarized in the same direction.