Revealing Color Forces with Transverse Polarized Electron Scattering

TL;DR

This study measures spin asymmetries in polarized electron-proton scattering to extract the color Lorentz force, revealing unexpected sign and scale dependence, and suggesting flavor independence when combined with neutron data.

Contribution

First measurement of the twist-3 matrix element $ ilde{d}_2^p$ over a range of $Q^2$, providing new insights into color forces in the nucleon.

Findings

Opposite sign compared to quark models and lattice QCD.

Scale dependence of the twist-3 matrix element observed.

Results imply flavor-independent color Lorentz force when combined with neutron data.

Abstract

The Spin Asymmetries of the Nucleon Experiment (SANE) measured two double spin asymmetries using a polarized proton target and polarized electron beam at two beam energies, 4.7 GeV and 5.9 GeV. A large-acceptance open-configuration detector package identified scattered electrons at 40$^{\circ}$ and covered a wide range in Bjorken $x$ ($0.3 < x < 0.8$). Proportional to an average color Lorentz force, the twist-3 matrix element, $\tilde{d}_2^p$, was extracted from the measured asymmetries at $Q^2$ values ranging from 2.0 to 6.0 GeV$^2$. The data display the opposite sign compared to most quark models, including the lattice QCD result, and an apparently unexpected scale dependence. Furthermore when combined with the neutron data in the same $Q^2$ range the results suggest a flavor independent average color Lorentz force.