Precision measurement of the neutron twist-3 matrix element d2n: Probing color forces

Abstract

Double-spin asymmetries and absolute cross sections were measured at large Bjorken x(0.25≤x≤0.90), in both the deep-inelastic and resonance regions, by scattering longitudinally polarized electrons at beam energies of 4.7 and 5.9 GeV from a transversely and longitudinally polarized He3 target. In this dedicated experiment, the spin structure function g2He3 was determined with precision at large x, and the neutron twist-3 matrix element d2n was measured at 〈Q2〉 of 3.21 and 4.32GeV2/c2, with an absolute precision of about 10-5. Our results are found to be in agreement with lattice QCD calculations and resolve the disagreement found with previous data at 〈Q2〉=5GeV2/c2. Combining d2n and a newly extracted twist-4 matrix element f2n, the average neutron color electric and magnetic forces were extracted and found to be of opposite sign and about 30MeV/fm in magnitude. © 2014 American Physical Society.