Precision Measurement of the Neutron Twist-3 Matrix Element dn 2: Probing Color Forces

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 gHe32 was determined with precision at large x, and the neutron twist-3 matrix element dn2 was measured at ⟨Q2⟩ of 3.21 and 4.32  GeV2/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⟩=5  GeV2/c2. Combining dn2 and a newly extracted twist-4 matrix element fn2, the average neutron color electric and magnetic forces were extracted and found to be of opposite sign and about 30  MeV/fm in magnitude.