  Mechanisms of spin-flavor SU(6) symmetry breaking in Quantum Chromodynamics
(QCD) are studied via an extraction of the free neutron structure function from
a global analysis of deep inelastic scattering (DIS) data on the proton and on
nuclei from $A = 2$ (deuterium) to 208 (lead). Modification of the structure
function of nucleons bound in atomic nuclei (known as the EMC effect) are
consistently accounted for within the framework of a universal modification of
nucleons in short-range correlated (SRC) pairs. Our extracted neutron-to-proton
structure function ratio $F_2^n/F_2^p$ becomes constant for $x_B \ge 0.6$,
equalling $0.47 \pm 0.04$ as $x_B \rightarrow 1$, in agreement with theoretical
predictions of perturbative QCD and the Dyson Schwinger equation, and in
disagreement with predictions of the Scalar Diquark dominance model. We also
predict $F_2^{^3\mathrm{He}}/F_2^{^3\mathrm{H}}$, recently measured, yet
unpublished, by the MARATHON collaboration, the nuclear correction function
that is needed to extract $F_2^n/F_2^p$ from
$F_2^{^3\mathrm{He}}/F_2^{^3\mathrm{H}}$, and the theoretical uncertainty
associated with this extraction.

Nuclear Theory;High Energy Physics - Experiment;High Energy Physics - Phenomenology;Nuclear Experiment