Origin of SU(2)-flavor-symmetry breaking in antiquark distributions

The New Muon Collaboration has recently measured the electromagnetic structure functions F₂ through deep-inelastic muon scattering from protons and deuterons. Structure functions [F₂ᵉᵖ(x)-F₂ᵉⁿ(x)] have been measured to very small Bjorken x. The results seem to violate the Gottfried sum rule. We discuss various possible reasons for this result, including significant contributions from valence quarks at very small x and SU(2)-flavor breaking in the sea. We show that the pionic contribution to the sea-quark distributions gives qualitative agreement with estimates of the SU(2)-flavor-breaking contribution. We discuss various experiments which could give a direct measurement of ū-d̄. We make qualitative estimates of shadowing effects in the deuteron and their impact on the extracted neutron structure functions.