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

The New Muon Collaboration has recently measured the electromagnetic structure functions $F_2$ through deep-inelastic muon scattering from protons and deuterons. Structure functions [$F_2^{\mathrm{ep}}\mathrm{}\left(x\right)-F_2^{\mathrm{en}}\mathrm{}\left(x\right)\mathrm{}$] 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 $\overline{u}-\overline{d}$. We make qualitative estimates of shadowing effects in the deuteron and their impact on the extracted neutron structure functions.