SU(2)-Flavor-Symmetry Breaking in Nuclear Antiquark Distributions
  SU(2)-flavor-symmetry breaking in antiquark distributions of the nucleon was suggested by the New Muon Collaboration (NMC) in deep inelastic muon scattering. As an independent test, Drell-Yan data for the tungsten target have been used for examining the asymmetry. We investigate whether there exists significant modification of the $\bar u -\bar d$ distribution in nuclei in a parton recombination model. It should be noted that a finite $\bar u-\bar d$ distribution is theoretically possible in nuclei even if the sea is $SU(2)_f$ symmetric in the nucleon. In neutron-excess nuclei such as the tungsten, there exist more $d$-valence quarks than $u$-valence quarks, so that more $\bar d$-quarks are lost than $\bar u$-quarks are due to parton recombinations in the small $x$ region. Our results suggest that the nuclear modification in the tungsten is a 2--10 % effect on the $\bar u-\bar d$ distribution suggested by the NMC data. Nuclear effects on the flavor asymmetric distribution could be an interesting topic for future theoretical and experimental investigations. 
S. Kumano (Saga University and the ECT*)