Nuclear modification of the flavor asymmetry ubar - dbar
  In order to test the NMC finding of flavor asymmetry $\overline u-\overline d$ in the nucleon, existing Drell-Yan data for the tungsten target are often used. However, we have to be careful in comparing nuclear data with the nucleon ones. We investigate whether there exists significant nuclear modification of the $\overline u - \overline d$ distribution in a parton-recombination model. It should be noted that a finite $\overline u - \overline d$ distribution is theoretically possible in nuclei even if the sea is 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 $\overline d$-quarks are lost than $\overline u$-quarks due to parton recombinations. Our results suggest that the nuclear modification in the tungsten is a 2--10 \% effect on the NMC $\overline u - \overline d$ distribution. The nuclear modification of the flavor asymmetry should be an interesting topic in connection with ongoing Drell-Yan experiments. 
S. Kumano (Saga Univ.)