New Measurements of the Nuclear Dependence of the EMC Effect

The European Muon Collaboration discovered that structure functions in nuclei are modified compared to those for a free nucleon. Even after twenty five years of experimental and theoretical investigations, the origin of the effect is still not well explained. I will present preliminary results from Jefferson Lab experiment E03-103 $\--$ a precision measurement of the EMC effect on light to medium-heavy nuclei with emphasis on the large $x_{Bj}$ region. Our data will directly measure the $A$ - dependence of the EMC effect. Data on light nuclei, $^3$He and $^4$He, allow direct comparison to `exact' few-body calculations of the EMC effect, and will also provide guidance for calculations of nuclear effects in deuterium. The large $x_{Bj}$ data are particularly sensitive to the details of nuclear structure, and our data will provide a reliable base-line to constrain models that incorporate conventional nuclear effects such as binding and Fermi motion. Conventional nuclear effects lead to modifications of the structure functions at all $x_{Bj}$ values, so that a quantitative understanding is important before the addition of more exotic effects that might be required to explain the observed nuclear dependence.