Nuclear-Target Effects in $\frac{J}{\psi }$ Production in 125-GeV/c Antiproton and ${\pi }^{-}$ Interactions

The production of the $\frac{J}{\psi }$ resonance in 125-GeV/c $\overline{p}$ and ${\phi }^{-}$ interactions with Be, Cu, and W targets has been measured. The cross section per nucleon for $\frac{J}{\psi }$ production is suppressed in W interactions relative to the lighter targets, especially at large values of Feynman $x$, which is opposite to the expectation from the various explanations of the European Muon Collaboration effect. Models incorporating modifications of the gluon structure functions in heavy targets show qualitative agreement with the data.