Observation of Structure in theϒRegion

The properties of the dimuon enhancement seen in 400-GeV proton-nucleus collisions have been clarified by a threefold increase in data. We find two peaks whose widths are consistent with our resolution: $M_1=9.4\mathrm{}$ GeV with $B{\frac{d\sigma }{\mathrm{dy}}|}_{y=0\mathrm{}}=1.8\mathrm{}\times {10}^{-37\mathrm{}}$ ${\mathrm{cm}}^2$/nucleon and $M_2=10.0\mathrm{}$ GeV with $B{\frac{d\sigma }{\mathrm{dy}}|}_{y=0\mathrm{}}=0.7\mathrm{}\times {10}^{-37\mathrm{}}$ ${\mathrm{cm}}^2$/nucleon. Evidence for the possible existence of a third peak near 10.4 GeV is discussed as are the comparisons with the properties of a $q\overline{q}$ system, where $q$ is a new heavy quark.