Observation of Structure in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>ϒ</mml:mi></mml:math>Region

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: ${M}_{1}=9.4$ GeV with $B{\frac{d\ensuremath{\sigma}}{\mathrm{dy}}|}_{y=0}=1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}37}$ ${\mathrm{cm}}^{2}$/nucleon and ${M}_{2}=10.0$ GeV with $B{\frac{d\ensuremath{\sigma}}{\mathrm{dy}}|}_{y=0}=0.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}37}$ ${\mathrm{cm}}^{2}$/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.