The rise and fall of the ridge in heavy ion collisions

Recent data from heavy ion collisions at RHIC show unexpectedly large near-angle correlations that broaden longitudinally with increasing centrality. The amplitude of this ridge-like correlation rises rapidly, reaches a maximum, and then falls in the most central collisions. In this Letter we explain how this behavior can be uniquely explained by initial-state coordinate-space anisotropies converted into final-state momentum-space correlations. We propose vn2/εn,part2 as a useful way to study length scales and provide a prediction for the ridge in Pb+Pb collisions at sNN=2.76 TeV.