Observation of a dramatic transition in the same-side $(\eta,\phi)$ correlation data from STAR

Two-dimensional angular correlations on relative pseudorapidity $\eta$ and azimuth $\phi$ are presented for charged particles from Au-Au collisions at $\sqrt{s_{NN}}$ = 62 and 200~GeV, with transverse momentum $p_t \geq 0.15$~GeV/$c$ and $|\eta| \leq 1$. Significant correlations are observed, including a peaked structure for same-side pairs (relative $\phi < \pi/2$) and a closely related away-side ridge. The same-side peak, associated with semihard parton scattering and fragmentation (minijets) in peripheral Au-Au and p-p collisions, follows binary-collision scaling in Au-Au collisions until mid-centrality where an abrupt transition to a qualitatively different centrality trend is observed. The transition (especially the large increase in $\eta$ width) leads to a manifestation at lower $p_t$ of the ridge phenomenon observed in trigger-associated particle correlations at higher $p_t$. Above the transition the number of same-side correlated particles increases rapidly relative to binary-collision scaling. The transition point at both energies occurs at a common transverse density of $\sim$2.5~particles/unit-$\eta$/fm$^{2}$. Our results contradict heavy ion collision scenarios which invoke rapid formation of an opaque, locally thermalized medium.