Transition in same-side $(\eta,\phi)$ correlation for Cu-Cu data from STAR

Two-dimensional angular correlations on relative pseudorapidity $\eta$ and azimuth $\phi$ are presented for charged particles from Cu-Cu collisions at $\sqrt{s_{NN}}$ = 62 and 200~GeV, with transverse momentum $p_t \geq 0.15$~GeV/$c$ and $|\eta| \leq 1$. For Au-Au data we observe a number of significant structures, including a peaked structure for same-side pairs (relative $\phi < \pi/2$) and a closely related away-side ridge. That peak follows binary-collision scaling in Au-Au until mid-centrality where an abrupt transition to a qualitatively different centrality trend is observed, leading to a manifestation at lower $p_t$ of the ridge phenomenon observed in trigger-associated particle correlations at higher $p_t$. Here we present results using the same analysis technique but on Cu-Cu collisions at $\sqrt{s_{NN}}$ = 62 and 200~GeV. Here we also observe a same-side peak and away side ridge. We focus on a comparison of the centrality trend of transition points in Cu-Cu to that of corresponding transitions previously observed in Au-Au.