System size and energy dependence of near-side di-hadron correlations
Two-particle azimuthal ($Δϕ$) and pseudorapidity ($Δη$) correlations using a trigger particle with large transverse momentum ($p_T$) in $d$+Au, Cu+Cu and Au+Au collisions at $\sqrt{s_{NN}}$ =\xspace 62.4 GeV and 200~GeV from the STAR experiment at RHIC are presented. The \ns correlation is separated into a jet-like component, narrow in both $Δϕ$ and $Δη$, and the ridge, narrow in $Δϕ$ but broad in $Δη$. Both components are studied as a function of collision centrality, and the jet-like correlation is studied as a function of the trigger and associated $p_T$. The behavior of the jet-like component is remarkably consistent for different collision systems, suggesting it is produced by fragmentation. The width of the jet-like correlation is found to increase with the system size. The ridge, previously observed in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, is also found in Cu+Cu collisions and in collisions at $\sqrt{s_{NN}}$ =\xspace 62.4 GeV, but is found to be substantially smaller at $\sqrt{s_{NN}}$ =\xspace 62.4 GeV than at $\sqrt{s_{NN}}$ = 200 GeV for the same average number of participants ($ \langle N_{\mathrm{part}}\rangle$). Measurements of the ridge are compared to models.