System size and energy dependence of near-side dihadron correlations

Abstract

Two-particle azimuthal ($\ensuremath{\Delta}\ensuremath{\varphi}$) and pseudorapidity ($\ensuremath{\Delta}\ensuremath{\eta}$) correlations using a trigger particle with large transverse momentum (${p}_{T}$) in $d$+Au, Cu+Cu, and Au+Au collisions at $\sqrt{{s}_{NN}}$ $=$ 62.4 GeV and 200 GeV from the STAR experiment at the Relativistic Heavy Ion Collider are presented. The near-side correlation is separated into a jet-like component, narrow in both $\ensuremath{\Delta}\ensuremath{\varphi}$ and $\ensuremath{\Delta}\ensuremath{\eta}$, and the ridge, narrow in $\ensuremath{\Delta}\ensuremath{\varphi}$ but broad in $\ensuremath{\Delta}\ensuremath{\eta}$. 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 $\sqrt{{s}_{NN}}$ $=$ 62.4 GeV, but is found to be substantially smaller at $\sqrt{{s}_{NN}}$ $=$ 62.4 GeV than at $\sqrt{{s}_{NN}}$ $=$ 200 GeV for the same average number of participants ($\ensuremath{\langle}{N}_{\mathrm{part}}\ensuremath{\rangle}$). Measurements of the ridge are compared to models.