Dihadron azimuthal correlations in Au$+$Au collisions at $\sqrt{s_{NN}}=$ 200 GeV
Phys.Rev.C 78 (2008) 014901, 2008.
The collaboration

Abstract (data abstract)
BNL-RHIC. Azimuthal angle ($\Delta\phi$) correlations are presented for a broad range of transverse momentum ($0.4 < p_T < 10$ GeV/$c$) and centrality (0-92\%) selections for charged hadrons from di-jets in Au+Au collisions at $\sqrt{s}_{NN}$ = 200 GeV. With increasing $p_T$, the away-side $\Delta\phi$ distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons to p+p data suggest that the away-side distribution can be divided into a partially suppressed $\Delta\phi$ region centered at $\pi$, and an enhanced $\Delta\phi$ region centered at $\pi \pm 1.1$. The $p_T$ spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger $p_T$ . The properties of the near-side distributions are also modified relative to those in p+p collisions, reflected by the broadening of the jet shape in $\Delta\phi$ and $\Delta\eta$, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to $p_T < 4$ GeV/$c$, above which both the hadron pair shape and per-trigger yield become similar to p+p collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for $p_T > 5$ GeV and a medium-induced component which is important for $p_T < 4$ GeV/$c$. We also quantify the role of jets at intermediate and low $p_T$ through the yield of jet-induced pairs in comparison to binary scaled p+p pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the $p_T$ magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower $p_T$.