Azimuthal angle ($\Delta \varphi$) 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 dijets in Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. With increasing $p_T$, the away-side $\Delta \varphi$ distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons with $p+p$ data suggest that the away-side distribution can be divided into a partially suppressed “head” region centered at $\Delta \varphi ~\pi$, and an enhanced “shoulder” region centered at $\Delta \varphi ~\pi \pm 1.1$. 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 $\Delta \varphi$ and $\Delta \eta$, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to $p_T\lesssim 4$ 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 $p_T\gtrsim 5$ GeV/$c$ and a medium-induced component which is important for $p_T\lesssim 4$ GeV/$c$. We also quantify the role of jets at intermediate and low $p_T$ through the yield of jet-induced pairs in comparison with 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 $p_T$.

• Received 30 January 2008

DOI:https://doi.org/10.1103/PhysRevC.78.014901