Transition in Yield and Azimuthal Shape Modification in Dihadron Correlations in Relativistic Heavy Ion Collisions

Hard-scattered parton probes produced in collisions of large nuclei indicate large partonic energy loss, possibly with collective produced-medium response to the lost energy. We present measurements of ${\pi }^0$ trigger particles at transverse momenta $p_T^t=4–12\mathrm{GeV}/c$ and associated charged hadrons ($p_T^a=0.5–7\mathrm{GeV}/c$) vs relative azimuthal angle $\Delta \varphi$ in $\mathrm{Au}+\mathrm{Au}$ and $p+p$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The $\mathrm{Au}+\mathrm{Au}$ distribution at low $p_T^a$, whose shape has been interpreted as a medium effect, is modified for $p_T^t<7\mathrm{GeV}/c$. At higher $p_T^t$, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured $p_T^a$, which quantitatively challenges some medium response models. The associated yield of hadrons opposing the trigger particle in $\mathrm{Au}+\mathrm{Au}$ relative to $p+p$ ($I_{AA}$) is suppressed at high $p_T$ ($I_{AA}\approx 0.35–0.5$), but less than for inclusive suppression ($R_{AA}\approx 0.2$).

Figure 1

Per-trigger jet pair yield vs $\Delta \varphi$ for selected ${\pi }^0$ trigger and $h^{\pm }$ partner $p_T$ combinations ($p_T^t\otimes p_T^a$) in $\mathrm{Au}+\mathrm{Au}$ and $p+p$ collisions. Depicted $\mathrm{Au}+\mathrm{Au}$ systematic uncertainties include point-to-point correlated background level and modulation uncertainties (gray bands and open boxes, respectively). For shape comparison insets show away-side distributions scaled to match at $\Delta \varphi =\pi$.

Figure 2

Away-side jet widths from a Gaussian fit by $h^{\pm }$ partner momentum for various ${\pi }^0$ trigger momenta in $p+p$, midcentral 20%–60% $\mathrm{Au}+\mathrm{Au}$, and central 0%–20% $\mathrm{Au}+\mathrm{Au}$ collisions. For comparison, an interpolation of the $p+p$ is depicted (curve). In cases where the best fit ${\sigma }_{\mathrm{away}}>\pi /2$ radians, the point is off the plot.

Figure 3

Away-side $I_{AA}$ for a narrow head $|\Delta \varphi -\pi |<\pi /6$ selection and the entire away side, $|\Delta \varphi -\pi |<\pi /2$ vs $h^{\pm }$ partner momentum for various ${\pi }^0$ trigger momenta. Calculations from two different predictions are shown for the head region in applicable $p_T$ ranges. A point-to-point uncorrelated 6% normalization uncertainty applies to all measurements. For comparison, ${\pi }^0$ $R_{AA}$ [24] bands are included where $p_T^t>5\mathrm{GeV}/c$.