Measurement of jet-medium interactions via direct photon-hadron correlations in $\mathrm{Au}+\mathrm{Au}$ and $d+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV

We present direct photon-hadron correlations in 200 GeV/$A$ $\mathrm{Au}+\mathrm{Au}$, $d+\mathrm{Au}$, and $p+p$ collisions, for direct photon $p_T$ from 5–12 $\mathrm{GeV}/c$, collected by the PHENIX Collaboration in the years from 2006 to 2011. We observe no significant modification of jet fragmentation in $d+\mathrm{Au}$ collisions, indicating that cold nuclear matter effects are small or absent. Hadrons carrying a large fraction of the quark's momentum are suppressed in $\mathrm{Au}+\mathrm{Au}$ compared to $p+p$ and $d+\mathrm{Au}$. As the momentum fraction decreases, the yield of hadrons in $\mathrm{Au}+\mathrm{Au}$ increases to an excess over the yield in $p+p$ collisions. The excess is at large angles and at low hadron $p_T$ and is most pronounced for hadrons associated with lower momentum direct photons. Comparison to theoretical calculations suggests that the hadron excess arises from medium response to energy deposited by jets.

Figure 1

Side view of the PHENIX central arm spectrometers in 2011.

Figure 2

Per-trigger yield of hadrons associated with direct photons in $\mathrm{Au}+\mathrm{Au}$ collisions (closed [black] circles) for direct photon $p_T$ 5–9 $\mathrm{GeV}/c$, compared with $p+p$ baseline (open [blue] squares), in various $\xi$ bins.

Figure 3

Per-trigger yield of hadrons associated with direct photons in $d+\mathrm{Au}$ collisions (closed [black] circles) for direct photon $p_T$ 7–9 $\mathrm{GeV}/c$, compared with $p+p$ baseline (open [blue] squares), in various $\xi$ bins.

Figure 4

(a) Integrated away-side ${\gamma }_{\mathrm{dir}}$-h per-trigger yields of $\mathrm{Au}+\mathrm{Au}$ (closed [black] circles), $d+\mathrm{Au}$ ([purple] crosses) and $p+p$ (open [blue] squares), as a function of $\xi$. The $p+p$ and $d+\mathrm{Au}$ points have been shifted to the left for clear viewing, as indicated in the legend. (b) $I_{AA}$ (closed [black] circles) and $I_{dA}$ ([purple] crosses).

Figure 5

$I_{AA}$ vs $\xi$ for direct photon $p_T^{\gamma }$ of 5–7 $\mathrm{GeV}/c$ (closed [black] circles), 7–9 $\mathrm{GeV}/c$ (closed [red] squares), and 9–12 $\mathrm{GeV}/c$ (closed [green] triangles).

Figure 6

$I_{AA}$ as a function of $\xi$ for direct photon $p_T^{\gamma }$ of (a) 5–7, (b) 7–9, and (c) 9–12 $\mathrm{GeV}/c$. Three away-side integration ranges are chosen to calculate the per-trigger yield and the corresponding $I_{AA}$: $|\mathrm{\Delta }\varphi -\pi |<\pi /2$ (closed [black] circles), $|\mathrm{\Delta }\varphi -\pi |<\pi /3$ (closed [blue] squares), and $|\mathrm{\Delta }\varphi -\pi |<\pi /6$ (closed [red] triangles).

Figure 7

Ratios of $I_{AA}$ as a function of direct photon $p_T$

for three different away-side integration ranges.

Figure 8

Measured $I_{AA}$ for direct photon $p_T$ of (a) 5–7, (b) 7–9, and (c) 9–12 $\mathrm{GeV}/c$, as a function of $\xi$, are compared with theoretical model calculations.