Medium Modification of Jet Fragmentation in $\mathrm{Au}\mathbf{+}\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}\mathbf{=}200\mathrm{GeV}$ Measured in Direct Photon-Hadron Correlations

The jet fragmentation function is measured with direct photon-hadron correlations in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The $p_T$ of the photon is an excellent approximation to the initial $p_T$ of the jet and the ratio $z_T=p_T^h/p_T^{\gamma }$ is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in $\mathrm{Au}+\mathrm{Au}$ collisions while a photon isolation cut is applied in $p+p$. $I_{\mathrm{AA}}$, the ratio of hadron yield opposite the photon in $\mathrm{Au}+\mathrm{Au}$ to that in $p+p$, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high $z_T$. The associated hadron yield at low $z_T$ is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.

Figure 1

$\Delta \varphi$ distributions for (a) $2.0<\xi <2.4$, (b) $1.6<\xi <2.0$, (c) $1.2<\xi <1.6$, (d) $0.8<\xi <1.2$, (e) $0.4<\xi <0.8$, and (f) $0.0<\xi <0.4$ for direct photons (circles) for the 0%–40% most central $\mathrm{Au}+\mathrm{Au}$ collisions and the $p+p$ reference (squares) in all panels. The data shown in panels (b), (d), and (f) are multiplied by a factor of 2 as indicated.

Figure 2

Panel (a) shows per trigger yield as a function of $\xi$ for $p+p$ collisions (squares) and 0%–40% most central $\mathrm{Au}+\mathrm{Au}$ collisions (circles). The points are shifted for clarity. For reference, the dependence on $z_T$ is also indicated. Panel (b) shows $I_{\mathrm{AA}}$, the ratio of $\mathrm{Au}+\mathrm{Au}$ to $p+p$ fragmentation functions. Also shown are predictions from BW-MLLA [18] (dashed line), calculated at $E_{\mathrm{jet}}=7\mathrm{GeV}$ with $f_{\mathrm{med}}=0.8$ selected for 0%–10% central $\mathrm{Au}+\mathrm{Au}$ and from YaJEM-DE [27, 28] (dot-dashed curve) for 0%–40% centrality and trigger photons from $9–12\mathrm{GeV}/c$, both for the full away side ($|\Delta \varphi -\pi |<\pi /2$).

Figure 3

Panel (a) shows the $I_{\mathrm{AA}}$ for the full away side ($|\Delta \varphi -\pi |<\pi /2$) (circles) and for two restricted away-side integration ranges, $|\Delta \varphi -\pi |<\pi /3$ (squares) and $|\Delta \varphi -\pi |<\pi /6$ (triangles). The points are shifted for clarity. Panel (b) shows the ratio of the $I_{\mathrm{AA}}$ for $|\Delta \varphi -\pi |<\pi /2$ to $|\Delta \varphi -\pi |<\pi /6$.