Parton energy loss in heavy-ion collisions via direct-photon and charged-particle azimuthal correlations

Charged-particle spectra associated with direct photon (${\gamma }_{\mathrm{dir}}$) and ${\pi }^0$ are measured in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at center-of-mass energy $\sqrt{s_{{}_{\mathit{NN}}}}=200$ GeV with the STAR detector at the Relativistic Heavy Ion Collider. A shower-shape analysis is used to partially discriminate between ${\gamma }_{\mathrm{dir}}$ and ${\pi }^0$. Assuming no associated charged particles in the ${\gamma }_{\mathrm{dir}}$ direction (near side) and small contribution from fragmentation photons (${\gamma }_{\mathrm{frag}}$), the associated charged-particle yields opposite to ${\gamma }_{\mathrm{dir}}$ (away side) are extracted. In central $\mathrm{Au}+\mathrm{Au}$ collisions, the charged-particle yields at midrapidity ($\left|\eta \right|<1$) and high transverse momentum ($3<p_T^{\mathrm{assoc}}<16$ GeV/$c$) associated with ${\gamma }_{\mathrm{dir}}$ and ${\pi }^0$ ($\left|\eta \right|<0.9$, $8<p_T^{\mathrm{trig}}<16$ GeV/$c$) are suppressed by a factor of 3–5 compared with $p+p$ collisions. The observed suppression of the associated charged particles is similar for ${\gamma }_{\mathrm{dir}}$ and ${\pi }^0$ and independent of the ${\gamma }_{\mathrm{dir}}$ energy within uncertainties. These measurements indicate that, in the kinematic range covered and within our current experimental uncertainties, the parton energy loss shows no sensitivity to the parton initial energy, path length, or color charge.

Figure 1

Probability distributions of the shower-shape quantity [as defined in Eq. (1)] for particles measured as BEMC clusters. The solid circles are neutral clusters above trigger threshold ($E_T>8$ GeV) in $p+p$ data, the open circles are neutral clusters above trigger threshold in central $\mathrm{Au}+\mathrm{Au}$ data, and the squares are identified electrons with $E>4$ GeV. The vertical line is the upper cut value used for identifying ${\pi }^0$.

Figure 2

Correlations (without background subtraction) of neutral-cluster triggers (${\pi }^0$ and inclusive $\gamma$’s) with associated charged hadrons measured in central (0%–10$\%$) and peripheral (40%–80$\%$) $\mathrm{Au}+\mathrm{Au}$ collisions at different bins of $p_T^{\mathrm{trig}}$ and $p_T^{\mathrm{assoc}}$.

Figure 3

Correlations (without background subtraction) of neutral-cluster triggers, ${\pi }_{\mathrm{rich}}^0$, and ${\gamma }_{\mathrm{rich}}$ samples with associated charged hadrons measured in peripheral (40%–80$\%$) and central (0%–10$\%$) $\mathrm{Au}+\mathrm{Au}$ collisions.

Figure 4

The $z_T$ dependence of ${\pi }^0-h^{\pm }$ and $h^{\pm }-h^{\pm }$ [10] near-side (left panel) and away-side (right panel) associated particle yields. The bin centers are shifted for clarity. The shaded boxes show the systematic errors correlated in $z_T$, and the brackets show the point-to-point systematic errors.

Figure 5

The $z_T$ dependence of away-side associated-particle yields for ${\pi }^0$ triggers (triangles) and ${\gamma }_{\mathrm{dir}}$ triggers (circles) for $p+p$ (open symbols) and 0%–10% $\mathrm{Au}+\mathrm{Au}$ (solid symbols) collisions. The trigger particle has $8<E_T^{\mathrm{trig}}<16$ GeV/$c$. Boxes show the $z_T$-correlated systematic errors, and brackets show the point-to-point systematic errors. Data are compared to theoretical calculations. For $p+p$ collisions, the solid line is a calculation by Zhang et al. [16] and the dashed line is by Qin et al. [17]. For central $\mathrm{Au}+\mathrm{Au}$ collisions, the dotted line is by Zhang et al. [16] and the dash-dotted line is by Qin et al. [17].

Figure 6

The $z_T$ dependence of $I_{\mathrm{AA}}$ for ${\gamma }_{\mathrm{dir}}$ triggers (circles) and ${\pi }^0$ triggers (triangles). Boxes show the $z_T$-correlated systematic errors, and brackets show the point-to-point systematic errors. The bin centers are shifted for clarity. Data are compared to theoretical calculations (see text).

Figure 7

$I_{\mathrm{AA}}$ as a function of $p_T$ for ${\gamma }_{\mathrm{dir}}$ triggers, measured in 0%–10% $\mathrm{Au}+\mathrm{Au}$ collisions. The associated charged particles have $z_T=0.4$–0.9. The shaded boxes show the systematic errors correlated in $p_T^{\mathrm{trig}}$, and the brackets show the point-to-point systematic errors.