Distributions of Charged Hadrons Associated with High Transverse Momentum Particles in $pp$ and $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

Charged hadrons in $0.15<p_{\perp }<4\mathrm{GeV}/c$ associated with particles of $p_{\perp }^{\mathrm{trig}}>4\mathrm{GeV}/c$ are reconstructed in $pp$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The associated multiplicity and $p_{\perp }$ magnitude sum are found to increase from $pp$ to central $\mathrm{Au}+\mathrm{Au}$ collisions. The associated $p_{\perp }$ distributions, while similar in shape on the nearside, are significantly softened on the awayside in central $\mathrm{Au}+\mathrm{Au}$ relative to $pp$ and not much harder than that of inclusive hadrons. The results, consistent with jet quenching, suggest that the awayside fragments approach equilibration with the medium traversed.

Figure 1

Background-subtracted (a),(b) $\Delta \varphi$ and (c),(d) $\Delta \eta$ distributions for $pp$ and 5%–0% central $\mathrm{Au}+\mathrm{Au}$ for $4<p_{\perp }^{\mathrm{trig}}<6\mathrm{GeV}/c$ and two associated $p_{\perp }$ ranges. The subtracted background level for $p_{\perp }=0.15–4\mathrm{GeV}/c$ ($2–4\mathrm{GeV}/c$) is $\frac{1}{N_{\mathrm{trig}}}\frac{d{N}_{\mathrm{ch}}}{d\Delta \varphi }\approx 1.4(0.007)$ in $pp$ and $\approx 211$ (2.1) in 5%–0% $\mathrm{Au}+\mathrm{Au}$. The curve in (a) shows the shape of an $[A-B\cos (\Delta \varphi )]$ function. The curves in (c),(d) are Gaussian fits to the $pp$ data.

Figure 2

(a) ${\mathcal{N}}_{\mathrm{ch}}$ and (b) ${\mathcal{P}}_{\perp }$ for $p_{\perp }^{\mathrm{trig}}=4–6(6–10)\mathrm{GeV}/c$ with systematic errors in bands (caps). Systematic errors are strongly correlated between near and away sides and among the centralities. The leftmost set of data is for $pp$. Some of the open points are slightly displaced in $d{N}_{\mathrm{ch}}/d\eta$ for clarity.

Figure 3

Associated charged hadron $p_{\perp }$ distributions (a),(b) and $AA/pp$ ratios (c),(d) for $4<p_{\perp }^{\mathrm{trig}}<6\mathrm{GeV}/c$ on near and away sides. Errors shown are statistical. The bands show the systematic uncertainties for the 5%–0% central data. The lines show the inclusive spectral shape for central collisions.

Figure 4

Awayside associated hadron $⟨p_{\perp }⟩$ for $p_{\perp }^{\mathrm{trig}}=4–6(6–10)\mathrm{GeV}/c$ with systematic errors in bands (caps).