Event-plane-dependent dihadron correlations with harmonic $v_n$ subtraction in Au + Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV

STAR measurements of dihadron azimuthal correlations ($\Delta \varphi$) are reported in midcentral (20–60%) Au + Au collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=200$ GeV as a function of the trigger particle's azimuthal angle relative to the event plane, ${\varphi }_s=|{\varphi }_t-{\psi }_{\mathrm{EP}}|$. The elliptic ($v_2$), triangular ($v_3$), and quadratic ($v_4$) flow harmonic backgrounds are subtracted using the zero yield at minimum (ZYAM) method. The results are compared to minimum-bias d + Au collisions. It is found that a finite near-side ($\left|\Delta \varphi \right|<\pi /2$) long-range pseudorapidity correlation (ridge) is present in the in-plane direction (${\varphi }_s\sim 0$). The away-side ($\left|\Delta \varphi \right|>\pi /2$) correlation shows a modification from d + Au data, varying with ${\varphi }_s$. The modification may be a consequence of path-length-dependent jet quenching and may lead to a better understanding of high-density QCD.

Figure 1

Raw dihadron $\Delta \varphi$ correlations (data points) as a function of ${\varphi }_s=|{\varphi }_t-{\psi }_{\mathrm{EP}}|$, with a cut on the trigger-associated pseudorapidity difference of $\left|\Delta \eta \right|>0.7$. The triangle two-particle $\Delta \eta$ acceptance is not corrected. Statistical errors are smaller than the symbol size; systematic uncertainty is 5% (not shown). The curves are flow modulated ZYAM background by Eq. (1) (blue solid), its systematic uncertainty boundaries (dashed), and the $v_3$ (pink solid) and $v_4\left\{{\psi }_2\right\}$ (green solid) contributions.

Figure 2

Background-subtracted dihadron $\Delta \varphi$ correlations as a function of ${\varphi }_s=|{\varphi }_t-{\psi }_{\mathrm{EP}}|$, with a cut on the trigger-associated pseudorapidity difference of $\left|\Delta \eta \right|>0.7$. The triangle two-particle $\Delta \eta$ acceptance is not corrected. Flow background is subtracted by Eq. (1). Systematic uncertainties due to flow subtraction are shown as black histograms enclosing the shaded area; those due to the ZYAM normalization are shown in the horizontal shaded band around zero. Statistical errors are smaller than the point size. For comparison, the inclusive dihadron correlations from $d$ + Au collisions are superimposed as the green histogram with statistical errors.

Figure 3

Parameters of four-Gaussian fit to the background subtracted dihadron correlations at $\left|\Delta \eta \right|>0.7$ as a function of ${\varphi }_s$. The near-side (NS) jetlike correlation results are also shown; they are obtained by the difference in $\Delta \varphi$ correlations at small and large $\Delta \eta$ from Ref. [10]. (a) Correlated yields where the NS jetlike ($\left|\Delta \eta \right|<0.7$) and ridge ($\left|\Delta \eta \right|>0.7$) yields are obtained from bin counting within $\left|\Delta \varphi \right|<1$; (b) Gaussian peak widths in $\Delta \varphi$; and (c) the away-side (AS) double-peak Gaussian centroid. For comparison the centroid from a two-Gaussian fit to the AS correlation ($\left|\Delta \varphi \right|>0.7)$ is shown for the four out-of-plane slices. Error bars are statistical only. The curves correspond to the results with maximum flow subtraction by the two-particle cumulant method, which indicate the systematics.