Azimuthal di-hadron correlations in $d+$ Au and Au $+$ Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV measured at the STAR detector

Yields, correlation shapes, and mean transverse momenta $p_T$ of charged particles associated with intermediate- to high-$p_T$ trigger particles ($2.5<p_T<10$ GeV/$c$) in $d+$ Au and Au $+$ Au collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=200$ GeV are presented. For associated particles at higher $p_T\gtrsim 2.5$ GeV/$c$, narrow correlation peaks are seen in $d+$ Au and Au $+$ Au, indicating that the main production mechanism is jet fragmentation. At lower associated particle $p_T<2$ GeV/$c$, a large enhancement of the near- ($\Delta \varphi ~0$) and away-side ($\Delta \varphi ~\pi$) associated yields is found, together with a strong broadening of the away-side azimuthal distributions in Au $+$ Au collisions compared to $d+$ Au measurements, suggesting that other particle production mechanisms play a role. This is further supported by the observed significant softening of the away-side associated particle yield distribution at $\Delta \varphi ~\pi$ in central Au $+$ Au collisions.

Figure 1

Azimuthal distribution of associated charged particles with $1.0<p_T^{\mathrm{assoc}}<2.5$ GeV/$c$ with respect to trigger particles with 4.0 $<p_T^{\mathrm{trig}}<$ 6.0 GeV/$c$ in 0–12% central Au $+$ Au collisions. The curve shows the modulation of the background due to elliptic flow $v_2$, and the grey band indicates the uncertainty on the elliptic flow of the background (see text).

Figure 2

Background-subtracted azimuthal angle difference distributions for associated particles with $p_T$ between 1.0 and 2.5 GeV/$c$ and for different ranges of trigger particle $p_T$, ranging from 2.5–3.0 GeV/$c$ (left column) to 6–10 GeV/$c$ (right column). Results are shown for Au $+$ Au collisions (solid circles) with different centrality (rows) and $d+$ Au reference results (open circles). The rapidity range is $\left|\eta \right|<1$ and as a result the rapidity difference $\left|\Delta \eta \right|<2$. Open red squares show results for a restricted acceptance of $\left|\Delta \eta \right|<0.7$, using tracks within $\left|\eta \right|<1$. The solid and dashed histograms show the upper and lower range of the systematic uncertainty due to the $v_2$ modulation of the subtracted background.

Figure 3

Background-subtracted azimuthal angle difference distributions for different $p_T^{\mathrm{trig}}$ (columns) and $p_T^{\mathrm{assoc}}$ (rows) in 0–12% central Au $+$ Au collisions (solid circles) and $d+$ Au reference results (open circles). The rapidity range is $\left|\eta \right|<1$ and as a result the rapidity difference $\left|\Delta \eta \right|<2$. Open red squares show results for a restricted acceptance of $\left|\Delta \eta \right|<0.7$, using tracks with $\left|\eta \right|<1$. The solid and dashed histograms show the upper and lower range of the systematic uncertainty due to the $v_2$ modulation of the subtracted background.

Figure 4

Background-subtracted associated hadron distribution in 0–12% central Au $+$ Au collisions with $0.8<p_T^{\mathrm{assoc}}<1.0$ and $4<p_T^{\mathrm{trig}}<6$ GeV/$c$. The colored curves indicate fits with three different functional forms for the away-side shape: (left panel) symmetric identical Gaussian distributions (2 Gauss), (middle panel) adding a cos$\Delta \varphi$ background distribution (2 Gauss $+$ Cosine), and (right panel) adding a third Gaussian as the away-side jetlike component (2 $+$ 1 Gauss). The histograms indicate the uncertainty on the background shape due to elliptic flow.

Figure 5

Angle $\Delta$ between the away-side peaks and $\Delta \varphi =\pi$ for $4<p_T^{\mathrm{trig}}<6$ GeV/$c$ in 0–12% central Au $+$ Au collisions as a function of $p_T^{\mathrm{assoc}}$. Three different parametrizations of the away-side peak shape were used (see text). The lines show the systematic uncertainty from $v_2$ variation, while the errors are statistical errors from the fit.

Figure 6

Near-side ($\left|\Delta \varphi \right|<0.9$, left panels) and away-side ($\left|\Delta \varphi \right|>0.9$, right panels) associated yield per trigger particle for various $p_T^{\mathrm{trig}}$ selections as a function of $p_T^{\mathrm{assoc}}$. Results are shown for 0–12% central Au $+$ Au collisions and for $d+$ Au collisions. The bottom panels show the ratios of the per-trigger associated yields in central Au $+$ Au and $d+$ Au collisions. The error bars on the points indicate the statistical uncertainty, including the statistical uncertainty on the subtracted background level. The gray bands indicate the uncertainty from the elliptic flow modulation of the background.

Figure 7

Inclusive, azimuthal $\langle p_T\rangle$ of associated hadrons between 0.25 and 4.0 GeV/$c$ in all events containing various classes of trigger particles. The lines show the $v_2$-modulated background for the different trigger ranges, with the colors corresponding to the data points.

Figure 8

Mean transverse momentum $\langle p_T\rangle$ of associated particles with 0.25 $<p_T^{\mathrm{assoc}}<$ 4.0 GeV/$c$, for four different centrality selections. The shaded bands show the systematic uncertainty due to elliptic flow of the uncorrelated background. The dashed lines indicate the inclusive $\langle p_T\rangle$ in the same $p_T$ range in events with a trigger particle.

Figure 9

Mean transverse momentum $\langle p_T\rangle (\Delta \varphi )$ of associated hadrons in the range $0.25<p_T^{\mathrm{assoc}}<4.0$ GeV/$c$ in the core ($|\Delta \varphi -\pi |<\frac{\pi }{6}$) and cone ($\frac{\pi }{6}<|\Delta \varphi -\pi |<\frac{\pi }{2}$) azimuthal regions for $3<p_T^{\mathrm{trig}}<4$ and $4<p_T^{\mathrm{trig}}<6$ GeV/$c$ as a function of number of participants. The lines indicate the inclusive $\langle p_T\rangle$ in the same $p_T$ range for events with a trigger hadron.

Figure 10

Azimuthal angle difference distributions for associated particles with $p_T$ between 1.0 and 2.5 GeV/$c$ and for different ranges of trigger particle $p_T$, ranging from 2.5–3.0 GeV/$c$ (left column) to 6–10 GeV/$c$ (right column). Results are shown for Au $+$ Au collisions with different centrality (rows). The line and the grey band show the elliptic flow modulated background that was subtracted to obtain Fig. 2.

Figure 11

Azimuthal angle difference distributions for different $p_T^{\mathrm{trig}}$ (columns) and $p_T^{\mathrm{assoc}}$ (rows) in 0–12% central Au $+$ Au collisions. The line and the grey band show the elliptic flow modulated background that was subtracted to obtain Fig. 3.