Dihadron azimuthal correlations in Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV

Azimuthal angle ($\Delta \varphi$) correlations are presented for a broad range of transverse momentum ($0.4<p_T<10$ GeV/$c$) and centrality (0–92%) selections for charged hadrons from dijets in Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. With increasing $p_T$, the away-side $\Delta \varphi$ distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons with $p+p$ data suggest that the away-side distribution can be divided into a partially suppressed “head” region centered at $\Delta \varphi ~\pi$, and an enhanced “shoulder” region centered at $\Delta \varphi ~\pi \pm 1.1$. The $p_T$ spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger $p_T$. The properties of the near-side distributions are also modified relative to those in $p+p$ collisions, reflected by the broadening of the jet shape in $\Delta \varphi$ and $\Delta \eta$, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to $p_T\lesssim 4$ GeV/$c$, above which both the hadron pair shape and per-trigger yield become similar to $p+p$ collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for $p_T\gtrsim 5$ GeV/$c$ and a medium-induced component which is important for $p_T\lesssim 4$ GeV/$c$. We also quantify the role of jets at intermediate and low $p_T$ through the yield of jet-induced pairs in comparison with binary scaled $p+p$ pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the $p_T$ magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower $p_T$.

Figure 1

Cartoon of hadron pair distribution in $\Delta \varphi$ for $p+p$ collisions. It has two peaks corresponding to near- and away-side jet, and a flat component representing the underlying event pairs.

Figure 2

Correlation function for $2<p_T^a<3,1<p_T^b<2$ GeV/$c$ in 0–5% Au+Au collisions. The dashed line represents the estimated elliptic flow modulated combinatoric background using zero yield at minimum (ZYAM) method (see Sec. 3e).

Figure 3

$p+p$ jet-induced hadron-pair yield $\Delta \varphi$ distributions calculated from the per-trigger yield using low-$p_T$ hadrons as triggers (solid symbols) and high-$p_T$ hadrons as triggers (open symbols).

Figure 4

Same as Fig. 3, but for 0–20% Au+Au.

Figure 5

pythia simulation showing jet-induced hadron pair $\Delta \varphi$ distribution for $3<p_T^a,p_T^b<5$ GeV/$c$ with (top histogram) and without (bottom histogram) initial and final state radiation. The radiation accounts for the increase of the background level.

Figure 6

Per-trigger yield vs $\Delta \varphi$ for various trigger and partner $p_T$ ($p_T^a\otimes p_T^b$), arranged by increasing pair proxy energy (sum of $p_T^a$ and $p_T^b$), in $p+p$ and 0–20% Au+Au collisions. The data in several panels are scaled as indicated. Solid histograms (shaded bands) indicate elliptic flow (ZYAM) uncertainties. Arrows in Fig. 6 depict the “head” region (HR), the “shoulder” region (SR) and the “near-side” region (NR).

Figure 7

$R_{\mathrm{HS}}$ vs $p_T^b$ for $p+p$ (open) and Au+Au (filled) collisions for four trigger selections. Shaded bars (brackets) represent $p_T$-correlated uncertainties due to elliptic flow (ZYAM procedure).

Figure 8

$R_{\mathrm{HS}}$ vs $N_{\mathrm{part}}$ for $2\text{−}3\otimes 2\text{−}3$ GeV/$c$. Shaded bars (brackets) represent $p_T$-correlated uncertainties due to elliptic flow (ZYAM procedure). The left-most point is from $p+p$.

Figure 9

Per-trigger yield $\Delta \varphi$ distribution and corresponding fits for $2\text{−}3\otimes 2\text{−}3$ GeV/$c$ in 0–5% Au+Au collisions. FIT1 (FIT2) is shown in the left panel (right panel). The total fit function and individual components are shown relative to the κ level indicated by the horizontal line.

Figure 10

(a) $D$ vs $N_{\mathrm{part}}$ from FIT1 (solid circles) and FIT2 (open circles) for $2\text{−}3\otimes 2\text{−}3$ GeV/$c$ bin. The error bars are the statistical errors; shaded bars and brackets are the systematic errors due to $v_2$. (b) Fraction of the shoulder Gaussian yield relative to the total away-side yield as function of $N_{\mathrm{part}}$ determined from FIT2.

Figure 11

Values of $D$ determined from FIT1 (solid circles) and FIT2 (open circles) as function of partner $p_T$ for three trigger $p_T$ ranges in 0–20% Au+Au collisions. The error bars are the statistical errors. The shaded bars and brackets are the systematic errors due to elliptic flow.

Figure 12

Per-trigger yield as function of partner $p_T$ for the HR (left panels) and the SR region (right panels). Filled (open) circles represent the 0–20% Au+Au ($p+p$) collisions. Results for four trigger $p_T$ in 2–3, 3–4, 4–5, 5–10 GeV/$c$ are shown. The shaded bars represent the total systematic uncertainties.

Figure 13

$I_{\mathit{AA}}$ vs $p_T^b$ for four trigger $p_T$ bins in HR+SR ($|\Delta \varphi -\pi |<\pi /2$) and HR ($|\Delta \varphi -\pi |<\pi /6$) in 0–20% Au+Au collisions. The shaded bars and brackets represent the total systematic errors in the two regions. They are strongly correlated. Grey bands around $I_{\mathit{AA}}=1$ represent 12% combined uncertainty on the single particle efficiency in Au+Au and $p+p$.

Figure 14

Same as Fig. 13, but for 60–92% Au+Au collisions.

Figure 15

Per-trigger yield vs $N_{\mathrm{part}}$ in the HR (left panels) and the SR (right panels) for partner $p_T$ in 0.4–1, 1–2, 2–3, 3–4, 4–5 GeV/$c$ and fixed trigger in 3–4 GeV/$c$. The left-most points are from $p+p$ collisions. The shaded bars (brackets) represent uncertainties due to elliptic flow (ZYAM procedure).

Figure 16

Near-side Gaussian widths vs partner $p_T$ for four trigger $p_T$ ranges compared between 0–20% Au+Au (solid circles) and $p+p$ (open circles).

Figure 17

Near-side Gaussian widths vs $N_{\mathrm{part}}$ for five successively increasing $p_T^a\otimes p_T^b$. The left-most point in each panel represents the value from $p+p$.

Figure 18

Near-side yield in $\left|\Delta \varphi \right|<\pi /3$ vs partner $p_T$ for four trigger $p_T$ selections. The filled and open circles are for 0–20% Au+Au and $p+p$, respectively.

Figure 19

Near-side $I_{\mathit{AA}}$ in 0–20% Au+Au vs partner $p_T$ for four trigger $p_T$ bins. The shaded bars around the data points are the total systematic errors. Grey bands around $I_{\mathit{AA}}=1$ represent 12% combined uncertainty on the single particle efficiency in Au+Au and $p+p$.

Figure 20

Near-side $I_{\mathit{AA}}$ in 60–92% Au+Au vs partner $p_T$ for four trigger $p_T$ bins. The shaded bars around the data points are the total systematic errors. Grey bands around $I_{\mathit{AA}}=1$ represent 12% combined uncertainty on the single particle efficiency in Au+Au and $p+p$.

Figure 21

Correlation function [left panel, Eq. (8)] and background subtracted hadron-pair ratio [right panel, Eq. (13)] in $\Delta \eta$ and $\Delta \varphi$ space for $2\text{−}3\otimes 2\text{−}3$ GeV/$c$ in 0–20% Au+Au collisions.

Figure 22

Background subtracted hadron-pair ratio [via Eq. (13)] for $2\text{−}3\otimes 2\text{−}3$ GeV/$c$ in 60–92% Au+Au (left panel) and $p+p$ (right panel) collisions.

Figure 23

Per-trigger yield vs $\Delta \eta$ for $p+p$ (open symbols) and 0–20% central Au+Au (filled symbols) collisions. Results are shown for four $p_T^a\otimes p_T^b$ selections as indicated.

Figure 24

Projection of two-dimensional per-trigger yield in $\left|\Delta \eta \right|<0.7\otimes \left|\Delta \varphi \right|<0.7$ and 0–20% central Au+Au collisions onto $\Delta \varphi$ (open symbols) and $\Delta \eta$ (filled symbols) for four $p_T^a\otimes p_T^b$ selections.

Figure 25

Truncated mean $p_T,\langle p_T^{\text{'}}\rangle$, in $1<p_T^b<5$ GeV/$c$ vs $N_{\mathrm{part}}$ for the near-side (diamonds), away-side shoulder (squares), and head (circles) regions for Au+Au (filled) and $p+p$ (open) for four trigger $p_T$ bins. Solid lines represent values for inclusive charged hadrons ($~0.36$ GeV/$c$) [6]. Error bars represent the statistical errors. Shaded bars represent the sum of $N_{\mathrm{part}}$-correlated elliptic flow and ZYAM error.

Figure 26

Truncated mean $p_T,\langle p_T^{\text{'}}\rangle$, calculated for five partner-$p_T$ ranges (from top to bottom) in the HR. The left panels (right panels) show the results for triggers in 3–4 (4–5) GeV/$c$. Solid lines represent values for inclusive charged hadrons. Error bars represent the statistical errors. Shaded bars represent the sum of $N_{\mathrm{part}}$-correlated elliptic flow and ZYAM error.

Figure 27

(a) $I_{\mathit{AA}}$ vs partner $p_T$ when 5–10 GeV/$c$ hadrons are designated triggers. (b) $I_{\mathit{AA}}$ vs trigger $p_T$ when 5–10 GeV/$c$ hadrons are used as partners.

Figure 28

Near-side pair suppression factor $J_{\mathit{AA}}$ in 0–20% Au+Au collisions as function of $p_T^b$ for various ranges of $p_T^a$. The dashed line indicates the charged hadron $R_{\mathit{AA}}$. The grey band around unity is the systematic error due to the 20% combined single particle efficiency of the triggers and partners in Au+Au and $p+p$. The dark line around unity indicates the uncertainty on the $N_{\mathrm{coll}}$.

Figure 29

Near-side pair suppression factor $J_{\mathit{AA}}$ as function of pair transverse momentum, $p_T^{\mathrm{sum}}=p_T^a+p_T^b$ in 0–20% Au+Au collisions. The grey band around unity is the systematic error due to the 20% combined single particle efficiency of the triggers and partners in Au+Au and $p+p$. The dark line around unity indicates the uncertainty on the $N_{\mathrm{coll}}$.

Figure 30

Near-side $J_{\mathit{AA}}$ as function of $p_T^b$ (left panels) and $p_T^{\mathrm{sum}}$ (right panels) for four $p_T^a$ bins and three centrality bins. From top to bottom are 20–40%, 40–60%, and 60–92%. The dashed lines in the left panels indicate the charged hadron $R_{\mathit{AA}}$. The grey bands around unity are the systematic errors due to the 17% combined single particle efficiency of the triggers and partners in Au+Au and $p+p$. The dark lines around unity indicate the uncertainty on the $N_{\mathrm{coll}}$.

Figure 31

Pair suppression factor $J_{\mathit{AA}}$ for the away-side HR in Au+Au collisions as function of $p_T^b$ for various ranges of $p_T^a$. The dashed lines indicate the square of the charged hadron $R_{\mathit{AA}}$. The grey bands around unity are the systematic errors due to the combined single particle efficiency of the two particles in Au+Au and $p+p$. The dark lines around unity indicate the uncertainty on the $N_{\mathrm{coll}}$.

Figure 32

Per-trigger yield in 0–10% central Au+Au collisions from PHENIX at (a) $\sqrt{s_{\mathit{NN}}}=200$ GeV and (b) $\sqrt{s_{\mathit{NN}}}=62.4$ GeV. The histograms show the combined uncertainty of the elliptic flow and ZYAM errors.

Figure 33

Comparison of $I_{\mathit{AA}}$ with energy loss calculation from Ref. [76].

Figure 34

Distribution of the leading particle from the dijets relative to the event plane calculated from hijing only (left) and event plane from the embedded event (right).

Figure 35

False reaction plane $v_2$ of the leading hadron from the embedded dijet as function of centrality. The η range used to determine the event plane is indicated in each panel. The embedded dijet is required to have a trigger hadron above 6 GeV/$c$ with a midrapidity window of $\left|\eta \right|<0.35$.

Figure 36

Per-trigger yield vs $\Delta \varphi$ for successively increasing trigger and partner $p_T$ ($p_T^a\otimes p_T^b$) in $p+p$ (open circles) and 0–20% Au+Au (filled circles) collisions. Data are scaled to the vertical axes of the four left panels. Histograms indicate elliptic flow uncertainties for Au+Au collisions.

Figure 37

Same as Fig. 36, but for 20–40% Au+Au collisions.

Figure 38

Same as Fig. 36, but for 60–92% Au+Au collisions.