Measurement of inclusive charged-particle jet production in Au $+$ Au collisions at $\sqrt{s_{NN}}=200$ GeV

The STAR Collaboration at the Relativistic Heavy Ion Collider reports the first measurement of inclusive jet production in peripheral and central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. Jets are reconstructed with the anti-$k_T$ algorithm using charged tracks with pseudorapidity $\left|\eta \right|<1.0$ and transverse momentum $0.2<p_{T,\mathrm{jet}}^{\mathrm{ch}}<30$ $\mathrm{GeV}/c$, with jet resolution parameter $R=0.2$, 0.3, and 0.4. The large background yield uncorrelated with the jet signal is observed to be dominated by statistical phase space, consistent with a previous coincidence measurement. This background is suppressed by requiring a high-transverse-momentum (high-$p_T$) leading hadron in accepted jet candidates. The bias imposed by this requirement is assessed, and the $p_T$ region in which the bias is small is identified. Inclusive charged-particle jet distributions are reported in peripheral and central Au+Au collisions for $5<p_{T,\mathrm{jet}}^{\mathrm{ch}}<25 \mathrm{GeV}/c$ and $5<p_{T,\mathrm{jet}}^{\mathrm{ch}}<30 \mathrm{GeV}/c$, respectively. The charged-particle jet inclusive yield is suppressed for central Au+Au collisions, compared to both the peripheral Au+Au yield from this measurement and to the $pp$ yield calculated using the PYTHIA event generator. The magnitude of the suppression is consistent with that of inclusive hadron production at high $p_T$ and that of semi-inclusive recoil jet yield when expressed in terms of energy loss due to medium-induced energy transport. Comparison of inclusive charged-particle jet yields for different values of $R$ exhibits no significant evidence for medium-induced broadening of the transverse jet profile for $R <0.4$ in central Au+Au collisions. The measured distributions are consistent with theoretical model calculations that incorporate jet quenching.

Figure 1

Distribution of $p_{T,\mathrm{jet}}^{\mathrm{reco},\mathrm{ch}}$ and jet area for the inclusive charged-particle jet population ($p_{T,\mathrm{lead}}^{\min }=0$) in central Au+Au collisions. Upper panels: $p_{T,\mathrm{jet}}^{\mathrm{reco},\mathrm{ch}}$ vs. jet area for $R=0.2$ (left) and $R=0.4$ (right). Middle and lower panels: Projection onto the jet area axis. Also shown are area distributions for PYTHIA-generated (middle) and SP jets (lower) with $p_T^{\mathrm{emb}}=10$ and 20 $\mathrm{GeV}/c$, embedded into real Au+Au data for central collisions. The vertical dashed lines show the jet area cut.

Figure 2

Distribution of inclusive charged-particle jet candidates passing the jet area cut as a function of $p_{T,\mathrm{jet}}^{\mathrm{reco},\mathrm{ch}}$ for $R=0.2$, 0.3, and 0.4, in peripheral (upper) and central (lower) Au+Au collisions.

Figure 3

Distribution of $p_{T,\mathrm{jet}}^{\mathrm{reco},\mathrm{ch}}$ measured in peripheral (left) and central (right) Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, for $p_{T,\mathrm{lead}}^{\min }=0$, 3, 5, and 7 $\mathrm{GeV}/c$. Upper: $R=0.2$; middle: $R=0.3$; lower: $R=0.4$. The distributions for $p_{T,\mathrm{lead}}^{\min }=0$ are the same as those in Fig. 2.

Figure 4

Instrumental jet response: Distribution of $p_{T,\mathrm{jet}}^{\det }$ for u-quark jets generated by PYTHIA with various values of $p_{T,\mathrm{jet}}^{\mathrm{part}}$, with detector effects corresponding to those in central Au+Au collisions. Jets have $R=0.3$ and are selected with the requirement $p_{T,\mathrm{lead}}^{\min }=5\mathrm{GeV}/c$ at both the particle and detector levels. The red distributions show a Gaussian functional fit to the peak region of each distribution, with relative width of the fit as shown.

Figure 5

Jet reconstruction efficiency in peripheral (upper) and central (lower) Au+Au collisions, for $R=0.3$ and $p_{T,\mathrm{lead}}^{\min }$ = 5 $\mathrm{GeV}/c$, for u quarks and gluons in the ratio 2:1 (labeled “nominal”), pure u, pure g, and variation of the relative tracking efficiency by $\pm 5\%$ for the nominal population. The orange line shows the single-track efficiency.

Figure 6

Response matrices (RM) for charged jets with $p_{T,\mathrm{lead}}^{\min }$ = 5 $\mathrm{GeV}/c$ and $R=0.3$. Left: Detector effects $R_{\det }$; center: background fluctuations $R_{\mathrm{bkg}}$ (SP embedding); right: $R_{\mathrm{total}}=R_{\mathrm{bkg}}\times R_{\det }$.

Figure 7

$\delta p_T$ distributions calculated by embedding various types of simulated jet in central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, for $R=0.2$ (left) and $R=0.4$ (right). Upper panels: $p_T^{\mathrm{emb}}$ = 20 $\mathrm{GeV}/c$ for SP jets, SP jets with $v_2$-modulated background, and light quark jets generated by PYTHIA. Lower panels: SP jets for several values of $p_T^{\mathrm{emb}}$. See text for details.

Figure 8

Estimated magnitude of corrections for charged jets with $R=0.3$ and $p_{T,\mathrm{lead}}^{\min }$ = 5.0 $\mathrm{GeV}/c$, for central Au+Au collisions. See text for details.

Figure 9

(Quasi-)inclusive jet $p_{T,\mathrm{jet}}^{\mathrm{reco},\mathrm{ch}}$ distributions for various values of $p_{T,\mathrm{lead}}^{\min }\text{for} R=0.2$ (left) and $R=0.4$ (right), for PM-generated events and for the STAR measurements of central Au+Au collisions (data from Fig. 3). Lower panels show the ratio of the PM and data distributions, for $p_{T,\mathrm{lead}}^{\min }=0$ and $p_{T,\mathrm{lead}}^{\min }=5 \mathrm{GeV}/c$.

Figure 10

Closure test for PM-generated events corresponding to central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 11

Upper panel: Inclusive pion cross section in $pp$ collisions at $\sqrt{s}=200$ GeV from measurements [4, 55, 82] and from a PYTHIA simulation (see text for details). Lower panel: ratio of data and PYTHIA.

Figure 12

Corrected quasi-inclusive charged-particle jet distributions in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV for $R=0.2$, 0.3, and 0.4. Left: Peripheral; right: central Au+Au collisions. Upper: $p_{T,\mathrm{lead}}^{\min }$ = 5 $\mathrm{GeV}/c$; lower: $p_{T,\mathrm{lead}}^{\min }$= 7 $\mathrm{GeV}/c$. Correlated and shape systematic uncertainties are shown separately. The value of $p_{T,\mathrm{jet}}^{\mathrm{ch}}$ is shifted horizontally within each bin to account for the spectrum shape.

Figure 13

Ratio of quasi-inclusive charged-particle jet cross sections simulated by PYTHIA for $pp$ collisions at $\sqrt{s}=200$ GeV, ${\eta }_{\mathrm{jet}}=0$, for $R=0.2$ (red) and 0.4 (blue), for $p_{T,\mathrm{lead}}^{\min }$ = 5 $\mathrm{GeV}/c$ relative to the unbiased distribution (“5/0,” dashed) and $p_{T,\mathrm{lead}}^{\min }$ = 7 $\mathrm{GeV}/c$ relative to $p_{T,\mathrm{lead}}^{\min }$ = 5 $\mathrm{GeV}/c$ (“7/5,” solid).

Figure 14

Ratio of distributions from Fig. 12 for $p_{T,\mathrm{lead}}^{\min }$= 7 $\mathrm{GeV}/c$ and 5 $\mathrm{GeV}/c$, for $R$= 0.2, 0.3, and 0.4 in peripheral (upper) and central (lower) Au+Au collisions. The red lines show the corresponding ratios from a PYTHIA simulation of $pp$ collisions (Fig. 13).

Figure 15

$R_{\mathrm{CP}}$ [(0–10%)/(60–80%)] for Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, for $R=0.2$, 0.3 and 0.4. See text for details.

Figure 16

$R_{\mathrm{CP}}$ distributions from Fig. 15 compared to that measured in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV [34], for $R=0.2$ (left) and $R=0.3$ (right). Also shown are $R_{\mathrm{CP}}$ for inclusive charged hadrons in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV [15] and in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV [94]. Data from RHIC are in blue; data from the LHC are in red. The charged hadrons $R_{\mathrm{CP}}$ distributions are the same in the two panels. The different choices of centrality class are discussed in the text.

Figure 17

$R_{AA}^{\mathrm{Pythia}}$ for quasi-inclusive charged jets in peripheral (upper) and central (lower) Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, for $R=0.2$, 0.3, and 0.4. The reference spectrum for $pp$ collisions at $\sqrt{s}=200$ GeV is generated by PYTHIA; see text for details. The region where the bias due to the $p_{T,\mathrm{lead}}^{\min }$ cut is small is indicated by the vertical dashed line.

Figure 18

Comparison of $R_{AA}^{\mathrm{Pythia}}$ from Fig. 17 (stars) to charged hadron [15, 102] and ${\pi }^0$[19] $R_{AA}$ at $\sqrt{s_{NN}}=200$ GeV. Only points from the region where the bias in the data due to the $p_{T,\mathrm{lead}}^{\min }$ cut is small are shown.

Figure 19

Comparison of $R_{AA}^{\mathrm{Pythia}}$ from Fig. 17 (stars) to the theoretical calculations described in Sect. 10. Only data in the unbiased region are shown. The Hybrid, LBT, and LIDO calculations are for charged-particle jets, while NLO and SCET are for fully reconstructed jets. The LBT and LIDO calculations account for the effect of $p_{T,\mathrm{lead}}^{\min }$ = 5 $\mathrm{GeV}/c$ in the Au+Au spectrum.

Figure 20

Ratio of quasi-inclusive yields for charged jets with $R=0.2$ and $R=0.4$, for peripheral (left) and central (right) Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV and $p_{T,\mathrm{lead}}^{\min }=5$ GeV/$c$. Calculations are shown for charged jets in $pp$ collisions at $\sqrt{s}=200$ GeV simulated by PYTHIA [83] and HERWIG [110]; both panels show the same distributions from these calculations. Also shown are predictions for reconstructed jets in Au+Au collisions from the theoretical calculations described in Sect. 10. The region where the bias due to the $p_{T,\mathrm{lead}}^{\min }$ cut is small is indicated by the vertical dashed line.