Measurement of substructure-dependent jet suppression in $\mathrm{Pb}+\mathrm{Pb}$ collisions at 5.02 TeV with the ATLAS detector

The ATLAS detector at the Large Hadron Collider has been used to measure jet substructure modification and suppression in $\mathrm{Pb}+\mathrm{Pb}$ collisions at a nucleon–nucleon center-of-mass energy $\sqrt{s_{{}_{NN}}}=5.02\mathrm{TeV}$ in comparison with proton–proton ($pp$) collisions at $\sqrt{s}=5.02\mathrm{TeV}$. The $\mathrm{Pb}+\mathrm{Pb}$ data, collected in 2018, have an integrated luminosity of $1.72{\mathrm{nb}}^{-1}$, while the $pp$ data, collected in 2017, have an integrated luminosity of $260{\mathrm{pb}}^{-1}$. Jets used in this analysis are clustered using the anti-$k_t$ algorithm with a radius parameter $R=0.4$. The jet constituents, defined by both tracking and calorimeter information, are used to determine the angular scale $r_{\mathrm{g}}$ of the first hard splitting inside the jet by reclustering them using the Cambridge–Aachen algorithm and employing the soft-drop grooming technique. The nuclear modification factor, $R_{AA}$, used to characterize jet suppression in $\mathrm{Pb}+\mathrm{Pb}$ collisions, is presented differentially in $r_{\mathrm{g}}$, jet transverse momentum, and in intervals of collision centrality. The $R_{AA}$ value is observed to depend significantly on jet $r_{\mathrm{g}}$. Jets produced with the largest measured $r_{\mathrm{g}}$ are found to be twice as suppressed as those with the smallest $r_{\mathrm{g}}$ in central $\mathrm{Pb}+\mathrm{Pb}$ collisions. The $R_{AA}$ values do not exhibit a strong variation with jet $p_{\mathrm{T}}$ in any of the $r_{\mathrm{g}}$ intervals. The $r_{\mathrm{g}}$ and $p_{\mathrm{T}}$ dependence of jet $R_{AA}$ is qualitatively consistent with a picture of jet quenching arising from coherence and provides the most direct evidence in support of this approach.

Figure 1

Top: Self-normalized $r_{\mathrm{g}}$ distributions measured using detector-level TCCs from $pp$ data at $\sqrt{s}=5.02\text{TeV}$ (left) and Pb+Pb data at $\sqrt{s_{{}_{NN}}}=5.02\text{TeV}$ (second and third panels) for different centrality intervals, compared with the detector-level distributions predicted by the Pythia 8 generator. The error bars represent statistical uncertainties. Bottom: Data-to-MC ratios of $r_{\mathrm{g}}$ distributions at detector level. The legend applies to all the panels.

Figure 2

The relative systematic uncertainties in inclusive $p_{\text{T}}^{\text{jet}}$ cross-sections in $pp$ collisions at $\sqrt{s}=5.02\text{TeV}$ (left) and in per-event jet yields in Pb+Pb collisions at $\sqrt{s_{{}_{NN}}}=5.02\text{TeV}$ for different event centralities (middle and right panels). The legend applies to all of the panels.

Figure 3

The relative systematic uncertainties in inclusive $r_{\mathrm{g}}$ cross-sections in $pp$ collisions at $\sqrt{s}=5.02\text{TeV}$ (left) and in per-event jet yields in Pb+Pb collisions at $\sqrt{s_{{}_{NN}}}=5.02\text{TeV}$ for different event centralities (middle and right panels) shown for soft-drop parameters $z_{\mathrm{cut}}=0.2$ and $\beta =0$. The legend applies to all of the panels.

Figure 4

The relative systematic uncertainties in the $R_{AA}$ measurements as a function of $p_{\text{T}}^{\text{jet}}$ in different centrality intervals of Pb+Pb collisions at $\sqrt{s_{{}_{NN}}}=5.02\text{TeV}$. The legend applies to all of the panels.

Figure 5

The relative systematic uncertainties in the $R_{AA}$ measurements as a function of $r_{\mathrm{g}}$ in different centrality intervals of Pb+Pb collisions at $\sqrt{s_{{}_{NN}}}=5.02\text{TeV}$ shown for soft-drop parameters $z_{\mathrm{cut}}=0.2$ and $\beta =0$. The legend applies to all of the panels.

Figure 6

Differential cross-section of jets passing the soft-drop grooming condition in different $r_{\mathrm{g}}$ intervals in $pp$ collisions at $\sqrt{s}=5.02\text{TeV}$ as a function of $p_{\text{T}}^{\text{jet}}$. Distributions for inclusive jets (no grooming) and for those jets that fail the soft-drop requirement ($r_{\mathrm{g}}=0$) are also shown. For visual clarity, the distributions are scaled by factors specified in the legend. The statistical uncertainties are smaller than the symbols, while shaded bars represent systematic uncertainties. Note that the highest $p_{\text{T}}^{\text{jet}}$ bin does not contain the overflow and that the 1.6% luminosity uncertainty is not shown. The $pp$ jet cross-sections are compared with predictions from three MC generators, Pythia 8 (top), Herwig (bottom left), and Sherpa (bottom right). The ratios of jet cross-section predictions from different MC generators to the unfolded data are shown in the lower panels. The statistical uncertainties on the ratios are smaller than the line widths, while shaded bars in the lower panels represent systematic uncertainties in inclusive cross-section ratios.

Figure 7

Differential cross-section of jets passing the soft-drop grooming condition in $pp$ collisions at $\sqrt{s}=5.02\text{TeV}$ as a function of $r_{\mathrm{g}}$ is shown for different $p_{\text{T}}^{\text{jet}}$ intervals and also for $p_{\text{T}}^{\text{jet}}>158$ GeV. The error bars represent statistical uncertainties, and are smaller than the marker size here, while the shaded bars represent systematic uncertainties. The $1.6\%$ uncertainty in the $pp$ luminosity is not shown. The last bin does not contain the overflow. The ratios of jet cross-section predictions from different MC generators to the unfolded data are shown in the lower panels. The shaded bars in the lower panels represent systematic uncertainties in cross-section ratios.

Figure 8

The per-event inclusive jet yields in Pb+Pb collisions at $\sqrt{s_{{}_{NN}}}=5.02\text{TeV}$ normalized by $\langle T_{\text{AA}}\rangle$ as a function of $p_{\text{T}}^{\text{jet}}$ in four centrality intervals. Distributions for inclusive jets (no grooming) and for those jets that fail the soft-drop requirement (denoted by $r_{\mathrm{g}}=0$) are also shown. For visual clarity, the distributions are scaled by factors specified in the legend. The last bin does not contain the overflow. The statistical uncertainties are indicated by the error bars while the systematic uncertainties are indicated by the shaded bars. The normalization uncertainties of $\langle T_{\text{AA}}\rangle$ for each centrality interval are not shown, but are listed in Table 1.

Figure 9

The per-event jet yields in Pb+Pb collisions at $\sqrt{s_{{}_{NN}}}=5.02\text{TeV}$ normalized by $\langle T_{\text{AA}}\rangle$ as a function of soft-drop $r_{\mathrm{g}}$ in four centrality intervals and three $p_{\text{T}}^{\text{jet}}$ intervals as well for $p_{\text{T}}^{\text{jet}}>158$ GeV. The last bin does not contain the overflow. The statistical uncertainties are indicated by the error bars while the systematic uncertainties are indicated by the shaded bars. The normalization uncertainties of $\langle T_{\text{AA}}\rangle$ for each centrality interval are not shown, but are listed in Table 1.

Figure 10

Nuclear modification factor, $R_{AA}$, as a function of $p_{\text{T}}^{\text{jet}}$ for soft-drop groomed jets with $\left|y\right|<2.1$ in four centrality intervals and four intervals of $r_{\mathrm{g}}$. Groomed jet $R_{AA}$ values are compared with $R_{AA}$ values of jets without significant splitting identified by the soft-drop procedure ($r_{\mathrm{g}}=0$) and jets without grooming (inclusive). The error bars represent statistical uncertainties while the shaded bars represent bin-wise correlated systematic uncertainties. The uncertainties in the $pp$ luminosity ($1.6\%$) and $\langle T_{\text{AA}}\rangle$ are not included, but are listed in Table 1.

Figure 11

Nuclear modification factor, $R_{AA}$, as a function of $r_{\mathrm{g}}$ for soft-drop groomed jets with $\left|y\right|<2.1$ in four centrality intervals and three intervals of $p_{\text{T}}^{\text{jet}}$, in comparison with the $p_{\text{T}}$-inclusive results. The error bars represent statistical uncertainties while the shaded bars represent bin-wise correlated systematic uncertainties. The uncertainties in the $pp$ luminosity ($1.6\%$) and $\langle T_{\text{AA}}\rangle$ are not included, but are listed in Table 1.

Figure 12

Comparison of $R_{AA}$ as a function of (left) $p_{\text{T}}^{\text{jet}}$ for inclusive jets and for four intervals of $r_{\mathrm{g}}$, (right) $r_{\mathrm{g}}$ for inclusive jets and for three intervals of $p_{\text{T}}^{\text{jet}}$, and in three centrality intervals of Pb+Pb events with predictions from the JETSCAPE framework [43, 91]. The error bars and the open boxes around the data points represent the statistical and systematic uncertainties, respectively. The uncertainties in the $pp$ luminosity ($1.6\%$) and $\langle T_{\text{AA}}\rangle$ are not included, but are listed in Table 1. The widths of the bands representing the JETSCAPE predictions indicate their statistical uncertainties.

Figure 13

Comparison of $R_{AA}$ as a function of (left) $p_{\text{T}}^{\text{jet}}$ for inclusive jets and for four intervals of $r_{\mathrm{g}}$ and (right) as a function of $r_{\mathrm{g}}$ for inclusive jets in 0–10% centrality Pb+Pb events with theoretical predictions from the pQCD framework of Caucal et al., described in Refs. [41, 42, 92]. The error bars and the open boxes around the data points represent the statistical and systematic uncertainties, respectively. The uncertainties in the $pp$ luminosity ($1.6\%$) and $\langle T_{\text{AA}}\rangle$ are not included, but are listed in Table 1. The shaded areas around the theoretical predictions represent the uncertainties arising from the variation of the transport coefficient ($\widehat{q}$) and shower cutoff parameters.

Figure 14

Comparison of $R_{AA}$ as a function of $r_{\mathrm{g}}$ for three intervals of $p_{\text{T}}^{\text{jet}}$ in 0–10% centrality Pb+Pb events with theoretical predictions from Ringer et al. [93]. The error bars and the open boxes around the data points represent the statistical and systematic uncertainties, respectively. The uncertainties in the $pp$ luminosity ($1.6\%$) and $\langle T_{\text{AA}}\rangle$ are not included, but are listed in Table 1. The green curve (med q/g) represents the quark and gluon differential energy loss setting. The red curve (med q/g $+ p_{\text{T}}$ broadening) adds transverse momentum broadening effects for all subjets in the medium.