Measurement of substructure-dependent suppression of large-radius jets with charged particles in Pb+Pb collisions with ATLAS

TL;DR

This ATLAS study investigates how the suppression of large-radius jets in Pb+Pb collisions depends on jet substructure by using large-$R$ jets ($R=1.0$) formed from reclustering $R=0.2$ calorimeter jets and defining two leading track-based sub-jets with improved angular resolution. The jet suppression is quantified via $R_{AA}$ as a function of jet transverse momentum, the charged-splitting scale $\sqrt{d_{12}}$, and the angular separation $\Delta R_{12}$, with results enabling direct constraints on color-coherence effects in the quenching process. The analysis employs 1.72 nb$^{-1}$ Pb+Pb and 255 pb$^{-1}$ pp data at $\sqrt{s_{NN}}=5.02$ TeV, using Bayesian unfolding to correct detector effects and Soft Drop grooming to suppress underlying-event contributions. The results show stronger suppression for jets with larger $\sqrt{d_{12}}$ and a centrality- and angular-separation-dependent pattern in $R_{AA}$, with a notable inflection near $\Delta R_{12}\approx 0.1$. Comparisons with theoretical models indicate sensitivity to the medium's resolution scale, disfavoring extreme values of the coherence length and supporting a finite $L_{ ext{res}}$, thus providing valuable insights into color coherence in jet quenching and guiding future refinements of quenching models.

Abstract

Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions. This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of $R = 1.0$ and its dependence on the jet substructure. The measurement uses 1.72 nb$^{-1}$ of Pb+Pb data and 255 pb$^{-1}$ of $pp$ data, both at $\sqrt{s_{_\mathrm{NN}}} = 5.02$ TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering $R = 0.2$ calorimetric jets and are measured for transverse momentum above $200$ GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ($R_\mathrm{AA}$). The jet $R_\mathrm{AA}$ is measured as a function of jet $p_{\mathrm{T}}$, the charged $k_t$ splitting scale ($\sqrt{d_{12}}$), and the angular separation ($ΔR_{12}$) of two leading sub-jets. The jet $R_\mathrm{AA}$ gradually decreases with increasing $\sqrt{d_{12}}$, implying significantly stronger suppression of large-radius jets with larger $k_t$ splitting scale. The jet $R_\mathrm{AA}$ gradually decreases for $ΔR_{12}$ in the range $0.01{-}0.2$ and then remains consistent with a constant for $ΔR_{12} \gtrsim 0.2$. The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.

Figures (5)

• Figure 1: Measured cross-sections in collisions and $\langle T_{\text{AA}}\xspace\rangle$-normalized per-event yields in Pb+Pb collisions, evaluated as a function of (a) $\sqrt{d_{12}}$ and (b) $\Delta R_{12}$, for reclustered $R=1.0$ jets in the interval $200{-}251$ GeV. Normalized yields are shown for five centrality bins of Pb+Pb collisions. The vertical bars on the data points indicate statistical uncertainties, while the shaded boxes indicate systematic uncertainties. The shown systematic uncertainties do not include the uncertainties in the luminosity and $\langle T_{\text{AA}}\xspace\rangle$ determinations, which are correlated for all the data points and are listed in Section \ref{['sec:uncert']}.
• Figure 2: The $R_{\ce{AA}}$ of $R=1.0$ jets evaluated as a function of (a) $\sqrt{d_{12}}$ and (b) $\Delta R_{12}$ in five centrality bins and the interval $200{-}251$ GeV. The vertical bars on the data points indicate statistical uncertainties, while the shaded boxes indicate systematic uncertainties. The coloured boxes at $R_{\ce{AA}}\xspace\ = 1$ represent the fractional per-centrality uncertainty in $\langle T_{\text{AA}}\xspace\rangle$ and $pp$ luminosity (magenta), which both affect the overall normalization.
• Figure 3: The $R_{\ce{AA}}$ of $R=1.0$ jets in the most central 0--10% collisions evaluated as a function of jet in six intervals of (a) $\sqrt{d_{12}}$ and (b) $\Delta R_{12}$. The vertical bars on the data points indicate statistical uncertainties, while the shaded boxes indicate systematic uncertainties. The coloured boxes at $R_{\ce{AA}}\xspace\ = 1$ represent the fractional per-centrality uncertainty in $\langle T_{\text{AA}}\xspace\rangle$ and $pp$ luminosity (magenta), which both affect the overall normalization.
• Figure 4: The $R_{\ce{AA}}$ of $R=1.0$ jets evaluated as a function of $\Delta R_{12}\xspace$ compared with measurements from Ref. HION-2019-09 ($R=1.0$ jets with $R=0.2$ constituents used to define sub-jets) and Ref. HION-2021-09 ($R=0.4$ jets with charged-particle tracks matched to calorimeter energy clusters used to define sub-jets). Results are shown in the interval $200{-}251$ GeV for this measurement and the measurement in Ref. HION-2019-09, and $200{-}315$ GeV for the measurement in Ref. HION-2021-09. The inset displays the same distributions using a logarithmic scale, highlighting features at lower $\Delta R_{12}\xspace$ values. The vertical bars on the data points indicate statistical uncertainties, while the shaded boxes indicate systematic uncertainties. The coloured boxes at $R_{\ce{AA}}\xspace\ = 1$ represent the fractional uncertainty in $\langle T_{\text{AA}}\xspace\rangle$ in 0--10% Pb+Pb collisions (green) and $pp$ luminosity (magenta), which both affect the overall normalization.
• Figure 5: The $R_{\ce{AA}}$ of $R=1.0$ jets as a function of $\Delta R_{12}\xspace$ is compared to theoretical calculations from the Hybrid model Kudinoor:2025ilxCasalderrey-Solana:2014bpaHulcher:2017cpt. Results are shown for $200{-}251$ GeV jets with $|\eta|<1.3$ in 0--10% Pb+Pb collisions. The theory is displayed as coloured bands corresponding to resolution lengths, $L_{\mathrm{res}}=0$, $1/(\pi T)$, $2/(\pi T)$, and $L_{\mathrm{res}}\!\to\!\infty$. The vertical bars on the data points indicate statistical uncertainties, while the shaded boxes indicate systematic uncertainties. The coloured boxes at $R_{\ce{AA}}\xspace=1$ represent the fractional uncertainties in $\langle T_{\text{AA}}\xspace\rangle$ (green) and the $pp$ luminosity (magenta), which affect the overall normalisation.