Substructure grooming of inclusive and photon-tagged jets in heavy-ion collisions

TL;DR

This work analyzes how jet grooming techniques, specifically Dynamical Grooming and Soft-Drop Grooming, reveal medium-induced modifications of jet substructure in PbPb collisions. Using the SHELL transport model, the authors disentangle energy-loss effects and medium response from selection biases, showing that high-$k_{ m T,g}$ suppression in inclusive jets arises from reduced $p_{T,2}$ and $R_g$ narrowing, with jet-by-jet matching exposing genuine broadening. Photon-tagged jets ($\gamma+$jets) are shown to exhibit more direct evidence of substructure broadening via $R_g$, which grows with jet radius and is dominated by medium-induced radiation; hadronization effects temper the observed broadening. The results emphasize $R_g$ as a robust observable for probing jet–QGP interactions and provide comprehensive predictions across grooming parameters, jet radii, and hadronization levels to guide experimental measurements.

Abstract

Jet substructure provides a powerful probe of partonic interactions within the quark-gluon plasma (QGP) in heavy-ion collisions. In this paper, we present a systematic theoretical study of the groomed substructures for both inclusive jets and photon-tagged jets ($γ+$jets) utilizing the Dynamical and Soft-Drop Grooming algorithms in PbPb collisions by employing the SHELL transport model. Our theoretical calculations exhibit a suppression at high $k_{\rm T,g}$, the relative transverse momentum between the two subjets in the groomed substructure, consistent with the recent ALICE measurements. We show that the suppression of high $k_{\rm T,g}$ arises from the combined effects of the reduction of the subleading subjet transverse momentum due to partonic energy loss and the narrowing of the groomed jet radius $R_g$ induced by selection bias. Our findings demonstrate that no enhancement is observed at high $k_{\rm T,g}$, even in the complete absence of selection bias. Furthermore, we propose that the broadening of $R_g$ in photon-tagged jets, which are less susceptible to selection bias compared to inclusive jets, provides relatively direct evidence of the jet substructure broadening. Our analysis reveals that the $R_g$ broadening becomes more pronounced as the jet radius increases, where the medium-induced gluon radiation plays a dominant role in driving such broadening. In particular, we find that as the jet radius increases, the Soft Drop grooming algorithm exhibits a better resolving power for the contribution of the medium response to the jet substructure broadening.

Figures (9)

• Figure 1: (Color online) $k_{\rm T, g}$ distribution of the groomed inclusive jets in pp collisions at $\sqrt{s}=5.02$ TeV utilizing both the Dynamical Grooming ($a=1.0$) and Soft-Drop Grooming ($z_{\rm cut}=0.2$) algorithms, compared to the the ALICE data ALICE:2024fip.
• Figure 2: (Color online) 2-dimensional ($z_g, R_g$) distribution of inclusive jets in pp collisions at $\sqrt{s}=5.02$ TeV computed using the Dynamical Grooming ($a=1.0$) and Soft-Drop Grooming ($z_{\rm cut}=0.2$) in the top and bottom figures.
• Figure 3: (Color online) $k_{\rm T, g}$ distributions of the groomed inclusive jets in pp and PbPb (left panel: $0-10\%$, right panel: $30-50\%$) collisions at $\sqrt{s_{NN}}=5.02$ TeV utilizing both the Dynamical Grooming (top panels) and Soft-Drop Grooming (middle panels) algorithms, compared to the recent experimental data measured by the ALICE Collaboration ALICE:2024fip. Each distribution is normalized by the total jet cross section in the $p_T$ interval $[60, 80]$ GeV. The ratios of PbPb/pp are also shown in the bottom panels.
• Figure 4: (Color online) $k_{\rm T, g}$ distributions of the groomed inclusive jets in pp and $0-10\%$ PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV utilizing both the Dynamical Grooming (left plot) and Soft-Drop Grooming (right plot) algorithms. The calculations by the JBJ matching analysis are also shown for comparison. The ratios of PbPb/pp are also shown in the bottom panels.
• Figure 5: (Color online) Normalized distributions of $p_{T,2}$ and $R_g$ of the groomed inclusive jets in pp and $0-10\%$ PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV utilizing both the Dynamical Grooming (left plot) and Soft-Drop Grooming (right plot) algorithms. The calculations by the JBJ matching analysis are also shown for comparison.