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

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.

💡 Research Summary

The ATLAS Collaboration presents a detailed measurement of the nuclear modification factor (R_AA) for large‑radius (R = 1.0) jets in √sₙₙ = 5.02 TeV Pb+Pb collisions, using 1.72 nb⁻¹ of heavy‑ion data and 255 pb⁻¹ of pp reference data. Large‑R jets are built by reclustering anti‑kₜ jets of radius R = 0.2 that satisfy p_T > 35 GeV and |η| < 3.0. This reclustering technique mitigates underlying‑event (UE) contamination and provides a robust kinematic definition of the jet while preserving the full jet energy.

To probe jet substructure with unprecedented angular resolution, charged‑particle tracks (p_T > 4 GeV) reconstructed in the inner detector are ghost‑associated to the large‑R jets. The tracks do not affect the jet four‑momentum because they are assigned an infinitesimal p_T (1 MeV) during clustering. The tracks are then reclustered with the kₜ algorithm (R = 2.5) and subjected to Soft Drop grooming (z_cut = 0.15, β = 0). This procedure isolates the two leading track‑based sub‑jets of each large‑R jet, defining two key observables:

• ΔR₁₂ = √