Measurement of substructure-dependent jet suppression in Pb+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 Pb+Pb collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_{{}_{\mathrm{N}\mathrm{N}}}}=5.02\mathrm{T}\mathrm{e}\mathrm{V}$ in comparison with $pp$ collisions at $\sqrt{s}=5.02\mathrm{T}\mathrm{e}\mathrm{V}$. The Pb+Pb data, collected in 2018, have an integrated luminosity of $1.72\mathrm{n}{\mathrm{b}}^{-1}$, while the $pp$ data, collected in 2017, have an integrated luminosity of $260{\mathrm{p}\mathrm{b}}^{-1}$. 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, $R_{\mathrm{A}\mathrm{A}}$, used to characterize jet suppression in Pb+Pb collisions, is presented differentially in $r_{\mathrm{g}}$, jet transverse momentum, and in intervals of collision centrality. The $R_{\mathrm{A}\mathrm{A}}$ 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 Pb+Pb collisions. The $R_{\mathrm{A}\mathrm{A}}$ 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 $R_{\mathrm{A}\mathrm{A}}$ is qualitatively consistent with a picture of jet quenching arising from coherence and provides the most direct evidence in support of this approach.