Measurement of jet substructure in boosted $t\bar{t}$ events with the ATLAS detector using 140 fb$^{-1}$ of 13 TeV $pp$ collisions

Measurements of the substructure of top-quark jets are presented using top quarks produced in $pp$ interactions at 13 TeV and recorded with the ATLAS detector. Top-quark jets reconstructed with the anti-$kt$ algorithm with a radius parameter $R=1.0$ are selected in top-quark pair ($t\bar{t}$) events with one top quark decaying semileptonically and the other hadronically, and in $t\bar{t}$ events where both top quarks decay hadronically. The top-quark jets are required to have transverse momentum ($p_{\mathrm{T}}$) ${>}$ 350 GeV, creating high-statistics data samples for jets with $p_{\mathrm{T}}$ between 350 and 600 GeV. The one- and two-dimensional differential cross-sections for eight substructure variables, defined using only the charged components of the jets, are measured in a particle-level phase space by correcting for the smearing and acceptance effects induced by the detector. The differential cross-sections are compared to several Monte Carlo (MC) predictions obtained matching top-quark pair production quantum chromodynamic (QCD) predictions at next-to-leading order (NLO) precision in the strong coupling constant $\alpha_S$ to leading order (LO) parton shower and hadronization generators. The QCD predictions for measures of energy-flow are found to be in good agreement with the measurements while variables sensitive to the three-body structure of the top-quark jets exhibit some tensions with the measured distributions.