  The Lund jet plane (LJP) is measured for the first time in $t\overline{t}$
events, using 140 fb$^{-1}$ of $\sqrt{s}$ = 13 TeV pp collision data collected
with the ATLAS detector at the LHC. The LJP is a two-dimensional observable of
the sub-structure of hadronic jets that acts as a proxy for the kinematics of
parton showers and hadron formation. The observable is constructed from charged
particles and is measured for R = 1.0 anti-$k_t$ jets with transverse momentum
above 350 GeV containing the full decay products of either a top quark or a
daughter $W$ boson. The other top quark in the event is identified from its
decay into a b-quark, an electron or a muon and a neutrino. The measurement is
corrected for detector effects and compared with a range of Monte Carlo
predictions sensitive to different aspects of the hadronic decays of the heavy
particles. In the $W$-boson-initiated jets, all the predictions are
incompatible with the measurement. In the top quark initiated jets,
disagreement with all predictions is observed in smaller subregions of the
plane, and with a subset of the predictions across the fiducial plane. The
measurement could be used to improve the tuning of Monte Carlo generators, for
better modelling of hadronic decays of heavy quarks and bosons, or to improve
the performance of jet taggers.
