Measurement of jet substructure observables in $\mathrm{t \bar t}$ events from pp collisions at $\sqrt{s}=13~\mathrm{TeV}$

A measurement of differential jet substructure observables is presented using $\mathrm{t \bar t}$ lepton+jets events from proton-proton collisions at $\sqrt{s}=13~\mathrm{TeV}$ recorded by the CMS experiment at the LHC in 2016 corresponding to an integrated luminosity of $35.9~\mathrm{fb^{-1}}$. Multiple jet substructure variables, such as the particle multiplicity, width, eccentricity, $p_\mathrm{T}$ dispersion, N-subjettiness ratios, generalized angularities, and energy correlation functions, are measured for inclusive jets, as well as for identified bottom, light-quark, and gluon jets from the $\mathrm{t \bar t}$ final state. The results are unfolded to the stable-particle level and compared to predictions from POWHEG interfaced with PYTHIA 8 and HERWIG 7.1, as well as from SHERPA 2 and DIRE. A reasonable agreement between the data and the Monte Carlo predictions is found. From a comparison of the jet width distribution to the prediction, it is shown that a lower value of the effective strong coupling in the PYTHIA 8 final-state parton shower is preferred.