Measurements of $b$-jet moments sensitive to $b$-quark fragmentation in $t \bar{t}$ events at the LHC with the ATLAS detector

The measurement of several observables sensitive to the fragmentation of $b$-quarks with 36 fb${}^{-1}$ of $\sqrt s = 13$ TeV LHC data are presented. Jets containing $b$-hadrons are obtained from a sample of dileptonic $t \bar{t}$ events, and the associated set of charged-particle tracks is separated into those from the primary $pp$ interaction vertex and those from the displaced $b$-decay secondary vertex. This division is used to construct high-resolution observables which characterize the longitudinal and transverse momentum distributions of the $b$-hadron within the $b$-jet. The measurements have been corrected for detector effects and provide a test of heavy-quark-fragmentation modelling at the LHC with color-connection via the top-quark decay to the proton beam remnants. The unfolded distributions are compared to the predictions of modern Monte Carlo parton-shower generators, and these predictions are found to be largely in agreement with the observed data. These measurements complement similar measurements from $e^+e^-$ collider experiments in which the $b$-quarks originate from a color-singlet $Z/\gamma^*$.