Measurements of observables sensitive to colour reconnection in $t\bar{t}$ events with the ATLAS detector at $\sqrt{s}=13$ TeV

A measurement of observables sensitive to effects of colour reconnection intop-quark pair-production events is presented using 139 fb$^{-1}$ of 13$\,$TeVproton-proton collision data collected by the ATLAS detector at the LHC. Eventsare selected by requiring exactly one isolated electron and one isolated muonwith opposite charge and two or three jets, where exactly two jets are requiredto be $b$-tagged. For the selected events, measurements are presented for thecharged-particle multiplicity, the scalar sum of the transverse momenta of thecharged particles, and the same scalar sum in bins of charged-particlemultiplicity. These observables are unfolded to the stable-particle level,thereby correcting for migration effects due to finite detector resolution,acceptance and efficiency effects. The particle-level measurements are comparedwith different colour reconnection models in Monte Carlo generators. Thesemeasurements disfavour some of the colour reconnection models and provideinputs to future optimisation of the parameters in Monte Carlo generators.