Study of the underlying event in top quark pair production at 13 TeV

Normalized differential cross sections as functions of the multiplicity and kinematic variables of charged-particle tracks from the underlying event are measured in top quark and antiquark pair events produced in proton-proton collisions at a center-of-mass energy of 13 TeV. The analysis is based on data collected by the CMS experiment at the LHC in 2016 and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The selected events contain one electron, one muon, and two jets from the hadronization and fragmentation of b quarks. These measurements characterize, for the first time, the properties of the underlying event at a factorization scale which is typically above twice the top quark mass. The sensitivity of the measured cross sections to different parameters employed in state-of-the-art Monte Carlo simulation programs is demonstrated by comparing the results with different simulations.