Jet activity in top-quark events at $\sqrt {s}$ = 13 TeV using 3.2 fb$^{-1}$ data collected by the ATLAS detector

In this thesis, the measurement of the normalised differential cross-sections of top quark pair production in association with jets using 3.2 fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of 13 TeV by the ATLAS experiment at the LHC are presented. Jets are selected from top events which are defined by an opposite-charge electron-muon pair and two $b$-tagged jets in the final state. The cross-sections are measured as functions of several observables are sensitive to additional jets: jet multiplicities, transverse momentum of additional jets, transverse momentum sum of all objects in the event and spatial correlations of the two high- est momentum additional jets. The data are corrected to obtain particle-level fiducial cross-section. The resulting measurements are compared to several predictions allowing detailed studies of Monte Carlo QCD modelling.