Measurement of ttZ and ttW production at ATLAS in 13 TeV data, using trilepton and same charge dimuon final states

The production of $t \bar t Z$ serves as an important probe of the coupling of top quarks to Z bosons. Modifications to the $t \bar t Z$ vertex by physics beyond the Standard Model would have an effect on the $t \bar t Z$ production rates. The first measurements of $t \bar t Z$ production were performed in 8 TeV data by the ATLAS and CMS experiments at the LHC. At 13 TeV collision energy, the production rate of $t \bar t Z$ increases by almost a factor of 4 and the effects of new physics, if present, should become more prominent. The first measurement by ATLAS of $t \bar t Z$ production at 13 TeV is presented here, with focus on the selection region that requires three charged leptons. This signature constitutes a compromise between a higher branching ratio for hadronic top decay chains and the better separation from background of the leptonic top decays. Moreover, the $t \bar t W$ process is measured, using final states that contain either three charged leptons or two muons of the same charge. The first observation of $t \bar t W$ was performed by ATLAS using 8 TeV data. Measuring this process can probe several different models for physics beyond the Standard Model.