Modelling of rare top quark processes at $\sqrt{s}$ = 13 TeV in ATLAS

This note documents modelling studies for some rare processes involving top quarks carried out by the ATLAS Collaboration. Different Monte Carlo event generators for five processes: $t\bar{t}W$, $t\bar{t}Z/\gamma^*$, $t\bar{t}H$, $t\bar{t}t\bar{t}$ and $tWZ$ have been compared. Next-to-leading-order multileg merged configurations and rare electroweak production mechanisms have been studied. The impact of the choice of the renormalization and factorization scales on the predicted cross-sections and selected observables is presented and discussed. These studies aim to improve ATLAS Monte Carlo simulations and reduce the systematic theoretical modelling uncertainties, which are already a limiting factor in several experimental LHC results.