Recent highlights of top-quark physics with the ATLAS Experiment - FPCP2020

Four recent measurements in different areas of top-quark physics are presented. Comprehensive measurements of differential cross-sections of top-quark-antiquark pair-production are performed in the all-hadronic channel. The top-quarks and top-quark-pairs are fully reconstructed and kinematic distributions of these objects are measured at the particle and at the parton level. The measurements in the all-hadronic channel complement measurements previously performed in the lepton+jets channels in terms of range and resolution. The cross sections for the production of top quark pairs in association to a photon (ttgamma) or to a Z boson (ttZ) are measured both inclusively and differentially as a function of kinematic variables characterizing the tt+boson system. Both sets of measurements use the full Run2 data set consisting of 139/fb of integrated luminosity. Final states with three and four leptons and b-jets are used to extract ttZ rates, while tt+gamma cross sections are derived from final states with one photon, one electron and one muon of opposite sign and at least two jets. The measurements are compared to predictions obtained by NLO+PS Monte Carlo and fixed order NLO calculations. The hard scattering process in which two top-quark-antiquark pairs are produced is also called four-top-quarks production and is predicted to have a small cross-section of 12 fb in the standard model. This very rare process has not been observed yet. The background is mainly given by top-quark-antiquark production in association with heavy flavor jets. In this presentation, two analyses are presented which aim to establish experimental evidence for this process based on the full Run 2 dataset recorded with the ATLAS detector. The first analysis selects events with exactly one charged lepton and several jets or two charged leptons of opposite electric charge. The second analysis is based on a lepton pair with the same electric charge or events with more than two leptons. In both channels multivariate techniques are used to optimize the separation between signal and background events and enhance the sensitivity. Finally, both channels are combined.