Highlights of top-quark production cross-section measurements with ATLAS at LHC: from precision to rarity

Top-quark production in proton-proton collisions at a center-of-mass energy of 13 TeV is measured with the data collected by the ATLAS detector over five orders of magnitude. The relatively large inclusive cross-section for the production of top-quark pair production is deter- mined using events in the lepton+jets channel and it reaches a relative uncertainty of 4.6% allowing for experimental scrutiny to theoretical calculations at next-to-next-to-leading order. Measurements of differential cross-sections of top-quark-antiquark pair-production performed in the all-hadronic channel allow studies of the correlation between the top-quark pair system and additional jet radiation by exploiting fully-reconstructed final states. Such measurements are com- pared quantitatively with predictions from several setups of next-to-leading order matrix-element generators combined with parton-shower generators. Finally, the ATLAS collaboration recently established first evidence for the hard scattering process in which two top-quark-antiquark pairs are produced. This process is also called four-top-quarks production and is predicted to have a small cross-section of 12 fb in the standard model. Candidate events are selected if a lepton pair with the same electric charge is present or if there are at least three leptons in the event. The background is mainly given by top-quark-antiquark production in association with a W boson and heavy-flavour jets. A multivariate discriminant is used to optimize the separation between signal and background events and enhance the sensitivity.