Measurements of the production cross-section of top-antitop quark pairs with additional jets provide important tests of quantum chromodynamics (QCD) predictions.
The ATLAS experiment has measured several final state observables that are sensitive to additional radiation in top-quark-antiquark final states at 13 TeV. These measurements are compared to predictions of modern Monte Carlo generators based on NLO QCD matrix element or LO multi-leg matrix elements.
The process of a ttbar pair produced in association with jets originating from b-quarks (b-jets) is particularly important to measure, as there are many uncertainties in the calculation of the process due to the relevance of multiple energy scales. The ATLAS Collaboration has performed fiducial cross-section measurements in the dilepton and lepton-plus-jets ttbar decay channels. Results are presented at particle level in the form of inclusive cross-sections of ttbar final states with three and four b-jets as well as differential cross-sections as a function of global event properties and properties of b-jet pairs.