Measurement of the $t\bar{t}H$ production cross-section with a collimated $H\rightarrow b\bar{b}$ decay in pp collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A measurement of the Higgs boson production cross section in association with a top-quark pair ($t\bar{t}H$) and the subsequent Higgs boson decay into a b-quark pair is presented. The events used for this analysis are characterised by one or two electrons or muons. Also in the single-lepton channel, the “boosted topology” was studied, targeting events with a Higgs boson and possibly a hadronically decaying top quark produced at a transverse momentum above 300 GeV. The analysed data was collected in pp collisions with the ATLAS detector at the LHC, with an integrated luminosity of 139 $fb^{-1}$ and at a centre-of-mass energy of 13 TeV, corresponding to the full run-2 dataset (2015–2018). The signal strength, defined as the ratio of the measured $t\bar{t}H$ signal cross section to the Standard Model expectation, is measured, assuming a Higgs boson mass of 125 GeV. For the first time, the signal strength is measured differentially, as a function of the Higgs boson transverse momentum, in the simplified template cross section framework. Results for the inclusive and differential cross section will be presented.