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Theoretical predictions for ttbb production are of crucial importance for ttHmeasurements in the H->bb channel at the LHC. To address the largeuncertainties associated with the modelling of extra QCD radiation in ttbbevents, in this paper we present a calculation of pp->ttbbj at NLO QCD. Thebehaviour of NLO corrections is analysed in a variety of observables, and toassess theoretical uncertainties we use factor-two rescalings as well asdifferent dynamic scales. In this context, we propose a systematic alignment ofdynamic scales that makes it possible to disentangle normalisation and shapeuncertainties in a transparent way. Scale uncertainties at NLO are typically atthe level of 20-30% in fiducial cross sections, and below 10% for the shapes ofdistributions. The kinematics of QCD radiation is investigated in detail,including the effects of its recoil on the objects of the ttbb system. Inparticular, we discuss various azimuthal correlations that allow one tocharaterise the QCD recoil pattern in a precise and transparent way. Ingeneral, the calculation at hand provides a variety of precise benchmarks thatcan be used to validate the modelling of QCD radiation in ttbb generators.Moreover, as we will argue, pp->ttbbj at NLO entails information that can beused to gain insights into the perturbative convergence of the inclusive ttbbcross section beyond NLO. Based on this idea, we address the issue of the largeNLO K-factor observed in the ttbb cross section, and we provide evidence thatsupports the reduction of this K-factor through a mild adjustment of the QCDscales that are conventionally used for this process. The presented 2->5 NLOcalculations have been carried out using OpenLoops 2 in combination with Sherpaand Munich.

NLO QCD predictions for $t\bar{t}b\bar{b}$ production in association with a light jet at the LHC