New NLOPS predictions for $$\varvec{t \bar{t} +b}$$ t t ¯ + b -jet production at the LHC

Measurements of $$t \bar{t} H$$ production in the $$H\rightarrow b \bar{b} $$ channel depend in a critical way on the theoretical uncertainty associated with the irreducible $$t \bar{t} +b$$ -jet background. In this paper, analysing the various topologies that account for b-jet production in association with a $$t \bar{t} $$ pair, we demonstrate that the process at hand is largely driven by final-state $$g\rightarrow b \bar{b} $$ splittings. We also show that in five-flavour simulations based on $$t \bar{t} +$$ multi-jet merging, b-jet production is mostly driven by the parton shower, while matrix elements play only a marginal role in the description of $$g\rightarrow b \bar{b} $$ splittings. Based on these observations we advocate the use of NLOPS simulations of $$pp\rightarrow t \bar{t} b\bar{b} $$ in the four-flavour scheme, and we present a new Powheg generator of this kind. Predictions and uncertainties for $$t \bar{t} +b$$ -jet observables at the 13 TeV LHC are presented both for the case of stable top quarks and with spin-correlated top decays. Besides QCD scale variations we consider also theoretical uncertainties related to the Powheg matching method and to the parton shower modelling, with emphasis on $$g\rightarrow b \bar{b} $$ splittings. In general, matching and shower uncertainties turn out to be remarkably small. This is confirmed also by a consistent comparison against Sherpa+OpenLoops.

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