New Nlops Predictions For T(T)Over-Bar + B-Jet Production At The Lhc

Measurements of t (t) over barH production in the H -> b (b) over bar channel depend in a critical way on the theoretical uncertainty associated with the irreducible t (t) over bar + b-jet background. In this paper, analysing the various topologies that account for b-jet production in association with a t (t) over bar pair, we demonstrate that the process at hand is largely driven by final-state g -> b (b) over bar splittings. We also show that in five-flavour simulations based on t (t) over bar +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 -> b (b) over bar splittings. Based on these observations we advocate the use of NLOPS simulations of pp -> t (t) over barb (b) over bar in the four-flavour scheme, and we present a new POWHEG generator of this kind. Predictions and uncertainties for t (t) over bar + 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 -> b (b) over bar splittings. In general, matching and shower uncertainties turn out to be remarkably small. This is confirmed also by a consistent comparison against SHERPA+OPENLOOPS.