Measurements of tt H production in the H → bb channel depend in a critical way on the theoretical uncertainty associated with the irreducible tt + b-jet background. In this paper, analysing the various topologies that account for b-jet production in association with a tt pair, we demonstrate that the process at hand is largely driven by final-state g → bb splittings. We also show that in five-flavour simulations based on tt+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 → bb splittings. Based on these observations we advocate the use of NLOPS simulations of pp → ttbb in the four-flavour scheme, and we present a new Powheg generator of this kind. Predictions and uncertainties for tt + 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 → bb splittings. In general, matching and shower uncertainties turn out to be remarkably small. This is confirmed also by a consistent comparison against Sherpa+OpenLoops.
We present theoretical predictions for the production of top-quark pairs with up to three jets at the next-to leading order in perturbative QCD. The relevant calculations are performed with Sherpa and OpenLoops. To address the issue of scale choices and related uncertainties in the presence of multiple scales, we compare results obtained with the standard scale H T /2 at fixed order and the MiNLO procedure. Analyzing various cross sections and distributions for tt + 0, 1, 2, 3 jets at the 13 TeV LHC we find a remarkable overall agreement between fixed-order and MiNLO results. The differences are typically below the respective factor-two scale variations, suggesting that for all considered jet multiplicities missing higher-order effects should not exceed the ten percent level.
The nonperturbative approach to soft high-energy hadron-hadron scattering, based on the analytic continuation of Wilson-loop correlation functions from Euclidean to Minkowskian theory, allows to investigate the asymptotic energy dependence of hadron-hadron total cross sections in lattice QCD. In this paper we will show, using best fits of the lattice data with proper functional forms satisfying unitarity and other physical constraints, how indications emerge in favor of a universal asymptotic high-energy behavior of the kind B log 2 s for hadronic total cross sections. *
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