NLO QCD predictions for t t ¯ \[ \overline{t} \] b b ¯ \[ \overline{b} \] production in association with a light jet at the LHC

Abstract

Theoretical predictions for tt¯\[ \overline{t} \]bb¯\[ \overline{b} \] production are of crucial importance for tt¯\[ \overline{t} \]H measurements in the H → bb¯\[ \overline{b} \] channel at the LHC. To address the large uncertainties associated with the modelling of extra QCD radiation in tt¯\[ \overline{t} \]bb¯\[ \overline{b} \] events, in this paper we present a calculation of pp → tt¯\[ \overline{t} \]bb¯\[ \overline{b} \]j at NLO QCD. The behaviour of NLO corrections is analysed in a variety of observables, and to assess theoretical uncertainties we use factor- two rescalings as well as different dynamic scales. In this context, we propose a systematic alignment of dynamic scales that makes it possible to disentangle normalisation and shape uncertainties in a transparent way. Scale uncertainties at NLO are typically at the level of 20–30% in integrated cross sections, and below 10% for the shapes of distributions. The kinematics of QCD radiation is investigated in detail, including the effects of its recoil on the objects of the tt¯\[ \overline{t} \]bb¯\[ \overline{b} \] system. In particular, we discuss various azimuthal correlations that allow one to characterise the QCD recoil pattern in a precise and transparent way. In general, the calculation at hand provides a variety of precise benchmarks that can be used to validate the modelling of QCD radiation in tt¯\[ \overline{t} \]bb¯\[ \overline{b} \] generators. Moreover, as we will argue, pp → tt¯\[ \overline{t} \]bb¯\[ \overline{b} \]j at NLO entails information that can be used to gain insights into the perturbative convergence of the inclusive tt¯\[ \overline{t} \]bb¯\[ \overline{b} \] cross section beyond NLO. Based on this idea, we address the issue of the large NLO K-factor observed in σtt¯bb¯\[ {\sigma}_{t\overline{t}b\overline{b}} \], and we provide evidence that supports the reduction of this K-factor through a mild adjustment of the QCD scales that are conventionally used for this process. The presented 2 → 5 NLO calculations have been carried out using OpenLoops 2 in combination with Sherpa and Munich.