NLO QCD predictions for tt¯bb¯¯ production in association with a light jet at the LHC

Journal article

Theoretical predictions for tt¯bb¯¯ production are of crucial importance for tt¯ H measurements in the H → b b¯¯ channel at the LHC. To address the large uncertainties associated with the modelling of extra QCD radiation in tt¯ bb¯¯ events, in this paper we present a calculation of pp → tt¯ bb¯¯ 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¯ bb¯¯ 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¯bb¯¯ generators. Moreover, as we will argue, pp → tt¯bb¯¯ j at NLO entails information that can be used to gain insights into the perturbative convergence of the inclusive tt¯bb¯¯ cross section beyond NLO. Based on this idea, we address the issue of the large NLO K-factor observed in σtt¯bb¯, 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.

Authors: Buccioni, F; Kallweit, S; Pozzorini, S; Zoller, M

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Springer Verlag
• Journal: Journal of High Energy Physics
• Volume: 2019
• Issue: 12
• Article number: 15
• Publication date: 2019-12-03
• Acceptance date: 2019-11-12
• DOI: 10.1007/jhep12(2019)015
• EISSN: 1029-8479
• ISSN: 1126-6708
• Language: English