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

• Published in: The journal of high energy physics Vol. 2019; no. 12
• Main Authors: Buccioni, Federico, Kallweit, Stefan, Pozzorini, Stefano, Zoller, Max F.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 03.12.2019
• Subjects: Classical and Quantum Gravitation; Elementary Particles; Physics; Physics and Astronomy; Quantum Field Theories; Quantum Field Theory; Quantum Physics; Regular Article - Theoretical Physics; Relativity Theory; String Theory; NLO Computations; QCD Phenomenology
• ISSN: 1029-8479
• DOI: 10.1007/JHEP12(2019)015

A bstract Theoretical predictions for t t ¯ b b ¯ production are of crucial importance for t t ¯ 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 t t ¯ b b ¯ events, in this paper we present a calculation of pp → t t ¯ b 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 t t ¯ b 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 t t ¯ b b ¯ generators. Moreover, as we will argue, pp → t t ¯ b b ¯ j at NLO entails information that can be used to gain insights into the perturbative convergence of the inclusive t t ¯ b b ¯ cross section beyond NLO. Based on this idea, we address the issue of the large NLO K -factor observed in σ t t ¯ b 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.