New NLOPS predictions for -jet production at the LHC

Tomáš Ježo; Jonas M. Lindert; Niccolo Moretti; Stefano Pozzorini

doi:10.1140/epjc/s10052-018-5956-0

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2024 Impact factor 4.8

Particles and Fields

Open Access

Eur. Phys. J. C (2018) 78: 502
https://doi.org/10.1140/epjc/s10052-018-5956-0

Regular Article - Theoretical Physics

New NLOPS predictions for $\varvec{t \bar{t} +b}$ -jet production at the LHC

Tomáš Ježo¹^*, Jonas M. Lindert², Niccolo Moretti¹ and Stefano Pozzorini¹

¹ Physics Institute, Universität Zürich, Zurich, Switzerland
² Institute for Particle Physics Phenomenology, Durham University, South Rd, Durham, DH1 3LE, UK

^* e-mail: tomas.jezo@physik.uzh.ch

Received: 24 February 2018
Accepted: 31 May 2018
Published online: 18 June 2018

Abstract

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