Off-shell top quarks with one jet at the LHC: a comprehensive analysis at NLO QCD

Abstract

We present a comprehensive study of the production of top quark pairs in association with one hard jet in the di-lepton decay channel at the LHC. Our predictions, accurate at NLO in QCD, focus on the LHC Run II with a center-of-mass energy of 13 TeV. All resonant and non-resonant contributions at the perturbative order O(α 4 s α 4 ) are taken into account, including irreducible backgrounds to ttj production, interferences and off-shell effects of the top quark and the W gauge boson. We extensively investigate the dependence of our results upon variation of renormalisation and factorisation scales and parton distribution functions in the quest for an accurate estimate of the theoretical uncertainties. Additionally, we explore a few possibilities for a dynamical scale choice with the goal of stabilizing the perturbative convergence of the differential cross sections far away from the tt threshold. Results presented here are particularly relevant for searches of new physics as well as for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.

Off-shell vs on-shell modelling of top quarks in photon associated production

“…Because Helac-Nlo is designed to efficiently obtain one-loop helicity amplitudes and calculate total cross sections at NLO in QCD for multi-particle processes in the SM the top-down approach is more natural and conceptually easier to implement. Furthermore, it allows us to exploit highly optimised recursion algorithms that have already been employed to provide NLO QCD results for tt, ttj, ttγ and ttZ processes with the complete top quark off-shell effects included [30,[54][55][56][57][58].…”

Polarized double-virtual amplitudes for heavy-quark pair production

“…A more complete modelling of pair production including decay and off-shell effects is available to NLO accuracy in QCD [7,8] in the case of the di-lepton channel and more recently also for the semi-leptonic channel [10]. These results were extended to pair production in association with a jet [11,12], which is of relevance for inclusive production at NNLO.…”

On the impact of off-shell contributions on the top quark mass extraction in $t\bar{t} j$ events

“…The NLO QCD calculation, as described in detail in Refs. [22,23], is used and takes into account all double, single and non-resonant contributions as well as their interference contributions at O(α 4 s α 4 ). This calculation has been performed within the HELAC-NLO framework [29], which consists of the two building blocks HELAC-DIPOLES [30,31] and HELAC-1LOOP [32].…”

“…Therefore, we want to focus on the pp → tt j process which has already been studied in great detail in recent years [15,16,17,18,19,20,21,22,23]. The pp → tt j process is particulary interesting because it can be used to extract the top quark mass parameter.…”