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 ${\cal O}(\alpha_s^4 \alpha^4)$ are taken into account, including irreducible backgrounds to $t\bar{t}j$ 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 $t\bar{t}$ 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.
Highlights
Jet activity can be used to examine the underlying production and decay mechanisms even further and to design new methods for a sizeable reduction of QCD backgrounds [1,2,3]
We only simulate decays of the weak bosons to different lepton generations to avoid virtual photon singularities stemming from quasi-collinear γ∗ → ± ∓ decays. These interference effects are at the per-mille level for inclusive cuts, as checked by an explicit leading order calculation
For the transverse momentum distribution of the bottom-jet we have a different behaviour, namely scale uncertainties have increased in the tails and reached almost 10% while parton distribution functions (PDFs) uncertainties stayed below 6% (3%) for CT14
Summary
For the pp → e+νeμ−νμbbj+X production process at the leading order (LO) in perturbative expansion and at O(αs3α4), the contribution from the following partonic subprocesses need to be taken into account: gg → e+νeμ−νμbbg , gq → e+νeμ−νμbbq , gq → e+νeμ−νμbbq, qq → e+νeμ−νμbbg ,. The number of loop topologies rapidly increases with the number of external particles and puts serious challenges starting from 2 → 5 processes, where the efficient selection and bookkeeping of topologies becomes a critical issue for the feasibility of the calculation To this end a few optimizations have been introduced for the generation of the skeleton files in the Helac1Loop program. Using these optimisations we have achieved a reduction of one order of magnitude in the generation of skeleton files for the process under consideration Another improvement in Helac-NLO is the implementation of a new option for selecting automatically the desired perturbative order in α and αs, preserving at the same time the structure and the advantages of the DysonSchwinger recursion algorithm for the construction of the amplitudes. This typically results in about 1%–10% of the total number of events carrying a non-unit weight
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