Abstract
In light of recent discrepancies between the modeling of $t\bar{t} W^\pm$ signatures and measurements reported by the Large Hadron Collider (LHC) experimental collaborations, we investigate in detail theoretical uncertainties for multi-lepton signatures. We compare results from the state-of-the-art full off-shell calculation and its Narrow Width Approximation to results obtained from the on-shell $t\bar{t} W^\pm$ calculation, with approximate spin-correlations in top-quark and $W$ decays, matched to parton showers. In the former case double-, single-, and non-resonant contributions together with interference effects are taken into account, while the latter two cases are only based on the double resonant top-quark contributions. The comparison is performed for the LHC at $\sqrt{s} = 13$ TeV for which we study separately the multi-lepton signatures as predicted from the dominant NLO contributions at the perturbative orders $\mathcal{O}(\alpha_s^3\alpha^6)$ and $\mathcal{O}(\alpha_s\alpha^8)$. Furthermore, we combine both contributions and propose a simple way to approximately incorporate the full off-shell effects in the NLO computation of on-shell $pp\to t\bar{t} W^\pm$ matched to parton showers.
Highlights
The hadronic production of top-quark pairs in association with a W boson is one of the most massive signatures currently accessible at the LHC and allows us to study possible deviations from the Standard Model (SM) dynamics of top quarks in the presence of charged electroweak (EW) gauge bosons
We focus on the three-lepton signature, where we exclude τ leptons, as their decays produce very distinct signatures and can be distinguished from electron or muon final states
This is expected since for the dominant QCD contribution the results based on the narrow-width approximations (NWAs) and the full off-shell calculation are in very good agreement
Summary
The hadronic production of top-quark pairs in association with a W boson is one of the most massive signatures currently accessible at the LHC and allows us to study possible deviations from the Standard Model (SM) dynamics of top quarks in the presence of charged electroweak (EW) gauge bosons. Additional measurements of the ttW cross section [7,8,10,11], from analyses aimed at the measurement of ttH and tttt production and utilizing data corresponding to an integrated luminosity of up to L 1⁄4 139 fb−1, still see a persisting tension with respect to the corresponding SM predictions depending on the considered final-state signature, where the largest deviation of up to a factor of 1.7 has been found in multilepton signatures [7] Fueled by these deviations, tremendous progress has been made in the last decade in the theoretical description of the pp → ttWÆ processes.
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