Top-quark pair production in association with a W± gauge boson in the POWHEG-BOX

We present a new Monte Carlo event generator for the production of a top-quark pair in association with a W^± boson at hadron colliders in the POWHEG-BOX framework. We consider the next-to-leading-order QCD corrections to the pp→tt̄W^± cross section, corresponding to the O(αₛ³α) and O(αₛα³) terms in the perturbative expansion of the parton-level cross section, and model the decays of W and top quarks at leading order retaining spin correlations. The fixed-order QCD calculation is further interfaced with the pythia8 parton-shower event generator via the POWHEG method as implemented in the POWHEG-BOX. The corresponding code is now part of the public repository of the POWHEG-BOX. We perform a comparison of different event generators for both the case of inclusive production and the case of the two same-sign leptons signature at the Large Hadron Collider operating at a center-of-mass energy of 13 TeV. We investigate theoretical uncertainties in the modeling of the fiducial volume stemming from missing higher-order corrections, the different parton-shower-matching schemes, and the modeling of decays. We find that the subleading contribution at O(αₛα³) is particularly sensitive to differences in the matching scheme and higher-order parton-shower effects. We observe that in particular jet observables can differ quite visibly although these differences play only a subordinate role in the description of physical observables once all contributions are combined.