Hadronic production of top-quark pairs in association with a pair of leptons in the powheg box framework

We present an implementation of tt¯ℓ+ℓ- (ℓ=e, μ) hadronic production at next-to-leading order in QCD matched to parton-shower event generators in the powheg box framework. The program we developed includes all leading-order contributions of order αs2α2 for the specified final state, as well as the corresponding first-order QCD corrections. Decays of the top quarks have been simulated retaining spin-correlations in all tree-level matrix elements. We consider the case of the Large Hadron Collider at s=13 TeV and compare results for tt¯ℓ+ℓ- production in the fiducial volume where the invariant mass of the lepton pairs is centered around the Z-boson mass to corresponding predictions for tt¯Z on-shell production with Z→ℓ+ℓ-. We find that off-shell effects in tt¯ℓ+ℓ- are in general small at the level of the total cross section, but can decrease the tail of the leptons' transverse momentum distributions by 10%-20% and, in these regions, they are visible beyond the scale uncertainty due to renormalization and factorization scale variation. Moreover, we find that accounting for top-quark decays in the narrow-width approximation with tree-level spin correlations also gives origin to 10%-20% effects in specific regions of the kinematic distributions of the tt¯ℓ+ℓ- decayed final state.