Hadronic production of top-quark pairs in association with a pair of leptons in the POWHEG BOX framework

Abstract

We present an implementation of $t\bar{t}{\ell }^{+}{\ell }^{-}$ ($\ell =e,\mu$) 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 ${\alpha }_s^2{\alpha }^2$ 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 $\sqrt{s}=13$ TeV and compare results for $t\bar{t}{\ell }^{+}{\ell }^{-}$ production in the fiducial volume where the invariant mass of the lepton pairs is centered around the $Z$-boson mass to corresponding predictions for $t\bar{t}Z$ on-shell production with $Z\to {\ell }^{+}{\ell }^{-}$. We find that off-shell effects in $t\bar{t}{\ell }^{+}{\ell }^{-}$ 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 $t\bar{t}{\ell }^{+}{\ell }^{-}$ decayed final state.

Comments: 22 pages, 13 figures