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

TL;DR

This paper presents a next-to-leading order QCD implementation of top-quark pair production with leptons in the POWHEG BOX framework, including spin correlations and off-shell effects, for accurate LHC predictions.

Contribution

It introduces a novel NLO QCD matched simulation of $t\bar{t}\ell^+\ell^-$ production with spin correlations and off-shell effects in the POWHEG BOX, enhancing precision for LHC analyses.

Findings

Off-shell effects are small on total cross section but affect lepton momentum tails.

Accounting for top decays with spin correlations impacts kinematic distributions.

Off-shell and decay effects can alter distributions by 10-20% in certain regions.

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 $Z\rightarrow \ell^+\ell^-$. 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.