$t\bar tZ$ in the $4\ell$ channel at NLO in QCD

TL;DR

This paper presents NLO QCD calculations for the $t\bar t Z$ process in the four-lepton channel, including full off-shell effects, interference, and gauge boson contributions, with detailed uncertainty analysis and comparison to the narrow width approximation.

Contribution

The work provides the first comprehensive NLO QCD predictions for $t\bar t Z$ with full off-shell effects and interference, enhancing the accuracy of theoretical modeling for experimental analyses.

Findings

Full off-shell effects significantly impact the $t\bar t Z$ predictions.

Uncertainty analysis shows scale and PDF variations are crucial.

Comparison with NWA highlights differences in modeling the process.

Abstract

NLO QCD corrections to the process $pp \rightarrow e^+\nu_e \mu^- \bar{\nu}_{\mu} \tau^+\tau^- b \bar{b}+X$ are presented with the full off-shell effects included. The calculation includes all resonant and non-resonant Feynman diagrams, photon and $Z$-gauge boson contributions, as well as interference effects, all incorporated at the matrix element level. Furthermore all heavy intermediate particles are described via Breit-Wigner propagators. Theoretical uncertainties related to the scale variation and choice of PDF sets were investigated at the integrated and differential level. The modelling is studied by a direct comparison of the full off-shell process to the NWA, where tops, $Z$- and $W$-gauge bosons are on-shell. Moreover, motivated by experimental cuts, we also investigate the impact of imposing a window cut around the mass of the $Z$-gauge boson on the full off-shell predictions.