Modelling top-quark decays in $t\bar{t}t\bar{t}$ production at the LHC

TL;DR

This study benchmarks fixed-order NLO QCD predictions for pp -> ttbar ttbar + X in the 4-lepton channel against NLO predictions matched to parton showers using MC@NLO and POWHEG, focusing on spin correlations in top decays and the role of matrix-element corrections. By comparing integrated fiducial cross sections and a suite of differential observables at sqrt(s) = 13.6 TeV, the work quantifies how parton showers approximate higher-order decay effects and how MEC improves modelling. The results show that PS predictions are generally smaller than fixed-order, but MEC in decays brings PS results into closer agreement with fixed-order within ~1–3% for integrated quantities and ~5–10% differentially, with larger deviations in high-multiplicity b-jet bins due to LO treatment of extra b-jets. These insights inform precise interpretation of ttbar ttbar signatures and guide background modelling for HL-LHC analyses seeking new physics in multi-top final states.

Abstract

We compare the fixed-order NLO QCD predictions for the $pp\to t\bar{t}t\bar{t}+X$ process in the $4\ell$ decay channel with the parton-shower based results obtained with the POWHEG and MC@NLO matching methods. In the first case, NLO QCD corrections are consistently included in both the $t\bar{t}t\bar{t}$ production step and the decays of the four top quarks, preserving all spin correlations. In the second approach, higher-order effects in top-quark decays with approximate spin correlations are simulated in the PYTHIA parton-shower framework. Additionally, we analyse the impact of including the so-called matrix element corrections in top-quark decays in both parton-shower matched predictions. The comparison is performed at the integrated and differential fiducial cross-section level for the LHC centre-of-mass energy of $\sqrt{s}=13.6$ TeV.

Figures (5)

• Figure 1: Integrated fiducial cross-section predictions at NLO in QCD for the $pp\rightarrow t \Bar{t}t \Bar{t}+X$ process in the $4\ell$ channel as a function of the number of $b$-jets present. The parton-shower based results are given without (left) and with the MEC (right). Predictions without (solid line) and with (dashed line) the $b$-jet identification are also shown. The second panel from the top provides the ratio to the fixed-order results. The third panel shows the ratio to the Powheg predictions. The fourth panel presents the scale uncertainties, and the bottom panel depicts the parton-shower matching uncertainties.
• Figure 2: Differential cross-section distributions at NLO in QCD for the $pp\to t\bar{t}t\bar{t}+X$ process in the $4\ell$ channel as a function of the transverse momentum of the first and second hardest $b$-jets, $p_T(b_1)$ and $p_T(b_2)$, respectively. The parton-shower based results are given without (left) and with the MEC (right). Predictions without (solid line) and with (dashed line) the $b$-jet identification are also shown. The second panel from the top provides the ratios to $\sigma^{\rm NLO}_{\rm NWA_{exp}}$. The third panel depicts the scale uncertainties, and the bottom panel presents the parton-shower matching uncertainties.
• Figure 3: Same as Figure \ref{['fig:pT_b1b2']} but for the transverse momentum of the third and fourth hardest $b$-jets, $p_T(b_3)$ and $p_T(b_4)$, respectively.
• Figure 4: Same as Figure \ref{['fig:pT_b1b2']} but for the transverse momentum of the hardest charged lepton, $p_T(\ell_1)$ and for the missing transverse momentum, $p_T(miss)$.
• Figure 5: Same as Figure \ref{['fig:pT_b1b2']} but for the rapidity of the hardest $b$-jet, $y(b_1)$, and the angular distance between the first and second hardest (charged) leptons, $\Delta \phi(\ell_1\ell_2)$.