[Paper Review] Hadroproduction of four top quarks in the POWHEG BOX

Tomáš Ježo, Manfred Kraus|arXiv (Cornell University)|Oct 28, 2021

Particle physics theoretical and experimental studies90 references26 citations

TL;DR

This paper presents a new Monte Carlo event generator in the POWHEG Box framework for simulating hadroproduction of four top quarks at next-to-leading order QCD accuracy (O(α⁵ₛ)) and including leading-order electroweak contributions. It enables precise modeling of spin correlations and decay effects, and the first detailed study of single-lepton plus jets signatures reveals significant impacts from electroweak modes and spin correlations at the fiducial level, enhancing sensitivity to new physics beyond the Standard Model.

ABSTRACT

We present a new Monte Carlo event generator for the hadronic production of four top quarks in the POWHEG BOX framework. Besides the dominant next-to-leading order QCD corrections at $\mathcal{O}(\alpha_s^5)$ we also include all subleading electroweak productions channels at leading-order accuracy. We validate our theoretical predictions by comparing to parton-shower matched predictions obtained within the MC@NLO framework for stable top quarks. Furthermore, we investigate in detail the various sources of theoretical uncertainties. Finally, we investigate a single lepton plus jets signature to study for the first time the impact of the electroweak production modes as well as spin-correlation effects at the fiducial level.

Motivation & Objective

• To develop a high-precision Monte Carlo event generator for four-top quark production in hadronic collisions, incorporating next-to-leading order QCD corrections and leading-order electroweak contributions.
• To model top-quark decays with spin-correlation effects retained at leading order, improving the fidelity of simulated final states.
• To enable the first detailed investigation of the single-lepton plus jets signature for four-top production, assessing the impact of electroweak production modes and spin correlations at the fiducial level.
• To provide a publicly available tool for precision phenomenology at the LHC, supporting searches for new physics beyond the Standard Model.
• To quantify theoretical uncertainties from scale variations, parton shower matching, and higher-order corrections in a consistent framework.

Proposed method

• The event generator is implemented within the POWHEG Box V2 framework, using the POWHEG method to match fixed-order NLO QCD calculations with parton showers.
• It includes O(α⁵ₛ) QCD corrections and all subleading electroweak contributions at LO, including interference terms between QCD and EW amplitudes.
• Top-quark decays are modeled at leading order while preserving spin-correlation effects through the use of spin-density matrices in the matrix element generator.
• The simulation includes full parton-shower matching, ensuring realistic jet and lepton kinematics in the final state.
• Theoretical predictions are validated against MC@NLO results for stable top quarks, ensuring consistency across frameworks.
• Fiducial-level analyses are performed using dedicated cuts and jet reconstruction (anti-kₜ algorithm), with results compared across different theoretical setups.

Experimental results

Research questions

• RQ1 How significant are electroweak production modes in the single-lepton plus jets final state for four-top quark production?
• RQ2 To what extent do top-quark spin correlations affect the fiducial cross section in the single-lepton plus jets channel?
• RQ3 How do theoretical uncertainties from scale variations and parton shower matching impact the prediction of the four-top cross section?
• RQ4 What is the relative size of subleading electroweak contributions, particularly those involving intermediate resonances like the Higgs or Z boson?
• RQ5 How do the inclusion of O(α⁵ₛ) QCD corrections and LO electroweak modes affect the overall cross section and kinematic distributions?

Key findings

• The inclusion of electroweak contributions increases the total cross section by approximately 5%–15% depending on the renormalization scale, with significant cancellations between formally subleading terms.
• Electroweak contributions, particularly from t-t̄ scattering via Higgs or Z exchange, can reach up to ±40% relative to the leading-order QCD term, with large negative contributions from interference terms.
• Spin-correlation effects lead to measurable distortions in the angular distributions of decay products, especially in the single-lepton plus jets final state, with observable impacts at the fiducial level.
• The fiducial cross section for the single-lepton plus jets channel is sensitive to both electroweak production modes and spin correlations, with the latter modifying the kinematic distributions by up to 10% in certain regions.
• Theoretical uncertainties from scale variations are found to be within 5% for the total cross section, while matching uncertainties between POWHEG and MC@NLO frameworks are comparable and small.
• The event generator is validated against MC@NLO predictions for stable top quarks, showing good agreement in kinematic distributions and cross sections, confirming the reliability of the implementation.