Future collider constraints on top-quark operators

TL;DR

This work delivers a global SMEFT analysis of the top-quark sector using data from Tevatron, LEP, and the LHC, constraining 22 Wilson coefficients across two-fermion, four-quark, and two-quark–two-lepton operators within a $U(2)^5$ flavor framework and a linear $\Lambda^{-2}$ truncation. It demonstrates current constraints are SM-compatible but highlights strong sensitivity from differential top-quark measurements and quantum-entanglement observables, while revealing substantial correlations that limit global bounds on four-fermion operators. The authors provide comprehensive HL-LHC projections, showing typical improvements by a factor of $2$–$4$ and stressing theory/modelling uncertainties as a limiting factor, especially for rare processes. They then quantify the potential of future lepton colliders (including ILC, CLIC, FCC-ee/CEPC) to dramatically improve two-quark and two-quark–two-lepton operator bounds, and they extend the study to very high-energy muon colliders where vector-boson-fusion becomes dominant and yields the strongest sensitivity to several operators, including the top Yukawa sector. Overall, the paper maps out a path where HL-LHC and next-generation lepton- or muon-collider experiments collectively constrain the top-quark SMEFT parameter space far beyond current capabilities, with clear guidance on which measurements and energy stages provide the most leverage.

Abstract

In this paper we present updated constraints on the top-quark sector of the Standard Model Effective Field Theory using data available from Tevatron, LEP and the LHC. Bounds are obtained for the Wilson coefficients from a global fit including the relevant two-fermion operators, four-quark operators and two-quark two-lepton operators. We compare the current bounds with the prospects for the high luminosity phase of the Large Hadron Collider and future lepton colliders.

Figures (21)

• Figure 1: The 95% probability constraints on the WC divided by $\Lambda^2$ using the current LHC data in combination of the legacy data from Tevatron and LEP.
• Figure 2: Correlation matrix obtained for the global fit including the data of the LHC, Tevatron and LEP. Entries smaller than 5% are set to zero.
• Figure 3: Ranking of the individual 95% probability bounds on the WC divided by $\Lambda^2$ of $O_{tG}$ and the $q\bar{q}t\bar{t}$ operators. Each measurement in top-quark pair production is shown.
• Figure 4: The 95% probability constraints on the WC divided by $\Lambda^2$ affecting the top-quark entanglement measurement. Solid bars represent individual bounds derived from single-parameter fits, while the shaded regions (full bars) indicate global marginalised constraints obtained by simultaneously fitting all WC.
• Figure 5: The 95% probability constraints on the WC divided by $\Lambda^2$ for the two-quark dimension-six operators. Both the LHC Run 2 (dark red) and the projections for the HL-LHC (light red) are shown. Solid bars represent individual bounds derived from single-parameter fits, while the shaded regions (full bars) indicate global marginalised constraints obtained by simultaneously fitting all Wilson coefficients.