Could electron-top interactions spoil the measurement of the Higgs trilinear? -A quantitative estimate at future lepton colliders-

TL;DR

The paper assesses whether electron–top four-fermion interactions in SMEFT can bias the extraction of the Higgs self-coupling κ3 at future e+e− colliders, focusing on FCC‑ee. It combines SMEFT analyses of the Zh cross-section at NLO with EWPOs and high-energy fermion-pair measurements, and complements this with explicit leptoquark UV completions. The results show that high-precision EWPOs and above-pole ee observables lift degeneracies and keep κ3 largely robust: the baseline 1σ sensitivity is about $0.17$ (17%), with marginalised fits yielding around $0.17$–$0.23$ (17–23%) depending on the scenario. Overall, the Higgs trilinear measurement at FCC‑ee remains a robust probe of the SM and potential new physics in the electron–top sector, thanks to the rich, redundant observable program including Drell–Yan constraints and leptoquark UV completions.

Abstract

The measurement of the Higgs self-coupling is considered the next milestone in the study of the Higgs boson properties. At future $e^+e^-$ facilities below the double Higgs production threshold, this is extracted from the $Zh$ production cross-section, which is sensitive to the trilinear coupling at the one-loop level. At the same perturbative order, potential effects beyond the Standard Model (SM) may affect the Higgstrahlung rate and distort the self-coupling determination. We study the question focusing especially on contact interactions containing two electron and two top-quark fields. We conclude that, in the context of FCC-ee and its planned runs at different energies, $eett$ interactions change the Higgs self-coupling sensitivity below the percent level. Even in the most pessimistic scenarios, we confirm a robust sensitivity of the order of 17% at the 1$σ$ confidence level under the assumption of otherwise SM-like Higgs couplings. A crucial role in these results is played by the measurement of fermion pair production above the $Z$ resonance.

Figures (13)

• Figure 1: In red: allowed regions at 1$\sigma$ (solid line) and 2$\sigma$ (dashed line) confidence level from EWPOs and $e^+e^-\to Zh$ combined. In green: allowed regions for $\kappa_Z$.
• Figure 2: Contribution of the Higgs trilinear coupling to the $Zh$ cross-section.
• Figure 3: Example one-loop SMEFT diagrams involving $eett$ operators relevant for some of the observables discussed in the main text. Figure (a) exemplifies the contribution to Higgsstrahlung, (b) the RGE effects responsible for $Zee$-coupling modifications, and (c) NLO contributions in Drell--Yan production through valence quarks ($q$).
• Figure 4: The 2$\sigma$ C.L. allowed regions from different processes for the two coefficients with right handed leptons, $[C_{eu}]_{1133}$ and $[C_{qe}]_{1133}$. All other SMEFT coefficients are set to zero.
• Figure 5: The 2$\sigma$ C.L. allowed regions from different processes for two of the coefficients with left handed leptons, $[C_{lu}]_{1133}$ and $[C_{lq}^{(3)}]_{3311}$. All other SMEFT coefficients are set to zero.