The Price of a Large Electron Yukawa Modification

Abstract

The theoretical implications of an electron Yukawa modification are considered in the context of a possible Higgs pole run at FCC-ee, aimed at bounding this coupling. We start from an effective field theory viewpoint, considering the impact of renormalisation group effects on related observables and also examining assumptions on the broader UV flavour structure. We then give an overview of the landscape of simplified models, investigating phenomenological constraints arising at higher orders. A short discussion of fine-tuning is also included.

Figures (9)

• Figure 1: An example two-loop Barr-Zee diagram contributing to $\Delta a_\ell$. Crosses on scalar legs indicate Higgs components set to $v$.
• Figure 2: Current and projected upper limits on $|\Delta \kappa_e|$(defined as $|\kappa_e-1|$) at 95% CL, assuming coefficients are generated at 2 TeV and only one SMEFT coefficient is non-zero at a time. Current LHC bounds on $[\mathcal{C}_{lequ}^{(1)}]_{1133}$ and $[\mathcal{C}_{lequ}^{(3)}]_{1133}$ are taken from CMS:2023xyc, while those on $[\mathcal{C}_{lequ}^{(1)}]_{1122}$, $[\mathcal{C}_{lequ}^{(3)}]_{1122}$ and $[\mathcal{C}_{ledq}]_{1133}$ are from Allwicher:2022gkm. All HL-LHC and FCC-ee projections, shown by green bars, are taken from Greljo:2024ytg. Constraints from the decays $D_s^+\to e^+\nu$ and $B_s \to e^+e^-$ follow from App. \ref{['sec:mesondecays']}, where the hatching on the latter bar indicates the assumption that the coefficient is in the up mass basis. Future $\Delta a_e$ sensitivity is assumed to reach $\Delta a_e^{\text{future}} < 5 \times 10^{-14}$.
• Figure 3: Schematic diagrams showing how the EW dipole operators $O_{eW}$ and $O_{eB}$ are generically generated at one loop order higher than the $O_{eH}$ operator in UV models. The grey blob represents a diagram of arbitrary loop order involving exchange of heavy UV states. The left hand diagram matches to the dimension-six $O_{eH}$ SMEFT operator, while the right hand diagram matches to the dimension-six dipole operators. The gauge boson line connects to any charged particle in the diagram, including the $H$ loop and any charged particles within the grey blob.
• Figure 4: Parameter space for a $\varphi$ extension coupled to electrons. Blue shaded regions are consistent with the indicated $\Delta a_e$ measurement at 95% C.L. Dashed lines show the $\kappa_e$ enhancement corresponding to the given parameter values. Solid (dotted) lines represent constraints on the couplings from current (projected) measurements at 95% C.L. The $R_e$ constraints are taken from Greljo:2024ytg, $\kappa^{\text{LHC}}_\lambda$ and $\kappa^{\text{FCCee}}_\lambda$ are from terHoeve:2025omu and $\kappa^{\text{FCChh}}_\lambda$ assumes 6% uncertainty at 95% CL FCC:2025lppdeBlas:2944678.
• Figure 5: Parameter space for a second Higgs doublet $\varphi$ with Yukawa-like couplings to electrons and top quarks, and assuming $\lambda_\varphi = 1$. Shaded regions and line styles have the same meaning as in Fig. \ref{['fig:e_only_varphi']}. The $\kappa_t^{\text{LHC}}$ limit is from ATLAS:2022vkf, and the $\kappa_t^{\text{FCCee}}$ projection is from deBlas:2019rxi.