UV origins of CP-violating leptonic Yukawa couplings

TL;DR

The paper investigates CP-violating modifications of leptonic Higgs Yukawa couplings, using the SMEFT as a bridge to connect UV completions with collider and low-energy observables. It demonstrates that dipole moments, EDMs, and EWPT provide strong, complementary constraints to direct Higgs measurements on the CP structure, notably via Barr–Zee contributions to the electron EDM. By systematically analyzing single- and two-field UV completions that generate $[C_{eH}]$ at tree level, the authors show that realistic UV models introduce correlated effects, including one-loop dipoles and tree-level Higgs-current operators, amplifying the phenomenological reach beyond the SMEFT. Representative flavor scenarios reveal a rich interplay between collider data, EDMs, and LFV observables, with MFV implementations offering distinct but testable patterns. The study highlights the importance of combining high- and low-energy probes to robustly test CP-violating lepton Yukawas and guides future explorations in both leptonic and quark sectors.

Abstract

Probing the CP nature of the Higgs Yukawa couplings is a promising avenue for unraveling new physics effects. In this work, we investigate the possible single- and two-field UV origins of CP-violating leptonic Yukawa couplings, using the Standard Model Effective Field Theory as a stepping stone. We demonstrate a rich set of constraints on the UV model parameters, including direct Higgs measurements, electric and magnetic dipole moments of leptons, charged lepton flavor violating observables, and electroweak precision tests. Studying representative flavor assumptions we find that the precision constraints are often, but not always, more constraining than the dedicated LHC analyses of modified leptonic Yukawa couplings.

Figures (5)

• Figure 1: The $68\%$ CL (dark) and $95\%$ CL (light) constraints on the real and imaginary part of the ${\mathcal{C}}_{eH}$ Wilson coefficient under three flavor assumptions: $\tau$-specific (left plot), $\mu$-specific (middle plot) and electron-specific (right plot). See Sec. \ref{['sec:EFT_results']} for a discussion.
• Figure 2: Representative diagrams for the single field configuration involving $\varphi$ (upper row), and a two-field configuration featuring two fermionic mediators (lower row) generating the operators discussed in Sec. \ref{['sec:Matching']}. The portal interaction is indicated by the red dot, while $f$ denotes either $\ell$ or $e$. See sections \ref{['sec:single_field_extensions']} and \ref{['sec:two_field_extensions']} for the matching expressions.
• Figure 3: The 95% CL bounds on the mass $M_\varphi$ of the scalar mediator $\varphi$ (upper row) and on the common mass scale $M_{E,\Delta_3}\equiv M_E=M_{\Delta_3}$ in the $(E, \Delta_3)$ two-field scenario (lower row). The three columns correspond to scenarios where the mediators couple exclusively to the $\tau$, $\mu$ and $e$ leptons, respectively. See Sec. \ref{['sec:single_flavor_assumptions']} for a discussion.
• Figure 4: The 95% CL bounds on the parameters of the $\varphi$ scenario under two flavor assumptions: MFV (left plot) and a fine tuned (FT) scenario with a vanishing electron coupling, while the muon and tau couplings enter proportionally with a relative weight $x$ (right plot). See Sec. \ref{['sec:multiflavor']} for a discussion.
• Figure 5: The 95% CL bounds on the parameters of the $(E, \Delta_3)$ scenario under three flavor assumptions: universal (left plot), $\tau$ and $\mu$ specific (middle plot) and MFV (right plot). See Sec. \ref{['sec:multiflavor']} for a discussion.