Constraints on CP-violating Higgs couplings to the third generation

TL;DR

The paper investigates CP violation in Higgs couplings to third-generation fermions by combining low-energy electric dipole moment bounds with Higgs production and decay data. Using Barr-Zee two-loop diagrams and RG-improved perturbation theory, it derives constraints on CP-violating couplings $\tilde{\kappa}_t$, $\tilde{\kappa}_b$, and $\tilde{\kappa}_\tau$, along with their CP-even counterparts, and projects future sensitivities. It shows that electron EDM currently yields the strongest bound on $\tilde{\kappa}_t$ (|$\tilde{\kappa}_t$| ≲ 0.01 for SM-like first-generation couplings), while neutron and mercury EDMs provide complementary limits via the Weinberg operator, especially if the Higgs couples only to the third generation. The work highlights the complementary reach of low-energy precision and collider experiments, predicting that next-generation EDMs could probe CP-violating scales up to ~25 TeV and strongly constrain CP violation in the Higgs-top sector, with future HL-LHC data further refining the bottom and tau couplings.

Abstract

Discovering CP-violating effects in the Higgs sector would constitute an indisputable sign of physics beyond the Standard Model. We derive constraints on the CP-violating Higgs-boson couplings to top and bottom quarks as well as to tau leptons from low-energy bounds on electric dipole moments, resumming large logarithms when necessary. The present and future projections of the sensitivities and comparisons with the LHC constraints are provided. Non-trivial constraints are possible in the future, even if the Higgs boson only couples to the third-generation fermions.

Figures (10)

• Figure 1: Left: Two-loop Barr-Zee contributions to the EDM of the electron involving a virtual Higgs boson and a photon or $Z$ boson. Right: Two-loop contribution to the Weinberg operator.
• Figure 2: Left: Present constraints on $\kappa_t$ and $\tilde{\kappa}_t$ from the electron EDM (blue), the neutron EDM (red), the mercury EDM (brown), and Higgs physics (gray). Right: Projected future constraints on $\kappa_t$ and $\tilde{\kappa}_t$, see text for details.
• Figure 3: Left: Present constraints on $\kappa_t$ and $\tilde{\kappa}_t$ from the neutron EDM (red) and Higgs physics (gray), assuming that the Higgs only couples to the third generation. Right: Projected future constraints on $\kappa_t$ and $\tilde{\kappa}_t$, see text for details.
• Figure 4: Left: Present constraints on $\kappa_b$ and $\tilde{\kappa}_b$ from the electron EDM (blue), the neutron EDM (red), the mercury EDM (brown), and Higgs physics (gray), restricting all other Higgs couplings to their SM values. Right: Possible future constraints on $\kappa_b$ and $\tilde{\kappa}_b$, see text for details.
• Figure 5: Left: Present constraints on $\kappa_b$ and $\tilde{\kappa}_b$ from the neutron EDM (red) and Higgs physics (gray), assuming that the Higgs only couples to the third-generation fermions, $W$, and $Z$ bosons. Right: Possible future constraints on $\kappa_b$ and $\tilde{\kappa}_b$, see text for details.