Modified Higgs branching ratios versus CP and lepton flavor violation

TL;DR

The work investigates whether new thresholds beyond the SM can simultaneously modify Higgs decays to diphotons and dileptons while introducing CP and lepton flavor violation. It demonstrates that EDM constraints tightly bound CP-odd contributions to $h\to\gamma\gamma$ for simple contact-operator cases, but carefully crafted UV completions with near-degenerate scalar states can evade these bounds and yield noticeable diphoton enhancements. In contrast, CP and LFV observables generally impose weaker restrictions on Higgs dilepton decays, leaving room for sizable deviations in the $h\to\tau\tau$ channel given appropriate flavor structures. The study highlights the critical interplay between Higgs precision measurements and low-energy CP/LFV data, and it emphasizes future experimental tests—particularly for $h\to\tau\tau$ and LFV tau decays—that could reveal or constrain such new thresholds.

Abstract

New physics thresholds which can modify the diphoton and dilepton Higgs branching ratios significantly may also provide new sources of CP and lepton flavor violation. We find that limits on electric dipole moments impose strong constraints on any CP-odd contributions to Higgs diphoton decays unless there are degeneracies in the Higgs sector that enhance CP-violating mixing. We exemplify this point in the language of effective operators and in simple UV-complete models with vector-like fermions. In contrast, we find that electric dipole moments and lepton flavor violating observables provide less stringent constraints on new thresholds contributing to Higgs dilepton decays.

Figures (2)

• Figure 1: Left: the diagram that gives rise to fermionic EDMs via the insertion of the operator $hF\tilde{F}$ from Eq. (\ref{['Leff']}). Right: the two-loop diagram that leads to fermion EDMs in the model involving a VL lepton, $\psi$, coupled to a singlet, $S$, that mixes with the Higgs. The cross on the scalar line indicates that this contribution is proportional to the mixing term, $A$, in the scalar potential.
• Figure 2: The effective increase in the diphoton rate as a function of the electron EDM coming from a coupling of the Higgs to $F_{\mu\nu}\tilde{F}^{\mu\nu}$. The black dashed lines show the relationship in the case of the contact operator $hF_{\mu\nu}\tilde{F}^{\mu\nu}$ simply cut off at the scales $\Lambda_{\rm UV}=200$ GeV and 1 TeV. The solid lines show the relationship in the case of a scalar singlet, $S$, nearly degenerate with the Higgs coupled to a VL fermion, $\psi$. We choose a splitting between $m_S$ and $m_h$ of $\Delta M=1$ GeV (left panel) and 3 GeV (right panel) and a $CP$-odd Yukawa coupling of the singlet to the VL fermions of $\tilde{Y}_S=2$. The curve on the left of each panel (green) is for a mixing angle $\theta=0.1$ and that on the right of each panel (blue) for $\theta=\pi/4$. The dotted lines show the value of $d_e$ implied for the two mixing angles for $m_\psi=105$ GeV and 300 GeV. Values of the electron EDM that are excluded experimentally, $d_e>1.05\times10^{-27}~e$ cm, are in the shaded region. We observe that the degenerate scalar allows for a sizable apparent increase in the Higgs diphoton rate in the $CP$-odd channel while not conflicting with the electron EDM limit, unlike the simple contact operator case.