Flavor Violating Higgs Decays

TL;DR

This work analyzes flavor-violating Higgs couplings of a $125$ GeV scalar, parameterizing off-diagonal Yukawas $Y_{ij}$ in the mass basis and exploring both a single-Higgs EFT and multi-EWSB scenarios. It derives comprehensive constraints from low-energy observables (e.g., $\mu\to e\gamma$, $\mu\to e$ conversion, meson mixing) and translates them into limits on FV Higgs decays, finding that $h\to \tau\mu$ and $h\to \tau e$ can be as large as ${\sim}10\%$ without conflicting with data. It demonstrates that current LHC analyses recast from $h\to\tau\tau$ already place meaningful bounds on these decays and outlines dedicated search strategies that could significantly improve sensitivity. The results highlight flavor-violating Higgs decays as a powerful window into electroweak-scale new physics, including extended Higgs sectors, and identify the complementary roles of high-energy colliders and precision low-energy experiments in constraining FV couplings.

Abstract

We study a class of nonstandard interactions of the newly discovered 125 GeV Higgs-like resonance that are especially interesting probes of new physics: flavor violating Higgs couplings to leptons and quarks. These interaction can arise in many frameworks of new physics at the electroweak scale such as two Higgs doublet models, extra dimensions, or models of compositeness. We rederive constraints on flavor violating Higgs couplings using data on rare decays, electric and magnetic dipole moments, and meson oscillations. We confirm that flavor violating Higgs boson decays to leptons can be sizeable with, e.g., h -> tau mu and h -> tau e branching ratios of order 10% perfectly allowed by low energy constraints. We estimate the current LHC limits on h -> tau mu and h -> tau e decays by recasting existing searches for the SM Higgs in the tau-tau channel and find that these bounds are already stronger than those from rare tau decays. We also show that these limits can be improved significantly with dedicated searches and we outline a possible search strategy. Flavor violating Higgs decays therefore present an opportunity for discovery of new physics which in some cases may be easier to access experimentally than flavor conserving deviations from the Standard Model Higgs framework.

Figures (12)

• Figure 1: Diagrams contributing to the flavor violating decay $\tau \to \mu\gamma$, mediated by a Higgs boson with flavor violating Yukawa couplings.
• Figure 2: Diagrams leading to $\tau \to 3\mu$ decay. The tree level Higgs exchange contribution (left) is typically subdominant compared to higher-order contributions with the topology shown on the right. The blob represents loops of the form shown in Figs. \ref{['fig:tau-mu-gamma']} and \ref{['fig:2loop']}.
• Figure 3: Diagram leading to muonium--antimuonium oscillations.
• Figure 4: A diagram contributing to the anomalous magnetic moment $g-2$ of the muon through FV couplings of the Higgs to $\tau\mu$.
• Figure 5: Tree-level and one-loop diagrams contributing to $\mu\to e$ conversion in nuclei via the flavor violating Higgs Yukawa couplings $Y_{\mu e}$ and $Y_{e\mu}$. In addition, we also include numerically important two-loop diagrams, see Appendix \ref{['App:mutoe']} for details.