Higgs lepton flavour violation: UV completions and connection to neutrino masses

TL;DR

This paper investigates lepton flavor violating Higgs decays, analyzing effective operators and high-energy models, and finds that certain two Higgs doublet models can produce observable decay rates, unlike most other models.

Contribution

It identifies promising models, especially two Higgs doublet models, that can generate detectable lepton flavor violating Higgs decays and explores their connection to neutrino mass models.

Findings

Two Higgs doublet models can have BR(h→τμ) ~ 0.01.

Most models predict BR(h→τμ) < 10^{-4}.

Extended models like Zee and left-right symmetric models could explain observed excess.

Abstract

We study lepton violating Higgs (HLFV) decays, first from the effective field theory (EFT) point of view, and then analysing the different high-energy realizations of the operators of the EFT, highlighting the most promising models. We argue why two Higgs doublet models can have a $\mathrm{BR}(h\rightarrow \tau \mu)\sim 0.01$, and why this rate is suppressed in all other realizations including vector-like leptons. We further discuss HLFV in the context of neutrino mass models: in most cases it is generated at one loop giving always $\mathrm{BR}(h\rightarrow \tau \mu) < 10^{-4}$ and typically much less, which is beyond experimental reach. However, both the Zee model and extended left-right symmetric models contain extra ${\rm SU(2)}$ doublets coupled to leptons and could in principle account for the observed excess, with interesting connections between HLFV and neutrino parameters.