Lessons from LHC on the LFV Higgs decays $h \to \ell_a \ell_b$ in the Two-Higgs Doublet Models

TL;DR

This paper investigates lepton-flavor violating Higgs decays within Two-Higgs Doublet Models, analyzing constraints, potential detection at the LHC, and the effects of neutrino-induced loops, highlighting promising scenarios for future searches.

Contribution

It provides a comprehensive analysis of LFV Higgs decays in 2HDMs, including constraints, textures, and loop effects, with predictions for detectability at the LHC.

Findings

Detectable $h \to \tau\mu$ decay rates in certain 2HDM scenarios.

Loop-induced LFV decays are too small for current LHC detection.

Constraints significantly restrict the parameter space for LFV Higgs decays.

Abstract

The non-conservation of the lepton number has been explored at the LHC through the Lepton-Flavor Violating (LFV) Higgs decays $h\to\ell_a\ell_b$, with $\ell_{a,\,b}=e,\,\mu,\,\tau$ $(a \neq b)$. Current limits on these decays are a source of valuable information on the structure of the Yukawa and Higgs sectors. The LFV Higgs couplings can arise within the general Two-Higgs Doublet Model (2HDM); the predicted rates for these decay modes depend on the specific Yukawa structure being considered, ranging from a vanishing branching ratio at tree-level for some versions (2HDM-I, II, X, Y), up to large and detectable ratios within the general 2HDM-III. An attractive scenario is given by the texturized version of the model (2HDM-Tx), with the Yukawa matrices having some texture zeros, such as the minimal version with the so-called Cheng-Sher ansazt. We study the constraints on the parameter space of the 2HDM provided by experimental and theoretical restrictions, and use them to study the detection of LFV Higgs modes at LHC. We find several encouraging scenarios to the search for the decay $h \to\tau\mu$ that could be achieved in the High-Luminosity LHC. On the other hand, LFV Higgs couplings can also be induced at one-loop level in the 2HDM with neutrino masses, with the loops being mediated by neutrino interactions; we find that the resulting branching ratios are of order $10^{-7}$ at best, which is out of the reach of current and future phases of the LHC.