[Paper Review] Lepton flavor violating Higgs Boson Decays in Supersymmetric High Scale Seesaw Models

M. E. Gómez, S. Heinemeyer+1 more|arXiv (Cornell University)|Mar 7, 2017

Particle physics theoretical and experimental studies53 references3 citations

TL;DR

This paper investigates lepton flavor violating (LFV) Higgs boson decays (h→ℓᵢℓⱼ) in supersymmetric models with high-scale seesaw mechanisms, particularly Type-I seesaw. Using both model-independent and constrained MSSM+seesaw frameworks, it finds that stringent experimental bounds on charged LFV processes (like μ→eγ) severely suppress LFV Higgs decay rates, making it impossible to explain the CMS excess in h→τμ with these models alone.

ABSTRACT

Within the MSSM, we have evaluated the decay rates for the lepton flavour violating Higgs boson decays (LFVHD) $h ightarrow l_i l_j$ where $l_{i,j}$ are charged leptons and $i eq j$. This has been done in a model independent (MI) way as well as in supersymmetric high scale seesaw models, in particular Type I see-saw model. Lepton flavour violation (LFV) is generated by non-diagonal entries in the mass matrix of the sleptons. In a first step we use the model independent approach where LFV (off-diagonal entries in the mass matrix) is introduced by hand while respecting the direct search constraints from the charged lepton flavor violating (cLFV) processes. In the second step we use high scale see-saw models where LFV is generated via renormalization group equations (RGE) from the grand unification scale (GUT) down to electroweak scale. cLFV decays are the most restrictive ones and exclude a large part of the parameter space for the MI as well as the high scale see-saw scenarios. Due to very strict constraints from cLFV, it is difficult to find large corrections to LFVHD. This applies in particular to $h ightarrow τμ$ where hints of an excess have been observed. If this signal is confirmed, it could not be explained with the models under investigation.

Motivation & Objective

• To assess whether supersymmetric high-scale seesaw models can explain the CMS-reported excess in h→τμ decay.
• To evaluate lepton flavor violating Higgs boson decays (LFVHD) in the context of the MSSM with seesaw mechanisms.
• To determine the impact of charged lepton flavor violation (cLFV) constraints on LFVHD rates in model-independent and seesaw-motivated scenarios.
• To explore whether neutrino mass generation via seesaw mechanisms can generate sufficient LFV in Higgs decays to match experimental hints.
• To identify whether additional sources of LFV beyond seesaw-induced mixings are required to explain the observed excess.

Proposed method

• Uses a model-independent approach to introduce off-diagonal entries in the slepton mass matrix by hand, consistent with cLFV constraints.
• Applies renormalization group equations (RGEs) from the GUT scale down to the electroweak scale to generate LFV in Type-I seesaw models.
• Implements the LFV effects via a FeynArts model file extension to the MSSM, enabling numerical computation of decay rates.
• Performs numerical analysis in the constrained MSSM (CMSSM) framework with seesaw Type-I, using benchmark points consistent with neutrino data.
• Applies experimental bounds from μ→eγ and other cLFV processes to constrain allowed parameter space.
• Evaluates branching ratios (BR) for h→eμ, h→eτ, and h→τμ across the m₀–m₁/₂ parameter plane, with varying tanβ and A₀.

Experimental results

Research questions

• RQ1 Can the MSSM with high-scale seesaw mechanisms generate sufficiently large branching ratios for h→τμ to explain the CMS excess?
• RQ2 How do experimental constraints from cLFV processes (especially μ→eγ) limit the possible size of LFV Higgs decays?
• RQ3 To what extent do RGE-induced slepton mixings in seesaw models contribute to LFV Higgs decays?
• RQ4 Are there viable parameter regions in the CMSSM+seesaw I framework where LFV Higgs decays reach observable levels?
• RQ5 Can neutrino-motivated seesaw models, including Type-II and Type-III, enhance LFV Higgs decays beyond the limits found in this study?

Key findings

• The maximum predicted branching ratio for h→τμ is of the order of 10⁻⁹, far below the level needed to explain the CMS excess.
• Branching ratios for h→eμ are typically O(10⁻¹⁸) in the allowed parameter space due to constraints from μ→eγ.
• Even in the lower-left region of the m₀–m₁/₂ plane, BR(h→eτ) reaches only O(10⁻¹⁶), still too small to explain the excess.
• The BR(h→τμ) predictions in the CMSSM-seesaw I scenario remain below O(10⁻¹⁶) in the viable parameter space after applying cLFV bounds.
• High-scale seesaw models (Type-I, II, III) all yield LFV Higgs decay rates that are too small to account for the CMS signal, even in optimistic scenarios.
• The study concludes that additional sources of lepton flavor violation beyond those in seesaw models are likely required to explain the observed h→τμ excess.