[Paper Review] Muon $g-2$ in a two-Higgs-doublet model with a type-II seesaw mechanism

Chuan-Hung Chen, Cheng-Wei Chiang+1 more|arXiv (Cornell University)|Apr 7, 2021

Computational Physics and Python Applications9 citations

TL;DR

This paper proposes a two-Higgs-doublet model with a type-II seesaw mechanism to explain the muon $g-2$ anomaly while satisfying Higgs signal strength constraints. It shows that two-loop Barr-Zee diagrams involving a heavy Higgs triplet with a sizable vacuum expectation value (VEV) can generate a large contribution to $a_\mu$, enabling new physics at $\mathcal{O}(100)$ GeV scale without requiring light exotic scalars, and predicts novel decay patterns for the doubly-charged Higgs boson that demand new LHC search strategies.

ABSTRACT

We study the two-Higgs-doublet model with type-II seesaw mechanism. In view of constraints from the Higgs data, we consider the aligned two-Higgs-doublet scheme and its effects on muon anomalous magnetic dipole moment, $a_μ$, including both one-loop and two-loop Barr-Zee type diagrams. Thanks to a sizable trilinear scalar coupling, the Barr-Zee type diagrams mediated by the Higgs triplet fields have a dominant effect on $a_μ$. In particular, unlike the usual two-Higgs-doublet models that require exotic Higgs bosons light in mass, the masses of the corresponding particles in the model are of ${\cal O}(100)$~GeV. The doubly-charged Higgs boson presents a different decay pattern from the usual Higgs triplet model and thus calls for a new collider search strategy, such as multi-$τ$ searches at the LHC.

Motivation & Objective

• To resolve the long-standing $3.7\sigma$ discrepancy in the muon anomalous magnetic moment ($a_\mu$) using an extended scalar sector.
• To construct a model that satisfies stringent Higgs signal strength constraints from the LHC while allowing new physics at $\mathcal{O}(100)$ GeV scale.
• To incorporate the type-II seesaw mechanism for naturally generating light neutrino masses via a Higgs triplet.
• To explore the role of trilinear scalar couplings and loop-level contributions in generating a large $a_\mu$ without requiring light exotic scalars.
• To identify new collider signatures, particularly for the doubly-charged Higgs boson, that evade existing LHC bounds and require new search strategies.

Proposed method

• Adopt the aligned two-Higgs-doublet scheme (A2HDS), where one physical Higgs boson is SM-like, ensuring compatibility with Higgs signal data.
• Introduce a complex Higgs triplet to implement the type-II seesaw mechanism, generating Majorana masses for neutrinos via lepton number-violating couplings.
• Compute one-loop and two-loop Barr-Zee diagrams involving the Higgs triplet and the neutral and charged Higgs states, focusing on contributions to $a_\mu$.
• Derive the trilinear couplings between the neutral Higgs bosons ($h$, $H$) and the charged Higgs states ($\delta^{\pm\pm}$, $\delta^{\pm}$, $H^{\pm}$) from the scalar potential in the Higgs basis.
• Use the mass relations and mixing patterns to show that the $h$ and $H$ states decouple from the $\delta^0$ state at leading order in $v_\Delta$, simplifying the analysis.
• Analyze the mass spectrum and decay patterns of the Higgs triplet, particularly the doubly-charged Higgs boson, to identify new LHC search channels such as multi-$\tau$ final states.

Experimental results

Research questions

• RQ1 Can a two-Higgs-doublet model with a type-II seesaw mechanism generate a sufficiently large contribution to the muon anomalous magnetic moment ($a_\mu$) while remaining consistent with LHC Higgs signal data?
• RQ2 What is the role of two-loop Barr-Zee diagrams involving the Higgs triplet in enhancing $a_\mu$ when the new scalar masses are $\mathcal{O}(100)$ GeV?
• RQ3 How does the presence of multiple lepton-number-violating parameters in the Higgs triplet sector affect the vacuum expectation value (VEV) and the resulting phenomenology?
• RQ4 What are the distinctive decay patterns of the doubly-charged Higgs boson in this model, and how do they differ from the standard Higgs triplet model?
• RQ5 Can this model evade existing LHC bounds on the doubly-charged Higgs boson, particularly the 350 GeV limit from pair production, and what new search strategies are required?

Key findings

• The two-loop Barr-Zee diagrams mediated by the Higgs triplet fields dominate the contribution to $a_\mu$, enabling a large deviation even when the heavier neutral Higgs boson is $\mathcal{O}(100)$ GeV.
• The model achieves a significant $a_\mu$ contribution due to a sizable trilinear coupling between the Higgs triplet and the neutral Higgs bosons, which is enhanced by the triplet's VEV.
• Unlike conventional 2HDMs requiring exotic scalars below 10 GeV, this model allows new physics at $\mathcal{O}(100)$ GeV scale, satisfying LHC constraints on Higgs signal strengths.
• The doubly-charged Higgs boson in this model has a richer decay pattern than in the standard Higgs triplet model, including decays into $\tau\tau$, $\tau\mu$, and $\tau e$ final states, due to the extended scalar sector.
• The model evades the current ATLAS lower bound of 350 GeV on the doubly-charged Higgs boson by allowing decays involving the light charged Higgs boson, necessitating new search strategies such as multi-$\tau$ final states at the LHC.
• The trilinear couplings $\lambda_{h\delta^{--}\delta^{++}} = \Lambda_8$, $\lambda_{H\delta^{-}\delta^{+}} = -\Lambda_{12} - \frac{\Lambda'_{12}}{2}$, and $\lambda_{hH^{-}H^{+}} = \Lambda_3$ are key to the loop-induced $a_\mu$ contribution and are derived from the scalar potential in the Higgs basis.