[Paper Review] Neutrinophilic two Higgs doublet model with $U(1)$ global symmetry

Kingman Cheung, Hiroshi Okada+1 more|arXiv (Cornell University)|Jun 7, 2017

Particle physics theoretical and experimental studies21 references3 citations

TL;DR

This paper proposes a neutrinophilic two-Higgs-doublet model with a U(1) global symmetry, where the second Higgs doublet acquires a tiny vacuum expectation value (VEV) via one-loop radiative corrections involving exotic fermions. This mechanism naturally generates small active neutrino masses without requiring fine-tuned Yukawa couplings, and predicts a GeV-scale sterile neutrino detectable at the Future Circular Collider (FCC), offering a testable framework for neutrino mass generation and beyond-Standard-Model physics.

ABSTRACT

We propose a neutrinophilic two-Higgs-doublet model, where the vacuum expectation value (VEV) of the second Higgs doublet is only induced at one-loop level via several neutral fermions. Thus, the masses of active neutrinos arising from the Higgs doublets are naturally small via such a tiny VEV. We discuss various phenomenology of the model, including the neutrino masses and oscillations, bounds on non-unitarity, lepton-flavor violations, the oblique parameters, the muon anomalous magnetic moment, the GeV-scale sterile neutrino candidate arising from the tiny VEV, and collider signatures. We finally discuss the possibility of detecting the sterile neutrino suggested in the experiment of Future Circular Collider (FCC).

Motivation & Objective

• To explain the small masses of active neutrinos without relying on fine-tuned Yukawa couplings.
• To extend the two-Higgs-doublet model (THDM) with a U(1) global symmetry and exotic fermions to generate a tiny VEV for the second Higgs doublet via loop effects.
• To explore phenomenological constraints from neutrino oscillations, lepton-flavor violation, oblique parameters, and the muon g−2.
• To identify a viable GeV-scale sterile neutrino candidate arising from the tiny VEV, and assess its detectability at future colliders like FCC.
• To provide a testable model where the sterile neutrino is more accessible than in canonical seesaw scenarios due to a large 20×20 mixing matrix.

Proposed method

• Introduce a U(1) global symmetry with exotic fermions (Dirac and Majorana types) and a singlet scalar field to mediate loop-induced VEV for the second Higgs doublet.
• Implement a Yukawa interaction Lagrangian involving the SM-like Higgs doublet (Φ₁), the inert doublet (Φ₂), and exotic fermions, with the second doublet's VEV generated at one-loop level.
• Use one-loop Feynman diagrams involving exotic fermions to compute the radiative generation of v₂, the VEV of Φ₂, ensuring it is naturally small.
• Derive analytical expressions for neutrino masses, lepton-flavor violating processes (e.g., μ→eγ), oblique parameters (S, T, U), and the muon anomalous magnetic moment (g−2).
• Perform numerical analysis using constraints from neutrino oscillation data, LFV bounds, oblique parameters, and g−2 to identify viable parameter space.
• Assess collider signatures at the LHC and detectability of the sterile neutrino at the FCC via its mixing and decay modes.

Experimental results

Research questions

• RQ1 Can a tiny vacuum expectation value for the second Higgs doublet be naturally generated via one-loop corrections in a U(1)-symmetric extension of the THDM?
• RQ2 Does the model successfully reproduce the observed neutrino oscillation data and non-unitarity bounds while satisfying constraints from lepton-flavor violation and oblique parameters?
• RQ3 What is the mass and mixing pattern of the sterile neutrino state induced by the tiny VEV, and is it detectable at future colliders like FCC?
• RQ4 Can the model simultaneously accommodate the measured muon g−2 anomaly and other precision electroweak observables?
• RQ5 How does the large 20×20 neutral fermion mixing matrix affect the phenomenology and detectability of the sterile neutrino compared to canonical seesaw models?

Key findings

• The second Higgs doublet's vacuum expectation value (v₂) is generated at one-loop level via exotic fermion loops, naturally leading to a small value that explains the tiny active neutrino masses.
• The model satisfies all constraints from neutrino oscillation data, lepton-flavor violation (e.g., μ→eγ), oblique parameters (S, T, U), and the muon g−2, with the latter receiving negative contributions that may require further tuning.
• A GeV-scale sterile neutrino with mass in the range of 20–50 GeV and mixing angle squared θₛ² ≈ 10⁻¹² to 10⁻¹¹ is predicted, lying within the reach of the FCC experiment.
• The sterile neutrino region satisfying all experimental bounds is found to be detectable at FCC, with a significantly enhanced testability compared to canonical seesaw models due to the large 20×20 mixing matrix.
• The model predicts a detectable signature via the Drell-Yan process involving exotic charged fermions (E′) at the LHC, and the Goldstone boson G from the U(1) symmetry may be probed via gravitational waves from early-universe phase transitions.
• The model remains consistent with dark matter constraints when an additional inert doublet is introduced, with a 500 GeV dark matter candidate possible.