[Paper Review] Dark Matter, LFV and Neutrino Magnetic Moment in the Radiative Seesaw Model with Triplet Fermion

Wei Chao|arXiv (Cornell University)|Feb 28, 2012

Dark Matter and Cosmic Phenomena2 references6 citations

TL;DR

This paper proposes a radiative seesaw model with a triplet fermion Σ and a Z₂ symmetry, where the lightest neutral Σ component serves as a 2.594 TeV dark matter candidate stabilized by the symmetry. The model generates tiny neutrino masses via one-loop diagrams, predicts lepton flavor violation (LFV) processes and neutrino transition magnetic moments, and shows that LFV processes are constrained by current and future experiments, while magnetic moments remain below current bounds.

ABSTRACT

In this paper we work in the framework of a radiative seesaw model with triplet fermion $Σ$. Due to the $Z_2$ discrete flavor symmetry, the lightest neutral component of $Σ$ is stable and thus can be a dark matter candidate. Its mass can be solely determined by the dark matter relic abundance, which is bout 2.594 TeV. The model also predict a dark matter-nucleus scattering cross section that would be accessible with future dark matter direct detection searches. We further investigate constraints on the parameter space of the model from the lepton-flavor-violating processes and neutrino transition magnetic moments, induced by the Yukawa interaction of the $Σ$ with the left-handed lepton doublets.

Motivation & Objective

• To explore a radiative seesaw model with a fermion triplet Σ and Z₂ symmetry to simultaneously address neutrino masses, dark matter, and lepton flavor violation.
• To determine the dark matter relic abundance and its direct detection cross section via gauge-mediated annihilation.
• To constrain the model’s parameter space using lepton flavor violation (LFV) processes and neutrino transition magnetic moments.
• To assess the viability of the model under current and future experimental limits on LFV and magnetic moments.

Proposed method

• Introduces a radiative seesaw model with two Y=0 fermion triplets Σ, one inert Higgs doublet Φ, and a Z₂ symmetry under which only Σ and Φ are odd.
• Uses the Z₂ symmetry to stabilize the lightest neutral component of Σ as a cold dark matter candidate.
• Computes the dark matter relic density via thermal freeze-out, assuming gauge interactions dominate annihilation, leading to a mass of 2.594 TeV.
• Derives one-loop neutrino mass generation via exchange of Σ⁰ and Φ⁰, with the mass matrix expressed in terms of Yukawa couplings and loop functions.
• Evaluates lepton flavor violation processes (μ→eγ and μ-e conversion) using the Yukawa couplings of Σ with left-handed lepton doublets.
• Calculates neutrino transition magnetic moments at one-loop level using the same Yukawa interactions, with form factors derived from loop integrals.

Experimental results

Research questions

• RQ1 What is the mass of the dark matter candidate Σ⁰ in this model, and can it be determined solely from the observed relic density?
• RQ2 How do lepton flavor violation processes (μ→eγ and μ-e conversion) constrain the Yukawa couplings in the model?
• RQ3 What is the size of the neutrino transition magnetic moment predicted by the model, and how does it compare to current experimental bounds?
• RQ4 Can the model explain both dark matter and neutrino masses within a single framework, and is it testable via future LFV or direct detection experiments?

Key findings

• The dark matter candidate Σ⁰ has a mass of approximately 2.594 TeV, determined solely by fitting the observed dark matter relic density via gauge-mediated annihilation.
• The model predicts a dark matter-nucleus scattering cross section that is potentially accessible in future direct detection experiments.
• Lepton flavor violation processes such as μ→eγ and μ-e conversion in Au, Ti, and S are constrained by current experimental limits, with μ→eγ providing the strongest constraint.
• The neutrino transition magnetic moment is estimated to be of order 10⁻²³ μB, significantly below the current experimental upper bound of 0.32×10⁻¹⁰ μB at 90% CL.
• The decay rate of ν₃ → ν₂γ is estimated to be ~10⁻⁶⁹ GeV, indicating negligible contributions to the cosmic infrared background.
• The model’s parameter space is tightly constrained by LFV processes, especially μ→eγ, suggesting that future LFV experiments will probe the remaining viable regions.