Singlet-doublet dark matter induced radiative neutrino mass and TeV scale leptogenesis

This article proposes two TeV-scale singlet-doublet dark matter models (Majorana and Dirac) that simultaneously explain the origin of radiative neutrino masses, the baryon asymmetry of the universe via leptogenesis, and the dark matter relic density, while offering testable signatures for collider experiments and cosmology.

The Gist

Imagine the universe as a vast, complex machine currently running on three mysterious fuels that scientists cannot quite see or touch: Dark Matter, Neutrino Mass, and Matter-Antimatter Asymmetry.

• Dark Matter is the invisible glue holding galaxies together.
• Neutrinos are ghostly particles that barely interact with anything, yet they possess a tiny, mysterious weight.
• Matter-Antimatter Asymmetry is the reason we exist at all. In the beginning, there should have been equal amounts of matter and antimatter, which would have annihilated each other, leaving nothing but light. Yet somehow, a tiny remnant of matter survived to create stars, planets, and us.

This work proposes a single, elegant "solution" that explains all three puzzles simultaneously by employing a new type of particle configuration called Singlet-Doublet Dark Matter. Consider this configuration as a special two-particle team that can assume different roles depending on its construction.

The authors investigate two versions of this team: the Majorana Team and the Dirac Team.

The Two Versions of the Team

1. The Majorana Team (The "Self-Reflecting" Version)

Imagine a particle that is its own mirror image. In this version, the universe is populated by three generations of these "mirror" particles (heavy and light) as well as a special invisible scalar particle (a type of energy field).

• The Dark Matter: The lightest member of this team is stable and invisible. It is the "Dark Matter" that fills the universe.
• The Neutrino Mass: The heavy members of the team are too massive to be Dark Matter, but they interact with the invisible scalar field. Through a complex quantum dance (in physical terms, a "loop"), they generate a tiny weight for the neutrinos. It is as if the heavy particles lend a bit of their mass to the neutrinos through a hidden connection.
• The Matter-Antimatter Imbalance: When the heavier, unstable members of this team decay (break apart), they do so in a way that favors matter over antimatter. This generates an excess of matter. This excess is then passed on to the particles we know (such as electrons and protons) via a cosmic relay system, eventually creating the baryon asymmetry observed today.

The Big Win: The authors show that this entire process can occur even if the particles are relatively light (in the "sub-TeV" range, which is light for particle physics). This means our current particle accelerators, such as the Large Hadron Collider, might be able to detect them soon.

2. The Dirac Team (The "Partner" Version)

In this version, the particles are not their own mirror image; they have distinct partners (like a left and a right hand). The universe contains a pair of these particles, three generations of invisible scalar fields, and a new type of "right-handed" neutrino partner.

• The Dark Matter: The lightest partner in this pair becomes the Dark Matter.
• The Neutrino Mass: Similar to the first version, the heavy partners and the scalar fields interact in a loop to grant the neutrinos their tiny mass. However, since these are "Dirac" particles, the entire "lepton number" (a type of particle counting) is conserved.
• The Matter-Antimatter Imbalance: Here is where it gets clever. When the heavy scalar fields decay, they produce equal amounts of "left-handed" matter and "right-handed" antimatter.
  • The left-handed part interacts with the universe's "sphaleron" processes (a type of cosmic mixer) and is converted into the matter we see today.
  • The right-handed part is invisible to this mixer and remains inactive.
  • The result? A net excess of matter in the visible universe, even though the total number of particles remained balanced.

The Big Win: This scenario works at the "TeV scale" (a few trillion electronvolts). As in the first version, the particles lie exactly in the range where our current and future experiments are searching for them.

Why This Matters (The "So What?")

The work claims that by using only these specific particle configurations, we do not need to invent three different, independent theories to explain Dark Matter, neutrino mass, and the existence of the universe. One framework handles everything.

Furthermore, the authors point to two exciting possibilities for how we might catch these particles:

1. Signals in Accelerators: Since the particles are light enough, they could decay in a way that leaves a "displaced vertex"—a feature where a particle travels a tiny, measurable distance before decaying. It is like seeing a firework fly a few meters before exploding, rather than exploding immediately.
2. Cosmic Background: In the Dirac version, the new particles could leave a subtle fingerprint in the cosmic microwave background (the afterglow of the Big Bang). Future telescopes like CMB-S4 could detect this additional "heat" or energy density and confirm the theory.

Summary

Consider this work as a master key. Instead of needing three different keys to unlock the doors of Dark Matter, neutrino mass, and the origin of the universe, the authors have developed a single, sophisticated locking mechanism (the Singlet-Doublet model) that opens all three doors simultaneously. They have shown that this mechanism operates at energy levels we can actually test, making it a promising candidate for the next major discovery in physics.

Technical Summary

Technical Summary: Singlet-Doublet Dark Matter-Induced Radiative Neutrino Masses and Leptogenesis at the TeV Scale

Problem Statement
The Standard Model (SM) cannot explain three critical observational facts: the existence of dark matter (DM), the origin of non-vanishing neutrino masses, and the observed matter-antimatter asymmetry of the universe. While canonical seesaw models can explain neutrino masses and baryogenesis via leptogenesis, they typically require high energy scales (far above the TeV scale) and do not integrate a natural candidate particle for dark matter. Conversely, scotogenic models generate neutrino masses radiatively at the one-loop level, naturally link the dark sector to neutrino mass generation, but often lack a unified mechanism for successful leptogenesis at accessible energy scales. This work addresses the need for a unified framework that simultaneously explains radiative neutrino masses, the dark matter relic density, and the baryon asymmetry within the TeV scale, making the scenario testable for current and future accelerators.

Methodology
The authors investigate two distinct realizations of the Singlet-Doublet Dark Matter (SDDM) framework, both extending the SM with a fermionic singlet and an electroweak doublet that mix after electroweak symmetry breaking (EWSB). The models are constrained by neutrino oscillation data, the anomalous magnetic moment of the muon ($(g-2)_\mu$), charged lepton flavor violation (cLFV, specifically $\mu \to e\gamma$), and direct detection limits.

1. Model A: Singlet-Doublet Majorana Dark Matter

   • Field Content: The SM is extended by three generations of singlet fermions ($N_i$), three generations of doublet fermions ($\Psi_i$), and a singlet scalar field ($\phi$). A discrete $Z_2$ symmetry ensures the stability of the lightest particle.
   • Neutrino Mass: Neutrino masses are forbidden at tree level due to the $Z_2$ symmetry (since $\phi$ does not acquire a vacuum expectation value). Masses arise radiatively at the one-loop level involving $N_i$, $\Psi_i$, and $\phi$.
   • Dark Matter: The lightest singlet-doublet fermion pair ($N_1, \Psi_1$) forms the Majorana dark matter candidate ($\chi_3$).
   • Leptogenesis: The baryon asymmetry is generated by the CP-violating, out-of-equilibrium decays of the heavier singlets ($N_2, N_3$) into doublets ($N \to \Psi H$), followed by the decay of the doublets into SM leptons and scalars ($\Psi \to L \phi$). The resulting lepton asymmetry is converted into baryon asymmetry via electroweak sphalerons.

2. Model B: Singlet-Doublet Dirac Dark Matter

   • Field Content: The SM is extended by a vector-like singlet fermion ($\chi$), a vector-like doublet fermion ($\Psi$), three generations of complex scalars ($\phi_i$), and three generations of right-handed Dirac neutrinos ($\nu_{Ri}$). A discrete $Z_4$ symmetry is imposed.
   • Neutrino Mass: A soft symmetry-breaking term in the scalar potential breaks the $Z_4$ symmetry and enables the generation of Dirac neutrino masses at the one-loop level.
   • Dark Matter: The lightest mass eigenstate ($\chi_1$), a mixture of $\chi$ and the neutral component of $\Psi$, serves as the Dirac dark matter candidate.
   • Leptogenesis: The CP-violating decays of the scalar fields ($\phi_i$) generate equal and opposite asymmetries in the left-handed ($L$) and right-handed ($\nu_R$) sectors. While total lepton number is conserved, the left-handed asymmetry is partially converted into baryon asymmetry via sphalerons, while the right-handed sector remains inert.

Main Results
The authors perform a comprehensive numerical scan of the parameter space for both models, utilizing the Casas-Ibarra parametrization to fit neutrino mass data and micrOMEGAs to calculate the relic density.

• Majorana Scenario (Model A):

  • Successful leptogenesis is achievable even in the sub-TeV range.
  • The model satisfies constraints from $(g-2)_\mu$ and cLFV ($\mu \to e\gamma$) with Yukawa couplings $\lambda \sim \mathcal{O}(10^{-4})$ and mixing angles $\sin \theta \sim \mathcal{O}(10^{-7})$ to $\mathcal{O}(10^{-1})$.
  • The presence of additional states in the dark sector (heavier generations of fermions and scalars) significantly modifies the relic density calculation, allowing for the correct DM abundance in regions where the pure singlet-doublet scenario would yield an underabundance.
  • Reference points (MBP1, MBP2) show that the correct baryon asymmetry ($\eta_B \sim 6 \times 10^{-10}$) can be generated with DM masses in the range of 150–800 GeV.

• Dirac Scenario (Model B):

  • Successful leptogenesis requires a scale of several TeV. The authors note that sub-TeV leptogenesis in this setup would require an increase in the soft symmetry-breaking parameter $\mu_\phi$, currently fixed at 1% of the scalar mass, to balance neutrino mass generation and washout effects.
  • The model exhibits seesaw-like behavior between the couplings $\lambda_\Psi$ (constrained by cLFV) and $\lambda_\chi$ (required for neutrino mass).
  • Direct detection prospects are promising, with allowed mixing angles $\sin \theta$ in the range of $\mathcal{O}(10^{-5})$ to $\mathcal{O}(10^{-2})$, depending on the mass hierarchy.
  • Reference points (DBP1, DBP2) yield the correct DM relic density ($\Omega_{DM}h^2 \approx 0.12$) and baryon asymmetry ($\eta_B \approx 6 \times 10^{-10}$) with scalar masses in the multi-TeV range.

Significance and Claims
The work claims to demonstrate that the Singlet-Doublet Dark Matter framework offers a viable, unified explanation for neutrino masses, dark matter, and baryon asymmetry at the TeV scale.

• Testability: In contrast to high-scale seesaw models, the particle masses in both scenarios lie in the GeV to TeV range, making them accessible to accelerator experiments (e.g., LHC), for instance via signatures such as prompt decays and displaced vertices (due to the small mass splitting between charged and neutral doublet components).
• Cosmological Signatures: The Dirac scenario predicts a contribution to the effective number of relativistic species, $\Delta N_{eff} \sim 0.14$, resulting from the thermalization of light right-handed neutrinos. The authors claim this falls within the projected sensitivity of future CMB experiments such as CMB-S4 and CMB-HD.
• Unified Framework: The work establishes that the same particles responsible for the dark matter relic density and the radiative generation of neutrino masses can also drive the mechanism for leptogenesis, satisfying all current phenomenological constraints without requiring new physics at inaccessible energy scales.