Neutrino mass, Dark Matter and Baryon Asymmetry via TeV-Scale Physics without Fine-Tuning (0807.0361v2)

Abstract: We propose an extended version of the standard model, in which neutrino oscillation, dark matter, and baryon asymmetry of the Universe can be simultaneously explained by the TeV-scale physics without assuming unnatural hierarchy among the mass scales. Tiny neutrino masses are generated at the three loop level due to the exact $Z_2$ symmetry, by which stability of the dark matter candidate is guaranteed. The extra Higgs doublet is required not only for the tiny neutrino masses but also for successful electroweak baryogenesis. The model provides discriminative predictions especially in Higgs phenomenology, so that it is testable at current and future collider experiments.

• The paper presents a TeV-scale extension of the Standard Model that generates tiny neutrino masses via three-loop processes without fine-tuning.
• The model stabilizes a scalar dark matter candidate through an exact Z₂ symmetry, predicting a relic abundance consistent with experimental observations.
• It incorporates additional CP-violating phases in an extended Higgs sector to enable a strong first-order phase transition for successful electroweak baryogenesis.

Analysis of TeV-Scale Physics in Explaining Neutrino Mass, Dark Matter, and Baryon Asymmetry

The paper by Aoki, Kanemura, and Seto presents a compelling model extending the Standard Model (SM) to address three major phenomena unresolved by the SM: neutrino masses, dark matter (DM), and baryon asymmetry in the Universe. This model, rooted in TeV-scale physics, seeks to provide a cohesive explanation for these phenomena without relying on significant hierarchical mass scales.

Fundamental Aspects of the Model

1. Neutrino Mass Generation: Unlike traditional seesaw mechanisms that require introducing very heavy right-handed neutrinos, the proposed model generates tiny neutrino masses at the three-loop level due to an exact $Z_2$ symmetry. This symmetry prohibits tree-level Yukawa couplings, with the extra Higgs doublet playing a pivotal role in this generation. The absence of a large hierarchy between mass scales here is critical, aligning all new physics at accessible TeV scales.
2. Dark Matter Stability and Characteristics: The $Z_2$ symmetry also stabilizes the lightest neutral $Z_2$-odd state, potentially making it a viable DM candidate—specifically, the scalar singlet $\eta$. The model predicts a mass range for $\eta$ that aligns with experimentally accessible regions. The annihilation rates and resulting relic abundance are calculated, supporting $\eta$ as a credible DM candidate with sufficient annihilation strength to predict current DM density values accurately.
3. Baryon Asymmetry through Electroweak Baryogenesis: The model incorporates CP violations in the extended Higgs sector to generate baryon asymmetry during the electroweak phase transition (EWPT). With additional CP-violating phases, the paper argues electroweak baryogenesis as a feasible mechanism under these extensions, necessitating strong first-order phase transitions achievable through the scalar potential's structure.

Phenomenological Implications and Predictions

The model makes several testable predictions regarding Higgs physics that align with ongoing and future collider experiments:

• Higgs Phenomenology: Predictions related to the SM-like Higgs boson suggest deviations in decay routes, particularly involving invisible channels due to possible decays into DM particles ($\eta$). The modifications in Higgs coupling are significant enough to be observed at facilities like the LHC and ILC.
• Lepton Flavor Violation: The model’s parameters fall within constraints allowed by current data regarding processes such as $\mu \to e\gamma$, with predictions for other flavor-changing neutral current processes that could be probed in future experiments.
• Dark Matter Detection: The real scalar singlet $\eta$ interaction with nuclear matter presents opportunities for detection via direct DM searches. The predictability of the coupling strength due to model constraints offers robust bounds for potential detection signals.
• Colliders and Beyond: Further scrutiny could be achieved via collider processes involving new particles predicted by the model. Charge and multi-Higgs production at colliders would provide complementary verification avenues.

Conclusion and Theoretical Outlook

The work highlights a controlled and theoretically motivated expansion of the SM consistent with all current experimental data for neutrinos, DM, and baryon asymmetry, emphasizing the utility of TeV-scale new physics. It provides a fertile testing ground for TeV-scale physics theories that offer comprehensive solutions to these challenges in particle physics. If collider and non-collider experiments in the near to intermediate future confirm the unique predictions of this model, it could significantly sway the direction of particle physics and cosmology, pointing to accessible energy scales for new physics beyond the SM.