[Paper Review] KeV Scale Frozen-in Self-Interacting Fermionic Dark Matter

Haipeng An, Ran Huo+1 more|arXiv (Cornell University)|Dec 13, 2018

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper proposes a keV-scale fermionic dark matter model where dark matter is produced via the freeze-in mechanism with a MeV-scale vector boson mediator. The model naturally generates self-interactions that resolve the coredensity profile in dwarf galaxies and relaxes the Lyman-α constraint to m_D ≳ 2 keV, while Fermi pressure and self-scattering jointly produce a ~10 pc core, consistent with observations.

ABSTRACT

We present a model in which the dark matter particle is frozen-in at MeV scale. In this model, the mediator between the standard model sector and the dark sector can automatically provide a self-interaction for dark matter. The interaction strength is naturally to be the in the region in favor of the cluster mass deficit anomaly. Due to the self-scattering, the Lyman-$α$ constraint can be relaxed to $m_D \gtrsim 2 $ keV. In this region the self-interaction and the Fermi pressure both play roles on forming a dark matter core at the center of the dwarf galaxies.

Motivation & Objective

• To address the small-scale anomalies in cold dark matter, such as the coredensity profile in dwarf galaxies and mass deficit in clusters.
• To relax the Lyman-α forest constraint on keV-scale dark matter by enabling non-thermal production via freeze-in.
• To show that Fermi pressure and self-scattering can jointly produce a small, stable core in dwarf galaxy halos.
• To demonstrate that stellar constraints on keV-scale dark matter can be avoided in this freeze-in framework.
• To provide a viable alternative to warm or self-interacting dark matter by combining freeze-in production with natural self-interactions.

Proposed method

• Introduces a Dirac fermion dark matter candidate χ with mass m_D ~ keV and a massive vector boson V (dark photon) at MeV scale, coupled to the SM via kinetic mixing.
• Uses the freeze-in mechanism to generate dark matter abundance through e⁺e⁻ annihilation and plasmon decay processes mediated by the vector boson V.
• Models the self-interaction strength via the mediator V, which naturally yields a self-scattering cross section consistent with cluster mass deficit observations.
• Applies the Fermi-Dirac distribution to account for degeneracy pressure in keV-scale fermionic dark matter, modifying the halo density profile.
• Solves the hydrostatic equilibrium equation with a modified Poisson equation that includes both Fermi pressure and self-scattering effects.
• Uses a transition radius r_M to match the dark matter halo profile to the NFW profile, ensuring consistency with observed baryonic mass distributions.

Experimental results

Research questions

• RQ1 Can a keV-scale fermionic dark matter model with freeze-in production naturally generate self-interactions that resolve the core-cusp problem in dwarf galaxies?
• RQ2 To what extent can the Lyman-α forest constraint on dark matter mass be relaxed in a freeze-in scenario with self-interactions?
• RQ3 How do Fermi pressure and self-scattering jointly affect the size and structure of dark matter cores in dwarf galaxies?
• RQ4 Can stellar constraints on keV-scale dark matter be evaded in a freeze-in model with a MeV-scale mediator?
• RQ5 What is the predicted core size in dwarf galaxies when both Fermi pressure and self-scattering are included in a keV-scale fermionic dark matter model?

Key findings

• The model relaxes the Lyman-α forest constraint to m_D ≳ 2 keV, allowing for keV-scale dark matter without violating observational limits.
• For m_D ≈ 2.5 keV, the combined effects of Fermi pressure and self-scattering produce a dark matter core of approximately 10 parsecs in size.
• At m_D ≈ 1.5 keV, the core size is about 15 parsecs, demonstrating the stabilizing role of Fermi pressure in the low-mass regime.
• The self-scattering cross section per unit mass is naturally in the range favored by cluster mass deficit observations, particularly in Fornax.
• The model avoids strong stellar constraints because the dark matter is produced via freeze-in, not thermal relic abundance, minimizing interactions during stellar evolution.
• Numerical solutions show that the core size is sensitive to m_D in the low-mass regime (m_D < 2.5 keV), where Fermi pressure dominates, and becomes less sensitive at higher masses, where self-scattering dominates.