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Probing the Scotogenic Dirac Model with FIMP Dark Matter and $ΔN_{\rm eff}$

Shu-Yuan Guo, Man-Yu Zhao

Abstract

We study a feebly interacting massive particle realization of the Scotogenic Dirac Model in which the lightest neutral fermion $N_1$ serves as a dark matter candidate, produced via the freeze-in or super-WIMP mechanism. The model generates Dirac neutrino masses at one loop, resulting in a rank-2 mass matrix that predicts one nearly massless neutrino. We analyze the DM relic density for various next-to-lightest odd particles (NLOPs), finding that coannihilation effects and enhanced annihilation channels are crucial for achieving the correct thermal freeze-out abundance of the NLOP. We provide a detailed analysis of the model's implications for the effective number of relativistic species, $ΔN_{\mathrm{eff}}$, which receives contributions from both a thermal bath of right-handed neutrinos and non-thermal energy injection due to late NLOP decays. Through an extensive parameter scan, we identify viable parameter space for all NLOP candidates that satisfies constraints from DM relic density, lepton flavor violation, Big Bang Nucleosynthesis, Cosmic Microwave Background, and $ΔN_{\mathrm{eff}}$.

Probing the Scotogenic Dirac Model with FIMP Dark Matter and $ΔN_{\rm eff}$

Abstract

We study a feebly interacting massive particle realization of the Scotogenic Dirac Model in which the lightest neutral fermion serves as a dark matter candidate, produced via the freeze-in or super-WIMP mechanism. The model generates Dirac neutrino masses at one loop, resulting in a rank-2 mass matrix that predicts one nearly massless neutrino. We analyze the DM relic density for various next-to-lightest odd particles (NLOPs), finding that coannihilation effects and enhanced annihilation channels are crucial for achieving the correct thermal freeze-out abundance of the NLOP. We provide a detailed analysis of the model's implications for the effective number of relativistic species, , which receives contributions from both a thermal bath of right-handed neutrinos and non-thermal energy injection due to late NLOP decays. Through an extensive parameter scan, we identify viable parameter space for all NLOP candidates that satisfies constraints from DM relic density, lepton flavor violation, Big Bang Nucleosynthesis, Cosmic Microwave Background, and .

Paper Structure

This paper contains 8 sections, 15 equations, 9 figures, 2 tables.

Table of Contents

  1. Introduction
  2. THE MODEL
  3. Dark Matter RELIC DENSITY
  4. Constraints from $N_{\text{eff}}$
  5. Thermal Contribution
  6. Non-thermal Contribution from Delayed Decay of NLOP
  7. Combined Results
  8. Conclusion

Figures (9)

  • Figure 1: The one-loop neutrino mass in Scotogenic Dirac Model.
  • Figure 2: The evolution of dark matter through freeze-in. The parameters used are listed in Tab. \ref{['tab:params']}.
  • Figure 3: The evolution of dark matter and NLOP under the "super-WIMP" mechanism. $N_2$ is chosen as NLOP in the upper two subfigures, while in the lower two subfigures $\phi_R$ and $\chi$ are selected as NLOP, respectively. The parameters used are listed in Tab. \ref{['tab:params']}.
  • Figure 4: The projected detecting capabilities of MATHUSLA to a long-lived $\phi^\pm$, in the mass-decay length plane. The band shows the efficiencies vary from 0.5 to 1. Colored lines correspond to different strengths of Yukawa coupling between $\phi^\pm$ and the dark matter.
  • Figure 5: (Left panel) The ratio of $\Gamma_{\rm el}/H$ as a function of temperature for different choice of $\lambda_5$. Here we have chosen $m_\chi = 100~\rm{GeV}$ for demonstration. (Right panel) The evolution of ratio $T_{\nu_R}/T_\gamma$ for different dark plasma decoupling temperature.
  • ...and 4 more figures