Sterile neutrino dark matter from a TeV Scale Seesaw

Abstract

In this article, we investigate the phenomenological aspects of a feebly interacting sterile neutrino dark matter candidate within a low-scale seesaw framework. The Type-I seesaw model is augmented by a second complex scalar doublet ($Φ_ν$), which couples exclusively to the heavy right-handed neutrinos and the lepton doublet, thereby generating the neutrino Dirac mass term while the first scalar doublet is responsible for giving mass to the remaining Standard Model particles. The lightest sterile neutrino ($N_1$) acts as a feebly interacting massive particle (FIMP), produced via decays of $W^\pm$, $Z$ and extra scalars present in the setup. We point out that $W^\pm$ and $Z$ contributions were overlooked in the previous studies, which actually dominate the $N_1$ production by a factor of $\sim 10^{13}$ and solely determines the relic abundance. Incorporating them leads to several novel consequences for the DM phenomenology like a new non-thermal condition which leads to smaller Yukawa couplings. We thoroughly discuss about the enhancement possibilities of $N_1$'s mass which is controlled by the small vacuum expectation value ($v_ν$) of the second Higgs doublet. After incorporating the latest Lyman-$α$ forest observations, this setup can accommodate both warm and cold dark matter scenarios.

Figures (5)

• Figure 1: Abundance of $N_1$ as a function of temperature from different gauge boson decays. The solid red line represents the total abundance of $N_1$ obtained by summing all contributions at benchmark points $v_{\nu}=10$ MeV, $M_{N_{1}}=0.1$ MeV, and $Y^{\nu}_{i1} \sim 10^{-14}$. The small plot shows the variation of all scalars, which is suppressed by the factor of $\sim 10^{13}$ from the $W^\pm$ and $Z$ contributions to the total abundance of $N_1$.
• Figure 2: Evolution of the dark matter relic abundance $\Omega h^2$ as a function of $z = m_h/T$. The solid maroon curve denotes the total relic abundance from all decay channels, while the green dashed line shows the contribution from $W$ boson decays, the blue dash-dotted line indicates the $Z$ boson decays, and the dotted black line corresponds to combined scalar decays. The horizontal red solid line indicates the observed relic abundance, $\Omega h^2 \sim 0.12$, as reported by Planck.(benchmark same as Fig. \ref{['numbden']})
• Figure 3: The red solid curve denotes the values of Dirac Yukawa coupling $Y^{\nu}_{11}$ and sterile neutrino mass $M_{N_1}$ for which the observed relic abundance is satisfied via freeze-in production. For simplicity, it was assumed that $Y^{\nu}_{i1} \sim Y^{\nu}_{11}$ where $i$ goes from $1$ to $3$ and benchmark points are $v_\nu \sim 10$ MeV and $m_{S^\pm} \simeq m_{S^0} \simeq m_{A^0} \sim 200$ GeV.
• Figure 4: The solid red and green contours denote the regions satisfying the dark matter relic density with and without gauge bosons ($W^\pm, Z$), respectively. The corresponding $v_\nu$ values are indicated at the end of each contour in the $\theta^2_{1}$–$M_{N_1}$ plane. The blue-shaded region is excluded by X-ray observations arising from the decay $N_1 \rightarrow \gamma \nu$ with $m_{s^+} \simeq m_{S^0} \simeq m_{A^0} \sim 200$ GeV.
• Figure 5: The blue line (contour) shows free-streaming length ($r_{\mathrm{fs}}$) as a function of sterile neutrino dark matter mass ($M_{N_1}$), showing excluded hot dark matter (shaded brown), constrained warm dark matter (shaded yellow), further constrained by the latest Lyman-$\alpha$ forest bounds (2024) (shaded green) and allowed cold dark matter (shaded blue) regions. The vertical dashed line marks the $7.1~\mathrm{keV}$ scale relevant to the observed $3.55~\mathrm{keV}$ X-ray line. The plot classifies dark matter candidates by free-streaming horizon consistent with astrophysical constraints.(benchmark same as Fig. \ref{['numbden']})