Freeze-in and Freeze-out in Right-Handed Neutrino Extended MSSM with Seesaw

TL;DR

This work analyzes a MSSM extended with right-handed neutrinos at a low seesaw scale, examining a two-component dark sector in which higgsino freeze-out competes with freeze-in production of right-handed sneutrinos. It shows that direct-detection bounds exclude higgsino-dominated DM unless gauginos are unnaturally heavy, and that freeze-in requires extremely small Yukawa couplings $y^{\nu} \sim 10^{-12}$, which in turn implies light sterile neutrinos whose DW production typically overproduces DM. Consequently, type-I, linear, and inverse seesaws are effectively ruled out, because sterile neutrinos generated via DW yield too large a relic density, while only Dirac neutrino scenarios (with no sterile states) can survive relic-density constraints. The findings place strong constraints on electroweak-scale SUSY seesaw DM and favor Dirac neutrinos if freeze-in sneutrino DM is realized. $y^{\nu}$-dependent freeze-in and direct-detection interplay thus critically shapes viable SUSY seesaw DM scenarios.

Abstract

We investigate the possibility of saturating the relic density bound with light higgsinos. When the minimal supersymmetric Standard Model is extended with right-handed neutrino superfields and the seesaw scale is very low, right-handed sneutrinos can be produced via the freeze-in mechanism. In such a case we can have essentially two independent sources for dark matter, the traditional freeze-out of higgsinos and the freeze-in of right-handed sneutrinos. The heavier of these two will decay to the lighter species with a delay. We rule out such a scenario for all seesaw models as the lifetime of sterile neutrinos, produced over-abundantly via Dodelson-Widrow mechanism, exceeds the age of the universe and will contribute to the relic density.

Figures (6)

• Figure 1: Higgsinos annihilate mainly through the $W$-portal as the coupling is unsuppressed. Here SM denotes Standard Model fermions.
• Figure 2: Potential t- and s-channel processes contributing to the freeze-in production of the right-handed sneutrinos. The mediator can be (a) the SM Higgs, (b) the charged Higgs, (c) a neutral higgsino, (d) the charged higgsino, (e) a left-handed neutrino, or (f) a left-handed sneutrino. SM stands for Standard Model particles ($t$, $b$, $\tau$, $W$, $Z$).
• Figure 3: Relic density as a function of scanned parameters, $\mu$ and $M^2_{\nu,N}$ for BP1. The kinks indicate changes in the nature of the LSP from neutralino to sneutrino as we increase the parameter.
• Figure 4: Relic density as a function of scanned parameters, $y^\nu_{1,1}$ and $y^\nu_{2,1}$, demonstrating a quadratic dependence across BP1.
• Figure 5: Mixing angle–mass plane for a right-handed (sterile) neutrino: $\sin^{2}\!\bigl(2\theta\bigr)$ vs. $m_{N}\,(\mathrm{GeV})$. The model points are distributed based on the total relic abundance by freeze-in and freeze-out. The background colour map shows the sterile-neutrino relic abundance $\Omega_{N}h^{2}$ from the Dodelson–Widrow mechanism.