Impact of conversion-driven processes on singlet-doublet Majorana dark matter relic

TL;DR

This work investigates a Majorana singlet-doublet fermionic dark matter (SDDM) model, a minimal SM extension where the DM candidate $\chi_3$ emerges from mixing between a singlet $\chi$ and a doublet $\Psi$, controlled by $M_{\rm DM}$, $ΔM$, and $\sin\theta$. It incorporates conversion-driven freeze-out channels—co-scattering, decay, and inverse-decay—in addition to annihilation and co-annihilation by solving a three-sector Boltzmann system that tracks the singlet, doublet, and SM bath. The authors show that the Majorana realization broadens the thermal relic-compatible region to $1~{\rm GeV}\lesssim M_{\rm DM}\lesssim1750~{\rm GeV}$ and $2\times10^{-7}\lesssim\sin\theta\lesssim0.16$ for all $ΔM>1$ GeV, with co-scattering enabling points that would otherwise be excluded. Conversion-driven dynamics extend collider-relevant regions, yielding displaced-vertex signatures and enhancing prospects for LHC and MATHUSLA searches, while direct-detection constraints further shape the viable parameter space. The framework connects relic-density calculations with current experimental bounds, highlighting how co-scattering and conversion-driven processes modify the allowed region in a minimal Majorana SDDM scenario.

Abstract

The singlet-doublet dark matter model offers a rich framework for exploring the nature of dark matter (DM) through its unique fermion structure. In this model, the important parameters are singlet-doublet mass splitting $Δ{M}$, singlet-doublet mixing angle $\sinθ$, and DM mass $M_{\rm DM}$. If the DM is assumed to be of Dirac nature, then the annihilation, co-annihilation, and conversion driven processes combinedly allows a range of parameter space: $1~{\rm GeV} \lesssim M_{\rm DM}\lesssim750$ GeV and $10^{-6}\lesssim\sinθ\lesssim0.04$ for all $Δ{M}>1$ GeV. While the nature of DM either Dirac or Majorana is not known, in this work we assume the nature of singlet-doublet DM to be of Majorana type and find that the relic density and direct detection can be satisfied in a larger parameter space. In particular, the allowed ranges of DM mass and $\sinθ$ are: $1~{\rm GeV}\lesssim M_{\rm DM}\lesssim1750$ GeV and $2\times10^{-7}\lesssim\sinθ\lesssim0.16$ for all $Δ{M}>1$ GeV.

Figures (4)

• Figure 1: [Left:] correct dark matter relic points are shown in the plane of $\Delta{M}$ vs $M_{\rm DM}$ for different ranges of $\sin\theta$ as mentioned in the inset of the figure without considering co-scattering contributions while solving Eqs. (\ref{['eq:Y1']}) and (\ref{['eq:Y2']}). [Right:] correct dark matter relic points are shown in the plane of $\Delta{M}$ vs $M_{\rm DM}$ for different ranges of $\sin\theta$ as mentioned in the inset of the figure considering all the relevant contributions including annihilation, co-annihilation, co-scattering and decay, inverse-decay while solving Eqs. (\ref{['eq:Y1']}) and (\ref{['eq:Y2']}). The gray region is excluded from the LEP bound DELPHI:2003uqw.
• Figure 2: Cosmological evolution of abundances of sector 1 and sector 2 particles for two sets of parameters illustrating co-annihilation (left) and co-scattering and decay effects (right). The parameters are mentioned in table \ref{['tab:tab2']} .
• Figure 3: Correct dark matter relic points are shown in the plane of $\sin\theta$ vs $M_{\rm DM}$. The color code represents the parameter $\rho_2$. The gray shaded region is excluded by direct detection experiment LZ LZ:2024zvo. LEP DELPHI:2003uqw constraint on the doublet fermion excludes the black shaded region.
• Figure 4: Phase diagram illustrating the SDDM parameter space. The black dashed line separates the thermal regime (above) from the non-thermal regime (below).