Minimal Freeze-in Dark Matter: Reviving electroweak doublet dark matter with Boltzmann suppressed freeze-in

TL;DR

The paper investigates a minimal, electroweak-interacting dark matter scenario where an SU(2)$_L$ doublet fermion couples to Standard Model gauge bosons but never thermalizes due to Boltzmann-suppressed production when $m_{ m DM} > T_{ m rh}$. By computing near-threshold production cross sections, reaction densities, and freeze-in yields, the authors show that direct-detection constraints push the viable mass to $m_{ m DM} > 10^{10}$ GeV in the canonical case, with a high-dimension operator splitting the neutral state relaxing this to $ olinebreak \sim 300$ GeV$; the surviving relic is built from $oldsymbol{ extPsi^0}$ and decays of $oldsymbol{ extPsi^\pm}$ to $oldsymbol{ extPsi^0}$. In the pseudo-Dirac variant, tree-level $Z$ couplings vanish and loop-induced SI scattering reduces constraints, allowing EW-scale masses and extending discovery prospects to future experiments like Darwin. The framework is presented as the most minimalist freeze-in DM scenario, with robust predictions tied to the reheating history and clear experimental tests in the near future.

Abstract

Dark matter communicating with the Standard Model solely via electroweak interactions provides a compelling picture. However, thermal freeze-out of electroweak doublet dark matter is generically strongly excluded by direct detection. We show that SU(2)${}_L$ doublet fermion dark matter evades direct detection if its mass exceeds $10^{10}$ GeV. If the neutral Dirac fermion is split into a pseudo-Dirac pair (via high dimension operator) this limit can be relaxed to 300 GeV. Provided the dark matter mass is above the reheat temperature of the Universe, the production rate never exceeds the Hubble rate in cases of interest, thus the dark matter never thermalizes. We apply constraints from direct detection (e.g. LZ) and consider the discovery potential of Darwin. This scenario presents the most minimal model of freeze-in dark matter, and is both elegant and highly predictive.

Figures (3)

• Figure 1: The solid line is the reheat temperature $T_\text{rh}$ which gives the observed dark matter relic density for $\Psi^0$ of mass $m$ assuming instantaneous reheating. The vertical red areas mark exclusion from direct detection experiments, and the projection by Darwin. The black dashed line indicates $T_\text{rh} = m$, above which freeze-in is no longer Boltzmann suppressed. The red shaded "thermalization" region indicates parameter values for which $\Psi^0$ would enter equilibrium with the Standard Model.
• Figure 2: Constraints on Minimal Freeze-in Dark Matter. We apply the spin-independent limits from XENONnT XENON:2025vwd and LZ LZ:2024zvo. We also show the anticipated discovery reach of the proposed Darwin experiment DARWIN:2016hyl. The parameter space below the 'Darwin' line lies within the neutrino fog. For instantaneous reheating the dark matter mass uniquely determines the required $T_\text{rh}$ and we plot some characteristic contours. We show the cosmological limits (assuming single field inflationary reheating) from BICEP/Keck BICEP:2021xfz which constrain $T_\text{rh}$.
• Figure 3: Indirect detection bounds on the vector-like model. The total annihilation cross-section (for $v=10^{-3}$) is shown as the black solid curve. We also present the leading constraints, namely, LHAASO galactic plane bounds Boehm:2025qroLHAASO:2024lnz and CASA-MIA CASA-MIA:1997tns, assuming 100% annihilations to $W^+W^-$. The red dotted line marks the mass below which the (SI) scattering cross-section is excluded by direct detection. Note the main sensitivity of indirect detection experiments lies within the mass range that is already excluded by direct detection limits.