Next-to-Minimal Freeze-in Dark Matter

Abstract

If the dark matter mass exceeds the highest temperature of the thermal bath, then dark matter production is Boltzmann suppressed. This opens new possibilities for dark matter model building. In particular, WIMP models that are experimentally excluded can be revived in this setting; conversely, freeze-in models, which would typically be beyond experimental reach, are potentially discoverable in the Boltzmann suppressed regime. In a recent letter, we highlighted these aspects for the case of electroweak doublet fermion dark matter assuming instantaneous inflationary reheating. Due to its elegance and simplicity, we coin this {\em Minimal Freeze-in} (MFI) Dark Matter. Here we consider next-to-minimal extensions of MFI dark matter. We present the implications for non-instantaneous reheating, including scenarios beyond the standard picture in which the Universe is initially matter dominated prior to reheating. Furthermore, we explore model variations within the electroweak dark matter scenario. Specifically, we consider fermion dark matter in higher representations of SU(2)${}_L$, exploring the current limits and the near-future discovery potential.

Figures (6)

• Figure 1: The solid lines correspond to the reheat temperature $T_\text{rh}$ which gives the observed dark matter relic density of mass $m$ assuming instantaneous reheating, for the different SU(2)$_L$ representations. The inset plot shows a zoom of the different lines. The vertical red regions indicate the parameter space excluded by direct detection (LZ); the projected sensitivity of DARWIN is also shown. 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: We show the lifetimes of the quintuplet ($\boldsymbol{5}$) representations due to Planck induced higher dimension operators for the two distinct cases $Y=0,\pm1,+2$ and $Y=-2$. This is plotted as a function of mass $m$. We overlay the indirect detection limits for decaying dark matter coming from Auger's observation of galactic $\gamma$-rays in red, specialized for the decay channel $\Psi^0 \to h \nu$Deligny:2024fyx. For $Y=0,\pm1,+2$ Auger places an upper bound on the dark matter mass. We also show the anticipated reach of KM3NeT (based on the assumption of a single event over 10 years) Kohri:2025bsn as the red dotted curve.
• Figure 3: Constraints from spin-independent limits LZ, for the different representations. We also show the projections of the proposed DARWIN experiment. For instantaneous reheating the dark matter mass uniquely determines the required $T_\text{rh}$ and we plot some characteristic contours. We also collect together the cosmological limits from BICEP/Keck which constrain $T_\text{rh}$ and the bounds from dark matter annihilations (cf. Eq. (\ref{['eq:mID']})), marked 'ID'. We separate the selection of models which are constrained by dark matter decays limits, which we show on the right hand panel, marked 'decays' (cf. Figure \ref{['fig:ID']}).
• Figure 4: Non-instantaneous reheating. The shaded region indicates viable parameters. The upper boundary is set by instantaneous reheating ($T_I = T_\text{rh}$); the lower boundary corresponds to the maximal duration of reheating, specifically $H(T_I)< 6\times 10^{13}~{\rm GeV}$ consistent with BICEP/Keck BICEP:2021xfz .The 'thermalization' region indicates parameters for which the dark matter enters equilibrium with the thermal bath, in which case the observed relic abundance is not reproduced.
• Figure 5: Zoomed-in view of the top left panel of Figure \ref{['fig:non-instantaneous']}, overlaying contours corresponding to different values of $T_\text{max}$.