Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Ultra-relativistic freeze-out: a bridge from WIMPs to FIMPs

Stephen E. Henrich, Yann Mambrini, Keith A. Olive

TL;DR

The paper revisits ultra-relativistic freeze-out (UFO) as a dark matter production mechanism that can bridge the WIMP and FIMP regimes by relaxing the assumption of instantaneous reheating after inflation. It shows that UFO requires the interaction rate to have a steeper temperature dependence than the Hubble rate, with the condition $\frac{d \ln \Gamma_{\rm rel}}{d \ln T} > \frac{d \ln H}{d \ln T}$, and that during reheating this translates into $n > 1$ for typical cross sections. By analyzing representative interactions—a Higgs-scalar contact ($n=-2$) and heavy-mediator fermion production ($n=2$)—the authors demonstrate that UFO can be the intermediate stage in some WIMP-to-FIMP transitions and can yield cold dark matter across a vast range of masses and reheating temperatures. They provide analytic estimates of the UFO parameter space showing it is large and that sub-GeV UFO dark matter is accessible to skipper CCD experiments, while also noting compatibility with BBN and potential extensions to other reheating scenarios via a companion work.

Abstract

We re-examine the case for dark matter (DM) produced by ultra-relativistic freeze-out (UFO). UFO is the mechanism by which Standard Model (SM) neutrinos decouple from the radiation bath in the early universe at a temperature $T_{d} \approx 1$ MeV. This corresponds to chemical freeze-out without Boltzmann suppression, such that the freeze-out (decoupling) temperature $T_{d}$ is much greater than $m_ν$ and the neutrinos are therefore ultra-relativistic at freeze-out. While UFO has historically been rejected as a viable mechanism for DM production due to its association with hot DM and the accompanying incompatibility with $Λ$CDM, we show that when the approximation of instantaneous reheating after inflation is lifted, UFO can produce cold DM and account for the entire observed relic density in large regions of parameter space. In fact, DM with masses ranging from sub-eV to PeV scales can undergo UFO and be cold before structure formation, given only a simple perturbative, post-inflationary reheating period prior to radiation domination. For some interactions, such as a contact interaction between the Higgs and DM scalars, there is a seamless transition between the WIMP and FIMP regimes which excludes UFO. However, for many other interactions, such as SM fermions producing fermionic DM via a heavy scalar or vector mediator, the WIMP to FIMP transition occurs \textit{necessarily} via a large intermediate region corresponding to UFO. We characterize the general features of UFO in this paper, while we supply a more detailed analysis in a companion paper. We find that UFO during reheating can produce the correct relic density ($Ω_χh^2 = 0.12$) for DM masses spanning about 13 orders of magnitude, reheating temperatures spanning 17 orders of magnitude, and beyond the Standard Model (BSM) effective interaction scales spanning 11 orders of magnitude.

Ultra-relativistic freeze-out: a bridge from WIMPs to FIMPs

TL;DR

The paper revisits ultra-relativistic freeze-out (UFO) as a dark matter production mechanism that can bridge the WIMP and FIMP regimes by relaxing the assumption of instantaneous reheating after inflation. It shows that UFO requires the interaction rate to have a steeper temperature dependence than the Hubble rate, with the condition , and that during reheating this translates into for typical cross sections. By analyzing representative interactions—a Higgs-scalar contact () and heavy-mediator fermion production ()—the authors demonstrate that UFO can be the intermediate stage in some WIMP-to-FIMP transitions and can yield cold dark matter across a vast range of masses and reheating temperatures. They provide analytic estimates of the UFO parameter space showing it is large and that sub-GeV UFO dark matter is accessible to skipper CCD experiments, while also noting compatibility with BBN and potential extensions to other reheating scenarios via a companion work.

Abstract

We re-examine the case for dark matter (DM) produced by ultra-relativistic freeze-out (UFO). UFO is the mechanism by which Standard Model (SM) neutrinos decouple from the radiation bath in the early universe at a temperature MeV. This corresponds to chemical freeze-out without Boltzmann suppression, such that the freeze-out (decoupling) temperature is much greater than and the neutrinos are therefore ultra-relativistic at freeze-out. While UFO has historically been rejected as a viable mechanism for DM production due to its association with hot DM and the accompanying incompatibility with CDM, we show that when the approximation of instantaneous reheating after inflation is lifted, UFO can produce cold DM and account for the entire observed relic density in large regions of parameter space. In fact, DM with masses ranging from sub-eV to PeV scales can undergo UFO and be cold before structure formation, given only a simple perturbative, post-inflationary reheating period prior to radiation domination. For some interactions, such as a contact interaction between the Higgs and DM scalars, there is a seamless transition between the WIMP and FIMP regimes which excludes UFO. However, for many other interactions, such as SM fermions producing fermionic DM via a heavy scalar or vector mediator, the WIMP to FIMP transition occurs \textit{necessarily} via a large intermediate region corresponding to UFO. We characterize the general features of UFO in this paper, while we supply a more detailed analysis in a companion paper. We find that UFO during reheating can produce the correct relic density () for DM masses spanning about 13 orders of magnitude, reheating temperatures spanning 17 orders of magnitude, and beyond the Standard Model (BSM) effective interaction scales spanning 11 orders of magnitude.

Paper Structure

This paper contains 2 sections, 10 equations, 3 figures, 1 table.

Table of Contents

  1. Schematic Comparison of WIMP-like Freeze-Out vs. UFO
  2. UFO Parameter Space

Figures (3)

  • Figure 1: $\Gamma$ vs. $T$ plots illustrating the transition from WIMP-like to FIMP-like behavior for two different interactions and high temperature instantaneous reheating (left) vs. non-instantaneous reheating with $T_{\rm RH} = 100$ GeV (right). Freeze-out occurs at the low-temperature intersection of $\Gamma$ and $H$ (for both WIMP-like and UFO DM). UFO is observed for the FSF (or FVF) interaction (bottom panels) but not for the contact interaction between scalar DM and the Higgs (top panels). The vertical gray dashed lines correspond to $T=T_{\rm RH}$. Results are shown for $m_{\chi}=10^4$ GeV and $M_{s}=10^{6}$ GeV.
  • Figure 2: Evolution of the comoving DM number density ($Y_{\chi}$) during reheating, illustrating the FO/FI transition for three different cases. Top panel: A contact interaction between DM and SM scalars for $m_{\chi}>T_{\rm RH}$. UFO is absent. Middle panel: FSF/FVF interaction for $m_{\chi}>T_{\rm RH}$. WIMP-like FO, UFO, and FI are all observed. Lower panel: FSF/FVF interaction for $m_{\chi}<T_{\rm RH}$. Only UFO and FI are observed. The vertical gray lines correspond to $\frac{a_{\rm RH}}{a_{\rm end}}$, with $T_{\rm RH}=100$ GeV.
  • Figure 3: Schematic illustration of the regimes in which WIMP-like FO or UFO can occur for a contact interaction between DM and SM scalars (top) or an interaction via a heavy mediator of mass $M_{s}=10^6$ GeV (bottom), with $m_{\chi}=1$ TeV.