arXiv subjects

Stephen E. Henrich

Publications and source records attributed to Stephen E. Henrich.

7 recordsLinked to original sources

arXiv2026

Leptogenesis and Low Reheating Temperatures

We study leptogenesis during non-instantaneous reheating in the canonical type-I seesaw framework, with the dominant source of right-handed neutrino (RHN) production being non-thermal from inflaton decays ($ϕ\rightarrow NN$). While matter-like reheating ($w_ϕ=0$) fails to be compatible with standard leptogenesis for very low reheating temperatures, the situation is strikingly different for generalized Starobinsky potentials approximated by $V(ϕ)\proptoϕ^k$ with $k\geq4$ about the minimum. In the latter cases, the observed baryon asymmetry can readily be obtained for arbitrarily low reheating temperatures above the BBN bound of $\sim4$ MeV. We study radiation-like reheating ($w_ϕ=1/3$, $k=4$) in detail, showing that the evolving effective mass of the inflaton condensate leads to kinematic shutoff of the $ϕ\rightarrow NN$ channel, which qualitatively changes the leptogenesis dynamics. We include a detailed treatment of the effects of fragmentation of the inflaton condensate. The final baryon asymmetry depends primarily on only two parameters: the inflaton-RHN coupling, $y_{ϕNN}$, and the CP-violating parameter $|ε|$. Interestingly, the final asymmetry is largely insensitive to the RHN mass, the reheating temperature, and the RHN decay rate. While we focus on fermionic reheating, we show that the general features of these results also hold for bosonic reheating to scalars.

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arXiv2026

Gravitational ultra-relativistic freeze-out during general reheating

We investigate ultrarelativistic freeze-out (UFO) in the context of generic reheating scenarios. While the standard WIMP dark matter paradigm has been extensively studied, UFO has so far only been analyzed within the specific reheating channel $ϕ\rightarrow f\bar{f}$. Unlike in the standard WIMP mechanism, where dark matter can only be diluted after freeze-out at $T_\mathrm{FO} \sim m_χ/\mathcal{O}(10)$, UFO dark matter can undergo freeze-in like phases following the initial freeze-out, driven by the non-trivial temperature evolution. The exact temperature evolution then needs to be accounted for, as a change in the temperature scaling can modify the IR/UV nature of UFO, impacting the relic abundance. We first generalize UFO to an arbitrary temperature profile $T \sim a^{-ξ}$, making explicit the UV and IR regimes for a thermally averaged cross section $\langle σv \rangle \sim T^n / Λ^{n+2}$. Then, as a concrete example, we consider the minimal scenario in which gravitational particle production at the onset of reheating sources an initial radiation abundance, and show that this early hot bath changes the UFO parameter space. We refer to this effect as GUFO. Specializing to $n = 2$, we find that matter-like reheating ($V \sim ϕ^2$) accommodates dark matter masses up to $10^7~\mathrm{GeV}$ for $Λ\lesssim 10^9~\mathrm{GeV}$ as thermalization becomes less stringent, while radiation-like reheating ($V \sim ϕ^4$) is compatible with GUFO across all reheating channels only if gravitational processes are taken into account.

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arXiv2026

Searching for UFOs from the early universe: direct detection prospects for relativistically decoupling dark matter

Particles that decouple relativistically from the Standard Model bath during reheating represent a versatile class of well-motivated cold dark matter candidates. In fact, ultrarelativistic decoupling ($T_{\rm FO}\gg m_χ$) is quite generic for beyond the Standard Model (BSM) heavy portal interactions with strong couplings and relatively low reheating temperatures. In this work, we study the direct detection prospects for ultrarelativistically frozen-out (UFO) candidates, using $Z'$-portal dark matter as a case study. Although typical UFO cross sections are suppressed by a heavy mediator mass scale, we find that experiments such as LZ, XENONnT, PandaX, and DarkSide-50 have already excluded a large portion of the UFO parameter space and there remains viable space above the neutrino fog for $0.4 \text{ GeV} \lesssim m_{\rm DM}\lesssim 1$ TeV. Moreover, SuperCDMS SNOLAB, which is expected to begin collecting data in 2026, should access a large region of UFO parameter space in the 0.5-10 GeV mass range. For heavy BSM portal interactions ($M\gtrsim 1$ TeV), UFOs are typically more accessible to detection than freeze-in candidates due to the comparatively larger cross sections. We also carefully delineate regions of parameter space with degeneracy between UFO and non-relativistic freeze-out. In sum, UFOs are attractive candidates for ongoing and next-generation dark matter detection experiments in a looming post-WIMP era.

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arXiv2025

Z' portal dark matter from post-inflationary reheating: WIMPs, FIMPs, and UFOs

We investigate the production of dark matter (DM) via a heavy $Z'$ mediator during the post-inflationary reheating epoch. In particular, we study production from three mechanisms which are smoothly connected to one another: WIMP-like freeze-out, FIMP-like freeze-in, and ultra-relativistic freeze-out (UFO). This is the first systematic study of $Z'$ portal DM which includes UFO. We find that much of the available parameter space for keV to TeV DM lies in the UFO regime for $ 1 \text{ TeV}\lesssim M_{Z'} \lesssim 1 \text{ PeV}$. When the mediator mass $M_{Z'}$ is greater than both the DM mass and the reheating temperature, UFO is a robust mechanism for producing cold DM. Although UFO DM is initially "hot" after freeze-out, it can easily become cold before structure formation if freeze-out occurs during post-inflationary reheating. Compared to standard freeze-in, UFO can accommodate significantly stronger interaction strengths (stronger couplings and/or smaller mediator masses).

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arXiv2025

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

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.

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arXiv2025

Ultra-Relativistic Freeze-Out During Reheating

We perform a thorough investigation of (ultra)relativistic freeze-out (UFO) during reheating. While the standard WIMP (non-relativistic freeze-out) and FIMP (freeze-in) paradigms have been explored in detail during the reheating epoch, UFO has not been systematically studied, despite the fact that it is operative in a broad region of parameter space. Although dark matter (DM) is ``hot" at the time of relativistic freeze-out, we show that it can easily undergo enough cooling by the time of structure formation to be compatible with $Λ$CDM. Unlike standard WIMP-like freeze-out, there can be significant out-of-equilibrium DM production after UFO, similar to the freeze-in mechanism. However, unlike freeze-in, UFO can accommodate much stronger couplings. The UFO parameter space consistent with $Ω_χh^2=0.12$ is quite large, with DM masses spanning about 13 orders of magnitude ($10^{-7} \text{ GeV} \lesssim m_χ \lesssim 10^{6}$ GeV), reheating temperatures spanning 17 orders of magnitude ($10^{-2} \text{ GeV} \lesssim T_{\rm RH} \lesssim 10^{15} \text{ GeV}$) and Beyond the Standard Model (BSM) effective interaction scales spanning 11 orders of magnitude ($10^{3} \text{ GeV} \lesssim Λ\lesssim 10^{14}\text{ GeV}$). Interestingly, the most suitable range of couplings for UFO lies precisely between the typical couplings for WIMPs and FIMPs, rendering UFO quite attractive from the standpoint of detection. Particle physics models that are easily amenable to UFO include heavy vector or scalar portal interactions, along with nonrenormalizable effective interactions. Finally, we show there is a distinction between UV UFO and IR UFO, where the relic abundance for the former is sensitive to the freeze-out temperature, while the abundance for the latter is sensitive to the DM mass and the reheating temperature but insensitive to the freeze-out temperature.

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arXiv2024

Aspects of Gravitational Portals and Freeze-in during Reheating

We conduct a systematic investigation of freeze-in during reheating while taking care to include both direct and indirect production of dark matter (DM) via gravitational portals and inflaton decay. Direct production of DM can occur via gravitational scattering of the inflaton, while indirect production occurs through scattering in the Standard Model radiation bath. We consider two main contributions to the radiation bath during reheating. The first, which may dominate at the onset of the reheating process, is produced via gravitational scattering of the inflaton. The second (and more standard contribution) comes from inflaton decay. We consider a broad class of DM production rates parameterized as $R_χ \propto T^{n+6}/Λ^{n+2}$, and inflaton potentials with a power-law form $V(ϕ) \propto ϕ^{k}$ about the minimum. We find the relic density produced by freeze-in for each contribution to the Standard Model bath for arbitrary $k$ and $n$, and compare these with the DM density produced gravitationally by inflaton scattering. We find that freeze-in production from the gravitationally-produced radiation bath can exceed that of the conventional decay bath and account for the observed relic density provided that $m_χ > T_{\rm RH}$, with additional $k$- and $n$-dependent constraints. For each freeze-in interaction considered, we also find $m_χ$- and $T_{\rm RH}$-dependent limits on the BSM scale, $Λ$, for which gravitational production will exceed ordinary freeze-in production.

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