Yann Mambrini

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 _FO ≫ 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 GeV ≲ m _DM ≲ 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 ≳ 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.

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 TeV ≲ M _Z' ≲ 1 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).

We present the spectrum of primordial gravitational wave (GW) expected from the presence of primordial black holes (PBHs) and inflaton in the early Universe. For the first time, to our knowledge, we combine the waves produced by the PBH decay with their density fluctuation counterpart, as well as their effects on the GW produced by the inflaton (high-frequency modes) and (low-frequency modes) the end of inflation. We generalize our study for a potential V ( ϕ ) ∝ ϕ k during reheating. We also extend our study, taking into account a possible memory burden effect to see how it can affect the shape of the spectrum.

We reexamine the case for dark matter (DM) produced by ultrarelativistic freeze-out (UFO). UFO is the mechanism by which standard model neutrinos decouple from the radiation bath in the early Universe at a temperature T_{d}≈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 ultrarelativistic 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 ΛCDM, 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, postinflationary 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 weakly interacting massive particle and feebly interacting massive particle regimes that excludes UFO. However, for many other interactions, such as standard model fermions producing fermionic DM via a heavy scalar or vector mediator, the weakly interacting massive particle to feebly interacting massive particle transition occurs necessarily via a large intermediate regime corresponding to UFO. We characterize the general features of UFO in this Letter, while we supply a more detailed analysis in a companion paper. These results render the conventional mechanism for producing hot DM broadly compatible with cold DM.

One of the simplest possible candidates for dark matter is a stable scalar singlet beyond the Standard Model. If its mass is below the Hubble scale during inflation, long-wavelength modes of this scalar will be excited during inflation, and their subsequent evolution may lead to the correct relic density of dark matter. In this work, we provide a comprehensive analysis of the evolution of a spectator scalar. We examine three cases: (1) a non-interacting massive scalar, (2) a massive scalar with self-interactions of the form λ_χχ^p, and (3) a massive scalar coupled to the inflaton ϕ through an interaction term of the form σ _n,m ϕ ⁿ χ ^m . In all cases, we assume minimal coupling to gravity and compare these results with the production of short-wavelength modes arising from single graviton exchange. The evolution is tracked during the reheating phase. Our findings are summarized using (m _χ, T _RH) parameter planes, where m _χ is the mass of the scalar field and T _RH is the reheating temperature after inflation. The non-interacting scalar is highly constrained, requiring m _χ > 3 × 10¹² GeV and T _RH ≲ 7 TeV (in the absence of fine-tuning) for an inflationary potential with a quadratic minimum. However, when self-interactions or couplings to the inflaton are included, the viable parameter space expands considerably. In these cases, sub-GeV and even sub-eV scalar masses can yield the correct relic abundance, opening new possibilities for light dark matter candidates. In all cases, we also impose additional constraints arising from the production of isocurvature fluctuations, the prevention of a secondary inflationary phase triggered by the spectator field, and the fragmentation of scalar condensates.