arXiv subjects

Yann Mambrini

Publications and source records attributed to Yann Mambrini.

At least 19 recordsLinked to original sources

arXiv2026

PBH runaway during reheating

The growth of primordial black holes through the absorption of the surrounding plasma has recently been shown to exhibit a critical behavior during radiation domination. We extend this analysis to the reheating era and derive analytical solutions for general reheating histories. We show that reheating modifies the critical condition for runaway absorption, making it dependent on both the reheating dynamics and the black-hole formation time. We identify two distinct regimes: runaway growth occurring during reheating, or being triggered after the onset of radiation domination by the mass accumulated during reheating. More generally, we derive a simple composition law describing how independent mass- growth mechanisms combine across successive cosmological eras. Applying it to radiation absorption and inflaton accretion, we obtain analytical results in excellent agreement with the full numerical evolution.

astro-ph.CO↗

arXiv2026

Seesaw reheating

We introduce the Seesaw Reheating scenario, in which the inflaton transfers its energy to a long-lived intermediate scalar associated with the spontaneous breaking of lepton number before the Universe is reheated through its decay into right-handed neutrinos. As a result, the reheating temperature is no longer determined by the inflaton decay width but by the dynamics of the seesaw sector. We derive analytical solutions describing the complete reheating history, revealing two characteristic features of this scenario: the relativistic time dilation of the intermediate scalar, which suppresses its decay and delays the transfer of energy to the thermal bath, and its subsequent transition from a relativistic to a non-relativistic regime, introducing a new characteristic timescale in the thermal history. Together, these effects lead to simple analytical expressions for the reheating temperature. When the intermediate scalar is identified with the field responsible for the spontaneous breaking of lepton number, the same framework naturally connects the reheating temperature to the origin of neutrino masses and provides a well-motivated setting for sterile-neutrino dark matter.

hep-ph↗

arXiv2026

When direct detection constrains reheating temperature: freeze-in with stronger couplings and inflaton-seeded freeze-in

Recent results from the DAMIC-M and PandaX collaborations have excluded the standard freeze-in production of dark matter for masses in the range $3~\mathrm{MeV} \lesssim m_\chi \lesssim 1~\mathrm{GeV}$ in the context of extensions of the Standard Model featuring an additional ultra-light $U(1)_{\rm X}$ gauge boson. In this work, we analyze the constraints induced by DAMIC-M and PandaX results on the reheating temperature in freeze-in models at stronger coupling, or when a non-thermal source (such as inflaton decay) comes into play. We identify viable scenarios in which the DM relic abundance is correctly reproduced while evading current experimental bounds on the electron-scattering cross section, $\overline{\sigma}_\mathrm{e}$. In particular, we show that for reheating temperatures below the electroweak scale, Boltzmann suppressed production can be compensated by stronger couplings, bringing freeze-in scenarios within present experimental reach. Finally, we study a hybrid scenario in which a small branching ratio of inflaton decay seeds a nonzero initial dark-matter abundance. We show that such contributions can significantly modify freeze-in predictions across broad regions of parameter space, offering an additional pathway for probing extremely feeble interactions.

hep-ph↗

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.

hep-ph↗

arXiv2026

When Primordial Black Holes Absorb During the Early Universe

We study the evolution of primordial black holes (PBHs) formed in the early universe in the presence of a surrounding thermal bath. By incorporating the effects of thermal absorption, we show that PBHs can undergo significant mass growth, leading to extended lifetimes and substantial deviations from the standard Hawking evaporation scenario. We find a critical collapse efficiency, $γ_{\rm c} \simeq 0.395$, above which the PBH mass grows without bound. This correction has profound implications for both PBH-induced reheating and dark matter (DM) production. Specifically, we find that the reheating temperature can be suppressed, and the DM parameter space for the PBH reheating scenario can undergo $\mathcal{O}(10)$-$\mathcal{O}(10^4)$ corrections, depending on the PBH formation mass and collapse efficiency. Moreover, our results significantly shift the parameter space in which PBHs can account for the entirety of the DM. To the best of our knowledge, this is the first comprehensive phenomenological study to incorporate thermal absorption into PBH evolution and quantify its impact on cosmological observables.

astro-ph.CO↗

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).

hep-ph↗

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.

hep-ph↗

arXiv2025

Gravitational Production of Spin-3/2 Particles During Reheating

We compute the density of a spin-$\frac32$ particle, the raritron, produced at the end of inflation due to gravitational interactions. We consider a background inflaton condensate as the source of this production, mediated by the exchange of a graviton. This production greatly exceeds the gravitational production from the emergent thermal bath during reheating. The relic abundance limit sets an absolute minimum mass for a stable raritron, though there are also model dependent constraints imposed by unitarity. We also examine the case of gravitational production of a gravitino, taking into account the goldstino evolution during reheating. We compare these results with conventional gravitino production mechanisms.

hep-ph↗

arXiv2025

Burdening (or not) gravitational waves in the presence of primordial black holes

We present the spectrum of primordial gravitational wave (GW) expected from the presence of primordial black holes (PBH) and inflaton in the early Universe. For the first time, 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 {\it after} (high frequency modes) and {\it before} (low frequency modes) the end of inflation. We generalize our study for a potential $V(ϕ)\propto ϕ^k$ 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.

hep-ph↗

arXiv2025

Phenomenological Constraints on Higgs reheating

In many models of inflation, reheating is realized through a coupling between the inflaton and the Higgs boson. Often, the mass of the inflaton is of order $10^{13}~$GeV determined by the amplitude of the scalar fluctuation spectrum. However, in models where the inflaton potential is of the form $V \sim ϕ^k$ about its minimum, the inflaton is massless for $k\ge 4$ unless a bare mass term, $\frac12 m_ϕ^2 ϕ^2$, is present. In this case, the inflaton mass may be of order the electroweak scale and may be subject to existing collider constraints. In particular, we investigate the constraints on the inflaton mass and reheating temperature $T_{\rm rh}$ arising from the decay of $ϕ$ into $\mathcal{H}$ through an interaction term $μϕ|\mathcal{H}|^2$. We perform a renormalization group analysis to determine the relative values of $μ$ and $m_ϕ$ such that the Higgs potential remains stable (and perturbative) at high energy. Taking into account the running of the Higgs quartic self-coupling and the experimental constraints from the LHC via the $\texttt{HiggsTools}$ public code, we find that $3.4 \times 10^6 $ GeV $\lesssim T_{\rm rh}\lesssim 3.9 \times 10^{12} $ GeV with a corresponding constraint on the inflaton bare mass $260~{\rm GeV} \lesssim m_ϕ\lesssim 3.8 \times 10^{10}~{\rm GeV}$. The dependencies between $T_{\rm rh}$ and the inflaton bare mass $m_ϕ$ as well as between $μ$ and $m_ϕ$ are provided.

hep-ph↗

arXiv2025

Generalizing the Bogoliubov vs Boltzmann approaches in gravitational production

We investigate the spectral behavior of scalar fluctuations generated by gravity during inflation and the subsequent reheating phase. We consider a non-perturbative Bogoliubov treatment within the context of pure gravitational reheating. We compute both long and short-wavelength spectra, first for a massless scalar field, revealing that the spectral index in part of the infrared (IR) regime varies between $-6$ and $-3$, depending on the post-inflationary equation of state (EoS), $0\leq w_ϕ\leq1$. Furthermore, we study the mass-breaking effect of the IR spectrum by including finite mass, $m_χ$, of the daughter scalar field. We show that for $m_χ/H_{\rm e} \gtrsim 3/2$, where $H_{\rm e}$ is the Hubble parameter during inflation, the IR spectrum of scalar fluctuations experiences exponential mass suppression, while for smaller masses, $m_χ/H_{\rm e}<3/2$, the spectrum remains flat in the IR regime regardless of the post-inflationary EoS. For any general EoS, we also compute a specific IR scale, $k_m$, of fluctuations below which the IR spectrum will suffer from this finite mass effect. In the UV regime, oscillations of the inflaton background lead to interference terms that explain the high-frequency oscillations in the spectrum. Interestingly, we find that for any EoS, $1/9 \lesssim w_ϕ\lesssim 1$, the spectral behavior turns out to be independent of the EoS, with a spectral index $ -6$. We have compared this Bogoliubov treatment for the UV regime to perturbative computations with solutions to the Boltzmann equation and found an agreement between the two approaches for any EoS, $0 \lesssim w_ϕ\lesssim 1$. We also explore the relationship between the gravitational reheating temperature and the reheating EoS employing the non-perturbative analytic approach, finding that reheating can occur for $w_ϕ\gtrsim 0.6$.

gr-qc↗

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.

hep-ph↗

arXiv2025

Scalar Field Fluctuations and the Production of Dark Matter

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 $(m_χ, T_{\rm RH})$ parameter planes, where $m_χ$ is the mass of the scalar field and $T_{\rm RH}$ is the reheating temperature after inflation. The non-interacting scalar is highly constrained, requiring $m_χ> 3 \times 10^{12}~\rm {GeV}$ and $ T_{\rm RH} \lesssim 7~\text{TeV}$ 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.

hep-ph↗

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.

hep-ph↗

arXiv2024

Reheating and Leptogenesis after Vector inflation

We study the reheating and non-thermal leptogenesis in the case of a vector inflaton. We concentrate on particle production during the phase of oscillating background, especially gravitational production induced by the presence of non-minimal coupling imposed by an isotropic and homogeneous Universe. Including processes involving the exchange of graviton, we then extend our study to decay into fermions via direct or anomalous couplings. The necessity of non-minimal gravitational coupling and the gauge nature of couplings to fermions implies a much richer phenomenology than for a scalar inflaton.

hep-ph↗

arXiv2024

Gravitational Wave Production During Reheating: From the Inflaton to Primordial Black Holes

We calculate the gravitational waves (GWs) produced by primordial black holes (PBHs) in the presence of the inflaton condensate in the early Universe. Combining the GW production from the evaporation process, the gravitational scattering of the inflaton itself, and the density fluctuations due to the inhomogeneous distribution of PBHs, we propose for the first time a complete coherent analysis of the spectrum, revealing three peaks, one for each source. Three frequency ranges ($\sim$ kHz, GHz, and PHz, respectively) are expected, each giving rise to a similar GW peak amplitude $Ω_{\rm GW}$. We also compare our predictions with current and future GWs detection experiments.

hep-ph↗

arXiv2024

Leptogenesis, primordial gravitational waves, and PBH-induced reheating

We explore the possibility of producing the observed matter-antimatter asymmetry of the Universe uniquely from the evaporation of primordial black holes (PBH) that are formed in an inflaton-dominated background. Considering the inflaton $(ϕ)$ to oscillate in a monomial potential $V(ϕ)\proptoϕ^n$, we show, it is possible to obtain the desired baryon asymmetry via vanilla leptogenesis from evaporating PBHs of initial mass $\lesssim 10$ g. We find that the allowed parameter space is heavily dependent on the shape of the inflaton potential during reheating (determined by the exponent of the potential $n$), the energy density of PBHs (determined by $β$), and the nature of the coupling between the inflaton and the Standard Model (SM). To complete the minimal gravitational framework, we also include in our analysis the gravitational leptogenesis set-up through inflaton scattering via exchange of graviton, which opens up an even larger window for PBH mass, depending on the background equation of state. We finally illustrate that such gravitational leptogenesis scenarios can be tested with upcoming gravitational wave (GW) detectors, courtesy of the blue-tilted primordial GW with inflationary origin, thus paving a way to probe a PBH-induced reheating together with leptogenesis.

hep-ph↗

arXiv2024

Primordial black hole versus inflaton

We compare the dark matter(DM) production processes and its parameters space in the background of reheating obtained from two chief systems in the early Universe: the inflaton $ϕ$ and the primordial black holes (PBHs). We concentrated on the mechanism where DMs are universally produced only from the PBH decay and the generation of the standard model plasma from both inflton and PBHs. Whereas the distribution of Primordial Black Holes behaves like dust, the inflaton phenomenology depends strongly on its equation of state after the inflationary phase, which in turn is conditioned by the nature of the potential $V(ϕ)$. Depending upon the initial mass and population of PBHs, a large range of DM mass is shown to be viable if reheating is controlled by PBHs itself. Inflaton-dominated reheating is observed to further widen such possibilities depending on the initial population of black holes and its mass as well as the coupling of the inflaton to the standard model sector.

hep-ph↗