arXiv subjects

Mathias Pierre

Publications and source records attributed to Mathias Pierre.

At least 19 recordsLinked to original sources

arXiv2025

Fermion (non)reheating with a quartic inflaton potential

Any viable inflationary model must account for reheating of the universe prior to the onset of primordial nucleosynthesis. In this work, we study the (p)reheating mechanism for an inflaton field with a quartic minimum of the T-model kind with coupling $\lambda$, prior to and post fragmentation, making a clear distinction between the two regimes. We assume that the main particle production channel corresponds to the decay into a pair of spin 1/2 fermions via Yukawa-like interactions. On top of its decays, we also consider the self-interaction of the inflaton, which sources the resonant growth of inflaton inhomogeneities, possibly leading to its eventual fragmentation. By means of a combination of non-perturbative (Heisenberg/Bogoliubov) and perturbative (Boltzmann) methods, we find that for Yukawa couplings that seemed to be intuitively perturbative, such as $y\gtrsim 10^{-8}$ ($y^2/\lambda\gtrsim 3\times10^{-5}$), parametric resonance, kinematic blocking, and Pauli suppression effects cannot be ignored. Additionally, we show that achieving $\rho_\phi \sim \rho_\psi$ prior to fragmentation requires large couplings, $y\gtrsim 0.2$ ($y^2/\lambda\gtrsim 10^{10}$), which needs a detailed study of backreaction and radiative corrections. Thus the rest of our work constitutes studying post-fragmentation fermion production where we conclude that, in general, reheating in this setup is not possible and thus we conclude that in order to successfully reheat, one must invoke a coupling to a integer- and/or 0-spin particle like a scalar boson.

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arXiv2024

Intrinsic non-Gaussianity of ultra slow-roll inflation

We study the non-Gaussian tail of the curvature fluctuation, $\zeta$, in an inflationary scenario with a transient ultra slow-roll phase that generates a localized large enhancement of the spectrum of $\zeta$. To do so, we implement a numerical procedure that provides the probability distribution of $\zeta$ order by order in perturbation theory. The non-Gaussianities of $\zeta$ can be shown to arise from its non-linear relation to the inflaton fluctuations and from the intrinsic non-Gaussianities of the latter, which stem from its self interactions. We find that intrinsic non-Gaussianities, which have often been ignored to estimate the abundance of primordial black holes in this kind of scenario, are important. The relevance of the intrinsic contribution depends on the rapidity with which the transient ultra slow-roll phase occurs, as well as on its duration. Our method cannot be used accurately when the perturbative in-in formalism fails to apply, highlighting the relevance of developing fully non-perturbative approaches to the problem.

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arXiv2024

Non-Gaussian tails without stochastic inflation

We show, both analytically and numerically, that non-Gaussian tails in the probability density function of curvature perturbations arise in ultra-slow-roll inflation from the $\delta N$ formalism, without invoking stochastic inflation. Previously reported discrepancies between both approaches are a consequence of not correctly accounting for momentum perturbations. Once they are taken into account, both approaches agree to an excellent degree. The shape of the tail depends strongly on the phase space of inflation.

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arXiv2024

Gravitational wave signatures of post-fragmentation reheating

After cosmic inflation, coherent oscillations of the inflaton field about a monomial potential $V(\phi)\sim \phi^k$ result in an expansion phase characterized by a stiff equation-of-state $w\simeq(k-2)/(k+2)$. Sourced by the oscillating inflaton condensate, parametric (self)resonant effects can induce the exponential growth of inhomogeneities eventually backreacting and leading to the fragmentation of the condensate. In this work, we investigate realizations of inflation giving rise to such dynamics, assuming an inflaton weakly coupled to its decay products. As a result, the transition to a radiation-dominated universe, i.e. reheating, occurs after fragmentation. We estimate the consequences on the production of gravitational waves by computing the contribution induced by the stiff equation-of-state era in addition to the signal generated by the fragmentation process for $k=4,6,8,10$. We find that the signal generated during the fragmentation process gives a larger contribution than the one induced by the stiff equation-of-state era in given frequency ranges for all values of $k$. Our results are independent of the reheating temperature provided that reheating is achieved posterior to fragmentation. Our work shows that the dynamics of such weakly-coupled inflaton scenario can actually result in characteristic gravitational wave spectra with frequencies from Hz to GHz, in the reach of future gravitational wave observatories, in addition to the complementarity between upcoming detectors in discriminating (post)inflation scenarios. We advocate the need of developing high-frequency gravitational wave detectors to gain insight into the dynamics of inflation and reheating.

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arXiv2024

The Waning of the WIMP: Endgame?

Weakly Interacting Massive Particles (WIMPs) continue to be considered some of the best-motivated Dark Matter (DM) candidates. No conclusive signal, despite an extensive search program that combines, often in a complementary way, direct, indirect, and collider probes, has been however detected so far. This situation might change in the near future with the advent of even larger, multi-ton Direct Detection experiments. We provide here an updated review of the WIMP paradigm, with a focus on selected models that can be probed with upcoming facilities, all relying on the standard freeze-out paradigm for the relic density. We also discuss Collider and Indirect Searches when they provide complementary experimental information.

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arXiv2023

Scalar dark matter production from the inflaton

A curved space-time is known to act as a source for particle production in environments where gravity plays a significant role. We explore this effect in a minimal setup of cosmic inflation on the production of a scalar dark matter candidate during and after the inflationary stage of the universe. We consider the production of dark matter via direct coupling to the inflaton field or from pure gravitational interactions. Cosmological constraints from structure formation and dark matter isocurvature perturbations are discussed. A new analytical expression for the isocurvature power spectrum is provided.

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arXiv2023

Effects of Fragmentation on Post-Inflationary Reheating

We consider the effects of fragmentation on the post-inflationary epoch of reheating. In simple single field models of inflation, an inflaton condensate undergoes an oscillatory phase once inflationary expansion ends. The equation of state of the condensate depends on the shape of the scalar potential, $V(\phi)$, about its minimum. Assuming $V(\phi) \sim \phi^k$, the equation of state parameter is given by $w = P_\phi/\rho_\phi = (k-2)/(k+2)$. The evolution of condensate and the reheating process depend on $k$. For $k \ge 4$, inflaton self-interactions may lead to the fragmentation of the condensate and alter the reheating process. Indeed, these self-interactions lead to the production of a massless gas of inflaton particles as $w$ relaxes to 1/3. If reheating occurs before fragmentation, the effects of fragmentation are harmless. We find, however, that the effects of fragmentation depend sensitively to the specific reheating process. Reheating through the decays to fermions is largely excluded since perturbative couplings would imply that fragmentation occurs before reheating and in fact could prevent reheating from completion. Reheating through the decays to boson is relatively unaffected by fragmentation and reheating through scatterings results in a lower reheating temperature.

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arXiv2023

Reheating after Inflaton Fragmentation

In the presence of self-interactions, the post-inflationary evolution of the inflaton field is driven into the non-linear regime by the resonant growth of its fluctuations. The once spatially homogeneous coherent inflaton is converted into a collection of inflaton particles with non-vanishing momentum. Fragmentation significantly alters the energy transfer rate to the inflaton's offspring during the reheating epoch. In this work we introduce a formalism to quantify the effect of fragmentation on particle production rates, and determine the evolution of the inflaton and radiation energy densities, including the corresponding reheating temperatures. For an inflaton potential with a quartic minimum, we find that the efficiency of reheating is drastically diminished after backreaction, yet it can lead to temperatures above the big bang nucleosynthesis limit for sufficiently large couplings. In addition, we use a lattice simulation to estimate the spectrum of induced gravitational waves, sourced by the scalar inhomogeneities, and discuss detectability prospects. We find that a Boltzmann approach allows to accurately predict some of the main features of this spectrum.

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arXiv2023

A New Window into Gravitationally Produced Scalar Dark Matter

Conventional scenarios of purely gravitationally produced dark matter with masses below the Hubble parameter at the end of inflation are in tension with Cosmic Microwave Background (CMB) constraints on the isocurvature power spectrum. We explore a more general scenario with a non-minimal coupling between the scalar dark matter field and gravity, which allows for significantly lighter scalar dark matter masses compared to minimal coupling predictions. By imposing relic abundance, isocurvature, Lyman-$\alpha$, and Big Bang Nucleosynthesis (BBN) constraints, we show the viable parameter space for these models. Our findings demonstrate that the presence of a non-minimal coupling expands the parameter space, yielding a dark matter mass lower bound of $2 \times 10^{-4} \, \rm{eV}$.

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arXiv2023

Monomial warm inflation revisited

We revisit the idea that the inflaton may have dissipated part of its energy into a thermal bath during inflation, considering monomial inflationary potentials and three different forms of dissipation rate. Using a numerical Fokker-Planck approach to describe the stochastic dynamics of inflationary fluctuations, we confront this scenario with current bounds on the spectrum of curvature fluctuations and primordial gravitational waves. We also obtain purely analytical approximations that improve over previously used ones in the small dissipation regime for the amplitude of the spectrum and its tilt. We show that only our numerical Fokker-Planck method is accurate, fast and precise enough to test these models against current data. We advocate its use in future studies of warm inflation. We also apply the stochastic inflation formalism to this scenario, finding that the resulting spectrum is the same as the one obtained with standard perturbation theory. We discuss the origin and convenience of using a commonly implemented large thermal correction to the primordial spectrum and the implications of such a term for a specific scenario. Improved bounds on the scalar spectral index will further constrain warm inflation in the near future.

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arXiv2023

Isocurvature Constraints on Scalar Dark Matter Production from the Inflaton

We investigate the production of a spectator scalar dark matter field that is directly coupled to the inflaton during inflation and reheating. We consider two specific inflationary potentials, namely the Starobinsky and T-model of inflation, which satisfy the constraints on the scalar tilt, $n_s$, and tensor-to-scalar ratio, $r$, measured by the Planck satellite. Excitation of light scalar dark matter during inflation may result in large isocurvature perturbations, which can be avoided by inducing a sizable effective dark matter mass during the inflationary phase. For purely gravitational production, the Planck isocurvature constraints require the dark matter mass to be larger than the Hubble scale at horizon exit, with $m_{\chi} \gtrsim 0.5H_*$. For small bare dark matter masses $m_{\chi} \ll H_*$, these constraints translate into a lower bound on the dark matter coupling to the inflaton. We argue that these constraints can be applied to a wide class of single-field slow-roll inflation models. We also derive isocurvature, dark matter abundance, and Lyman-$\alpha$ constraints on the direct coupling and bare dark matter mass.

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arXiv2022

Primordial black holes and gravitational waves from dissipation during inflation

We study the generation of a localized peak in the primordial spectrum of curvature perturbations from a transient dissipative phase during inflation, leading to a large population of primordial black holes. The enhancement of the power spectrum occurs due to stochastic thermal noise sourcing curvature fluctuations. We solve the stochastic system of Einstein equations for many realizations of the noise and obtain the distribution for the curvature power spectrum. We then propose a method to find its expectation value using a deterministic system of differential equations. In addition, we find a single stochastic equation whose analytic solution helps to understand the main features of the spectrum. Finally, we derive a complete expression and a numerical estimate for the energy density of the stochastic background of gravitational waves induced at second order in perturbation theory. This includes the gravitational waves induced during inflation, during the subsequent radiation epoch and their mixing. Our scenario provides a novel way of generating primordial black hole dark matter with a peaked mass distribution and a detectable stochastic background of gravitational waves from inflation.

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arXiv2022

Higgs-boson visible and invisible constraints on hidden sectors

We investigate the impact of interactions between hidden sectors and the discovered Higgs boson $h_{125}$, allowing for additional invisible decay channels of $h_{125}$. We perform $\chi^2$-fits to the measurements of the Higgs-boson cross sections as a function of the invisible branching ratio and different combinations of coupling modifiers, where the latter quantify modifications of the couplings of $h_{125}$ compared to the predictions of the Standard Model. We present generic results in terms of exclusion limits on the coupling modifiers and the invisible branching ratio of $h_{125}$. Additionally, we apply our results to a variety of concrete model realizations containing a hidden sector: dark matter within Higgs- and singlet-portal scenarios, models featuring (pseudo) Nambu-Goldstone bosons and two Higgs doublet extensions. One of the main conclusions of our work is that in a wide class of models the indirect constraints resulting from the measurements of the cross sections of $h_{125}$ provide substantially stronger constraints on the invisible Higgs-boson branching ratio compared to the direct limits obtained from searches for the invisible decay of $h_{125}$. However, we demonstrate that the presence of an invisible decay mode of $h_{125}$ can also open up parameter space regions which otherwise would be excluded as a result of the indirect constraints. As a byproduct of our analysis, we show that in light of the new results from the LZ collaboration a fermionic DM candidate within the simplest Higgs-portal scenario is ruled out under standard assumptions.

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arXiv2022

Scalar Dark Matter Production from Preheating and Structure Formation Constraints

We investigate the out-of-equilibrium production of scalar dark matter (DM) from the inflaton condensate during inflation and reheating. We assume that this scalar couples only to the inflaton via a direct quartic coupling and is minimally coupled to gravity. We consider all possible production regimes: purely gravitational, weak direct coupling (perturbative), and strong direct coupling (non-perturbative). For each regime, we use different approaches to determine the dark matter phase space distribution and the corresponding relic abundance. For the purely gravitational regime, scalar dark matter quanta are copiously excited during inflation resulting in an infrared (IR) dominated distribution function and a relic abundance which overcloses the universe for a reheating temperature $T_\text{reh}>34 ~\text{GeV}$. A non-vanishing direct coupling induces an effective DM mass and suppresses the large IR modes in favor of ultraviolet (UV) modes and a minimal scalar abundance is generated when the interference between the direct and gravitational couplings is maximal. For large direct couplings, backreaction on the inflaton condensate is accounted for by using the Hartree approximation and lattice simulation techniques. Since scalar DM candidates can behave as non-cold dark matter, we estimate the impact of such species on the matter power spectrum and derive the corresponding constraints from the Lyman-$\alpha$ measurements. We find that they correspond to a lower bound on the DM mass of $\gtrsim 3\times 10^{-4} \, \rm{eV}$ for purely gravitational production, and $\gtrsim 20 \, \rm {eV}$ for direct coupling production. We discuss the implications of these results.

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arXiv2021

Inverse Seesaw, dark matter and the Hubble tension

We consider the inverse Seesaw scenario for neutrino masses with the approximate Lepton number symmetry broken dynamically by a scalar with Lepton number two. We show that the Majoron associated to the spontaneous symmetry breaking can alleviate the Hubble tension through its contribution to $\Delta N_\text{eff}$ and late decays to neutrinos. Among the additional fermionic states required for realizing the inverse Seesaw mechanism, sterile neutrinos at the keV-MeV scale can account for all the dark matter component of the Universe if produced via freeze-in from the decays of heavier degrees of freedom.

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arXiv2021

Metastable Conformal Dark Matter

We show that a metastable dark matter candidate arises naturally from the conformal transformation between the Einstein metric, where gravitons are normalised states, and the Jordan metric dictating the coupling between gravity and matter. Despite being secluded from the Standard Model by a large scale above which the Jordan metric shows modifications to the Einstein frame metric, dark matter couples to the energy momentum tensor of the Higgs field in the primordial plasma primarily. This allows for the production of dark matter in a sufficient amount which complies with observations. The seclusion of dark matter makes it long-lived for masses $\lesssim 1$ MeV, with a lifetime much above the age of the Universe and the present experimental limits. Such a dark matter scenario has clear monochromatic signatures generated by the decay of the dark matter candidate into neutrino and/or $\gamma-$rays.

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arXiv2020

How warm are non-thermal relics? Lyman-$\alpha$ bounds on out-of-equilibrium dark matter

We investigate the power spectrum of Non-Cold Dark Matter (NCDM) produced in a state out of thermal equilibrium. We consider dark matter production from the decay of scalar condensates (inflaton, moduli), the decay of thermalized and non-thermalized particles, and from thermal and non-thermal freeze-in. For each case, we compute the NCDM phase space distribution and the linear matter power spectrum, which features a cutoff analogous to that for Warm Dark Matter (WDM). This scale is solely determined by the equation of state of NCDM. We propose a mapping procedure that translates the WDM Lyman-$\alpha$ mass bound to NCDM scenarios. This procedure does not require expensive ad hoc numerical computations of the non-linear matter power spectrum. By applying it, we obtain bounds on several NCDM possibilities, ranging from $m_{\rm DM}\gtrsim {\rm EeV}$ for DM production from inflaton decay with a low reheating temperature, to sub-keV values for non-thermal freeze-in. We discuss the phenomenological implications of these results for specific examples which include strongly-stabilized and non-stabilized supersymmetric moduli, gravitino production from inflaton decay, $Z'$ and spin-2 mediated freeze-in, and non-supersymmetric spin-3/2 DM.

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arXiv2020

Disformal Dark Matter

We generalize dark matter production to a two-metric framework whereby the physical metric, which couples to the Standard Model (SM), is conformally and/or disformally related to the metric governing the gravitational dynamics. We show that this setup is naturally present in many Ultra Violet (UV) constructions, from K\"ahler moduli fields to tensor-portal models, and from emergent gravity to supergravity models. In this setting we study dark matter production in the early Universe resulting from both scatterings off the thermal bath and the radiative decay of the inflaton. We also take into account non-instantaneous reheating effects at the end of inflation. In this context, dark matter emerges from the production of the scalar field mediating the conformal/disformal interactions with the SM, i.e. realising a Feebly Interacting Matter Particle (FIMP) scenario where the suppression scale of the interaction between the scalar and the SM can be taken almost as high as the Planck scale in the deep UV.

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