arXiv subjects

Géraldine Servant

Publications and source records attributed to Géraldine Servant.

18 recordsLinked to original sources

arXiv2026

A universal bound on the duration of a kination era

We show that primordial adiabatic curvature fluctuations generate an instability of the scalar field sourcing a kination era. We demonstrate that the generated higher Fourier modes constitute a radiation-like component dominating over the kination background after about $11$ e-folds of cosmic expansion. Current constraints on the extra number of neutrino flavors $ΔN_{\rm eff}$ thus imply the observational bound of approximately 10 e-folds, representing the most stringent bound to date on the stiffness of the equation of state of the pre-Big-Bang-Nucleosynthesis universe.

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arXiv2026

New Source for QCD Axion Dark Matter Production: Curvature Induced

We discuss a novel mechanism for generating dark matter from a fast-rolling scalar field, relevant for both inflation and rotating axion models, and apply it specifically to the (QCD) axion. Dark matter comes from scalar field fluctuations generated by the product of the curvature perturbation and the fast-rolling background field. These fluctuations can explain the totality of dark matter in a vast axion parameter space, particularly for the QCD axion, which will be targeted by upcoming experiments. We review the constraints on this mechanism and potential gravitational-wave signatures.

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arXiv2024

Model implementations of axion dark matter from kinetic misalignment

The axion kinetic misalignment mechanism (KMM) opens the possibility of explaining dark matter for almost any axion mass and decay constant that are not accessible by the standard misalignment mechanism, in particular at low values of the axion decay constant (i.e. large coupling). This is a new opportunity for most axion experiments which could be sensitive to dark matter and probe new regimes of axion cosmology. We scrutinise UV completions that lead to the KMM mechanism. These mainly rely on the early dynamics of the axion partner, the radial mode of the complex scalar field, from which the axion inherits kinetic energy. The damping of the radial-mode energy density is then a necessary ingredient. We study in detail thermal damping from interactions in the plasma. A minimal and rather natural implementation consists of a KSVZ-type model with a nearly-quadratic potential for the radial mode extended by U(1)-breaking higher-dimensional operators. Furthermore, we study Higgs portal interactions as an alternative damping mechanism and improve upon previously proposed implementations based on quartic potentials. These implementations can lead to the QCD axion being dark matter and in the reach of IAXO, while MADMAX, IAXO and ALPS II can be sensitive to a generic Axion-Like-Particle (ALP) as dark matter. Such models typically feature a kination era. We also show that ALP dark matter from KMM points to a particular realization of inflation.

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arXiv2024

Ultrahigh frequency primordial gravitational waves beyond the kHz: The case of cosmic strings

We investigate gravitational-wave backgrounds (GWBs) of primordial origin that would manifest only at ultra-high frequencies, from kilohertz to 100 gigahertz, and leave no signal at either LIGO, Einstein Telescope, Cosmic Explorer, LISA, or pulsar-timing arrays. We focus on GWBs produced by cosmic strings and make predictions for the GW spectra scanning over high-energy scale (beyond $10^{10}$ GeV) particle physics parameters. Signals from local string networks can easily be as large as the Big Bang nucleosynthesis/cosmic microwave background bounds, with a characteristic strain as high as $10^{-26}$ in the 10 kHz band, offering prospects to probe grand unification physics in the $10^{14}-10^{17}$ GeV energy range. In comparison, GWB from axionic strings is suppressed (with maximal characteristic strain $\sim 10^{-31}$) due to the early matter era induced by the associated heavy axions. We estimate the needed reach of hypothetical futuristic GW detectors to probe such GWB and, therefore, the corresponding high-energy physics processes. Beyond the information of the symmetry-breaking scale, the high-frequency spectrum encodes the microscopic structure of the strings through the position of the UV cutoffs associated with cusps and kinks, as well as potential information about friction forces on the string. The IR slope, on the other hand, reflects the physics responsible for the decay of the string network. We discuss possible strategies for reconstructing the scalar potential, particularly the scalar self-coupling, from the measurement of the UV cutoff of the GW spectrum.

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arXiv2024

ALP Dark Matter Mini-Clusters from Kinetic Fragmentation

We show that very compact axion mini-clusters can form in models where axion-like-particle (ALP) dark matter is produced via the kinetic misalignment mechanism, which is well-motivated in pre-inflationary $U(1)$ symmetry breaking scenarios. This is due to ALP fragmentation. We predict denser halos than what has been obtained so far in the literature from standard misalignment in post-inflationary $U(1)$ breaking scenarios or from large misalignment. The main reason is that adiabatic fluctuations are significant at early times; therefore, even if amplification from parametric resonance effects is moderate, the final size of ALP fluctuations is larger in kinetic misalignment. We compare halo mass functions and halo spectra obtained in kinetic misalignment, large misalignment, and standard misalignment, respectively. Our analysis does not depend on the specific model realization of the kinetic misalignment mechanism. We present our results generally as a function of the ALP mass and the ALP decay constant only. We show that a sizable region of this ALP parameter space can be tested by future experiments that probe small-scale structures.

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arXiv2024

Gravitational signatures of ALP dark matter fragmentation

The misalignment mechanism for axion-like particles (ALPs) is a leading explanation for dark matter. In this work we investigate ALPs with non-periodic potentials, which allow for large misalignment of the field from the minimum and make it possible for ALPs to match the relic density of dark matter in a large part of the parameter space. Such potentials give rise to self-interactions which can trigger an exponential growth of fluctuations in the ALP field via parametric resonance, leading to the fragmentation of the field. The fluctuations later collapse to halos that can be dense enough to produce observable gravitational effects. These effects would provide a probe of dark matter even if it does not couple to the Standard Model (or too feebly). We determine the relevant regions of parameter space in the (ALP mass, decay constant)-plane and compare predictions in different axion fragmentation models. These proceedings are a short version of arXiv:2305.03756

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arXiv2023

Constraining Postinflationary Axions with Pulsar Timing Arrays

Models that produce Axion-Like-Particles (ALP) after cosmological inflation due to spontaneous $U(1)$ symmetry breaking also produce cosmic string networks. Those axionic strings lose energy through gravitational wave emission during the whole cosmological history, generating a stochastic background of gravitational waves that spans many decades in frequency. We can therefore constrain the axion decay constant and axion mass from limits on the gravitational wave spectrum and compatibility with dark matter abundance as well as dark radiation. We derive such limits from analyzing the most recent NANOGrav data from Pulsar Timing Arrays (PTA). The limits are similar to the $N_{\rm eff}$ bounds on dark radiation for ALP masses $m_a \lesssim 10^{-22}$ eV. On the other hand, for heavy ALPs with $m_a\gtrsim 0.1$ GeV and $N_{\rm DW}\neq 1$, new regions of parameter space can be probed by PTA data due to the dominant Domain-Wall contribution to the gravitational wave background.

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arXiv2023

ALP dark matter with non-periodic potentials: parametric resonance, halo formation and gravitational signatures

Axion-like particles (ALPs) are leading candidates to explain the dark matter in the universe. Their production via the misalignment mechanism has been extensively studied for cosine potentials characteristic of pseudo-Nambu-Goldstone bosons. In this work we investigate ALPs with non-periodic potentials, which allow for large misalignment of the field from the minimum. As a result, the ALP can match the relic density of dark matter in a large part of the parameter space. Such potentials give rise to self-interactions which can trigger an exponential growth of fluctuations in the ALP field via parametric resonance, leading to the fragmentation of the field. We study these effects with both Floquet analysis and lattice simulations. Using the Press-Schechter formalism, we predict the halo mass function and halo spectrum arising from ALP dark matter. These halos can be dense enough to produce observable gravitational effects such as astrometric lensing, diffraction of gravitational wave signals from black hole mergers, photometric microlensing of highly magnified stars, perturbations of stars in the galactic disk or stellar streams. These effects would provide a probe of dark matter even if it does not couple to the Standard Model. They would not be observable for halos predicted for standard cold dark matter and for ALP dark matter in the standard misalignment mechanism. We determine the relevant regions of parameter space in the (ALP mass, decay constant)-plane and compare predictions in different axion fragmentation models.

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arXiv2023

Relaxion Dark Matter from Stochastic Misalignment

Cosmological relaxation of the electroweak scale via Higgs-axion interplay, named as relaxion mechanism, provides a dynamical solution to the Higgs mass hierarchy. In the original proposal by Graham, Kaplan and Rajendran, the relaxion abundance today is too small to explain the dark matter of the universe because of the high suppression of the misalignment angle after inflation. It was then realised by Banerjee, Kim and Perez that reheating effects can displace the relaxion, thus enabling it to account for the dark matter abundance from the misalignment mechanism. However, this scenario is realised in a limited region of parameter space to avoid runaway. We show that in the regime where inflationary fluctuations dominate over the classical slow-roll, the "stochastic misalignment" of the field due to fluctuations can be large. We study the evolution of the relaxion after inflation, including the high-temperature scenario, in which the barriers of the potential shrink and destabilise temporarily the local minimum. We open new regions of parameter space where the relaxion can naturally explain the observed dark matter density in the universe, towards larger coupling, larger mass, larger mixing angle, smaller decay constant, as well as larger scale of inflation.

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arXiv2022

The Stochastic Relaxion

We revisit the original proposal of cosmological relaxation of the electroweak scale by Graham, Kaplan and Rajendran in which the Higgs mass is scanned during inflation by an axion field, the relaxion. We investigate the regime where the relaxion is subject to large fluctuations during inflation. The stochastic dynamics of the relaxion is described by means of the Fokker-Planck formalism. We derive a new stopping condition for the relaxion taking into account transitions between the neighboring local minima of its potential. Relaxion fluctuations have important consequences even in the "classical-beats-quantum" regime. We determine that for a large Hubble parameter during inflation, the random walk prevents the relaxion from getting trapped at the first minimum. The relaxion stops much further away, where the potential is less shallow. Interestingly, this essentially jeopardises the "runaway relaxion" threat from finite-density effects, restoring most of the relaxion parameter space. We also explore the "quantum-beats-classical" regime, opening large new regions of parameter space. We investigate the consequences for both the QCD and the non-QCD relaxion. The misalignment of the relaxion due to fluctuations around its local minimum opens new phenomenological opportunities.

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arXiv2022

Supercool Composite Dark Matter Beyond 100 TeV

Dark Matter could be a composite state of a confining sector with an approximate scale symmetry. We consider the case where the associated pseudo-Goldstone boson, the dilaton, mediates its interactions with the Standard Model. When the confining phase transition in the early universe is supercooled, its dynamics allows for Dark Matter masses up to $10^6$ TeV. We derive the precise parameter space compatible with all experimental constraints, finding that this scenario can be tested partly by telescopes and entirely by gravitational waves.

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arXiv2021

Revealing the Primordial Irreducible Inflationary Gravitational-Wave Background with a Spinning Peccei-Quinn Axion

The primordial irreducible gravitational-wave background due to quantum vacuum tensor fluctuations produced during inflation spans a large range of frequencies with an almost scale-invariant spectrum but is too low to be detected by the next generation of gravitational-wave interferometers. We show how this signal is enhanced by a short temporary kination era in the cosmological history (less than 10 e-folds), that can arise at any energy scale between a GeV and the inflationary scale $10^{16}$ GeV. We argue that such kination era is naturally generated by a spinning axion before it gets trapped by its potential. It is usually assumed that the axion starts oscillating around its minimum from its initially frozen position. However, the early dynamics of the Peccei-Quinn field can induce a large kinetic energy in the axion field, triggering a kination era, either before or after the axion acquires its mass, leading to a characteristic peak in the primordial gravitational-wave background. This represents a smoking-gun signature of axion physics as no other scalar field dynamics is expected to trigger such a sequence of equations of state in the early universe. We derive the resulting gravitational-wave spectrum, and present the parameter space that leads to such a signal as well as the detectability prospects, in particular at LISA, Einstein Telescope, Cosmic Explorer and Big Bang Observer. We show both model-independent predictions and present as well results for two specific well-motivated UV completions for the QCD axion dark matter where this dynamics is built-in.

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arXiv2020

Relaxion Fluctuations (Self-stopping Relaxion) and Overview of Relaxion Stopping Mechanisms

In implementations of the electroweak scale cosmological relaxation mechanism proposed so far, the effect of the quantum fluctuations of the homogeneous relaxion field has been ignored. We show that they can grow during the classical cosmological evolution of the relaxion field passing through its many potential barriers. The resulting production of relaxion particles can act as an efficient stopping mechanism for the relaxion. We revisit the original relaxion proposal and determine under which conditions inflation may no longer be needed as a source of friction. We review alternative stopping mechanisms and determine in detail the allowed parameter space for each of them (whether happening before, during and after inflation), also considering and severely constraining the case of friction from electroweak gauge boson production in models with large and Higgs-independent barriers.

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arXiv2020

BSM with Cosmic Strings: Heavy, up to EeV mass, Unstable Particles

Unstable heavy particles well above the TeV scale are unaccessible experimentally. So far, Big-Bang Nucleosynthesis (BBN) provides the strongest limits on their mass and lifetime, the latter being shorter than 0.1 second. We show how these constraints could be potentially tremendously improved by the next generation of Gravitational-Wave (GW) interferometers, extending to lifetimes as short as $10^{-16}$ second. The key point is that these particles may have dominated the energy density of the universe and have triggered a period of matter domination at early times, until their decay before BBN. The resulting modified cosmological history compared to the usually-assumed single radiation era would imprint observable signatures in stochastic gravitational-wave backgrounds of primordial origin. In particular, we show how the detection of the GW spectrum produced by long-lasting sources such as cosmic strings would provide a unique probe of particle physics parameters. When applied to specific particle production mechanisms in the early universe, these GW spectra could be used to derive new constraints on many UV extensions of the Standard Model. We illustrate this on a few examples, such as supersymmetric models where the mass scale of scalar moduli and gravitino can be constrained up to $10^{10}$ GeV. Further bounds can be obtained on the reheating temperature of models with only-gravitationally-interacting particles as well as on the kinetic mixing of heavy dark photons at the level of $10^{-18}$.

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arXiv2020

Beyond the Standard Models with Cosmic Strings

We examine which information on the early cosmological history can be extracted from the potential measurement by third-generation gravitational-wave observatories of a stochastic gravitational wave background (SGWB) produced by cosmic strings. We consider a variety of cosmological scenarios breaking the scale-invariant properties of the spectrum, such as early long matter or kination eras, short intermediate matter and inflation periods inside a radiation era, and their specific signatures on the SGWB. This requires to go beyond the usually-assumed scaling regime, to take into account the transient effects during the change of equation of state of the universe. We compute the time evolution of the string network parameters and thus the loop-production efficiency during the transient regime, and derive the corresponding shift in the turning-point frequency. We consider the impact of particle production on the gravitational-wave emission by loops. We estimate the reach of future interferometers LISA, BBO, DECIGO, ET and CE and radio telescope SKA to probe the new physics energy scale at which the universe has experienced changes in its expansion history. We find that a given interferometer may be sensitive to very different energy scales, depending on the nature and duration of the non-standard era, and the value of the string tension. It is fascinating that by exploiting the data from different GW observatories associated with distinct frequency bands, we may be able to reconstruct the full spectrum and therefore extract the values of fundamental physics parameters.

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arXiv2020

Baryogenesis from a dark first-order phase transition

We present a very minimal model for baryogenesis by a dark first-order phase transition. It employs a new dark $SU(2)_{D}$ gauge group with two doublet Higgs bosons, two lepton doublets, and two singlets. The singlets act as a neutrino portal that transfer the generated asymmetry to the Standard Model. The model predicts $ΔN_\text{eff} = 0.09-0.13$ detectable by future experiments as well as possible signals from exotic decays of the Higgs and $Z$ bosons and stochastic gravitational waves.

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arXiv2020

Axion Fragmentation

We investigate the production of axion quanta during the early universe evolution of an axion-like field rolling down a wiggly potential. We compute the growth of quantum fluctuations and their back-reaction on the homogeneous zero-mode. We evaluate the transfer of kinetic energy from the zero mode to the quantum fluctuations and the conditions to decelerate the axion zero-mode as a function of the Hubble rate, the slope of the potential, the size of the barriers and the initial field velocity. We discuss how these effects impact the relaxion mechanism.

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arXiv2011

Non-resonant New Physics in Top Pair Production at Hadron Colliders

We use top quark pair production as a probe of top-philic non-resonant new physics. Following a low energy effective field theory approach, we calculate several key observables in top quark pair production at hadron colliders (e.g., total cross section, ttbar invariant mass distribution, forward-backward asymmetry, spin correlations) including the interference of the Standard Model with dimension-six operators. We determine the LHC reach in probing new physics after having taken into account the Tevatron constraints. In particular, we show that the gluon fusion process gg -> ttbar which remains largely unconstrained at the Tevatron is affected by only one top-philic dimension-six operator, the chromo-magnetic moment of the top quark. This operator can be further constrained by the LHC data as soon as a precision of about 20% is reached for the total ttbar cross-section. While our approach is general and model-independent, it is particularly relevant to models of Higgs and top compositeness, which we consider in detail, also in connection with ttbar ttbar and ttbar bbbar production.

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