arXiv subjects

Olivier Deligny

Publications and source records attributed to Olivier Deligny.

At least 19 recordsLinked to original sources

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 $3~\mathrm{MeV} \lesssim m_χ\lesssim 1~\mathrm{GeV}$ 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, $\overlineσ_\mathrm{e}$. 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↗

arXiv2025

Multi-messenger constraints on transient accelerators of ultra-high energy cosmic rays

The origin of ultra-high-energy cosmic rays (UHECRs) remains an open questions in astrophysics. We explore two primary scenarios for the distribution of UHECR sources, assuming that their production rate follows either the cosmic star-formation-rate or stellar-mass density. By jointly fitting the UHECR energy spectrum and mass composition measured by the Pierre Auger Observatory above the ankle (10^{18.7} eV), we derive constraints on the acceleration mechanisms, source energetics, and elemental abundances at escape. Using these constraints, we generate sky maps above 40 EeV based on a catalog of over 400,000 galaxies out to 350 Mpc, providing a near-infrared flux-limited sample that maps the two stellar-activity tracers across the full sky. A crucial factor in understanding UHECR propagation is the influence of large-scale cosmic structures, particularly galaxy clusters, the largest gravitationally bound systems in the Universe, which are filled with magnetized diffuse plasma. Intermittent sources hosted in galaxies within such structures, coupled with cosmic magnetic fields, shape the observed UHECR arrival directions and provide insights into the burst rate of the sources. We show that these environments can significantly impact UHECR transport, making them particularly opaque to heavy nuclei. Additionally, we compute the expected secondary neutrino and photon fluxes from UHECR interactions in these environments and compare them with current experimental limits, constraining the maximum energy that particles can achieve. Finally, we assess the compatibility of these constraints with astrophysical candidates, identifying long gamma-ray bursts as the most promising sources.

astro-ph.HE↗

arXiv2024

Searches for ultra-high energy gamma-ray at the Pierre Auger Observatory and implications on super-heavy dark matter

The first interactions of photon-induced showers are of electromagnetic nature, and the transfer of energy to the hadron/muon channel is reduced with respect to the bulk of hadron-induced showers. This results in a lower number of secondary muons. Additionally, as the development of photon showers is delayed by the typically small multiplicity of electromagnetic interactions, their maximum of shower development is deeper in the atmosphere than for showers initiated by hadrons. These salient features have enabled searches for photon showers at the Pierre Auger Observatory. They have led to stringent upper limits on ultra-high-energy gamma-ray fluxes over four orders in magnitude in energy. These limits are not only of considerable astrophysical interest, but they also allow us to constrain beyond-standard-physics scenarios. For instance, dark matter particles could be superheavy, provided their lifetime is much longer than the age of the universe. Constraints on specific extensions of the Standard Model of particle physics that meet the lifetime requirement for a superheavy particle will be presented. They include limits on instanton strength as well as on mixing angle between active and sterile neutrinos.

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arXiv2024

Closing the net on transient sources of ultra-high-energy cosmic rays

Arrival directions of ultra-high-energy cosmic rays (UHECRs) observed above $4\times10^{19}\,$eV provide evidence of localized excesses that are key to identifying their sources. We leverage the 3D matter distribution from optical and infrared surveys as a density model of UHECR sources, which are considered to be transient. Agreement of the sky model with UHECR data imposes constraints on both the emission rate per unit matter and the time spread induced by encountered turbulent magnetic fields. Based on radio measurements of cosmic magnetism, we identify the Local Sheet as the magnetized structure responsible for the kiloyear duration of UHECR bursts for an observer on Earth and find that the turbulence amplitude must be within $0.5-20\,$nG for a coherence length of $10\,$kpc. At the same time, the burst-rate density must be above $50\,$Gpc$^{-3}\,$yr$^{-1}$ for Local-Sheet galaxies to reproduce the UHECR excesses and below $5\,000\,$Gpc$^{-3}\,$yr$^{-1}$ ($30\,000\,$Gpc$^{-3}\,$yr$^{-1}$) for the Milky Way (Local-Group galaxies) not to outshine other galaxies. For the transient emissions of protons and nuclei to match the energy spectra of UHECRs, the kinetic energy of the outflows responsible for UHECR acceleration must be below $4\times10^{54}\,$erg and above $5\times10^{50}\,$erg ($2\times10^{49}\,$erg) if we consider the Milky Way (or not). The only stellar-sized transients that satisfy both Hillas' and our criteria are long gamma-ray bursts.

astro-ph.HE↗

arXiv2024

Floor of cosmogenic neutrino fluxes above $10^{17}~$eV

The search for neutrinos with energies greater than $10^{17}~$eV is being actively pursued. Although normalization of the dominant neutrino flux is highly uncertain, a floor level is guaranteed by the interactions of extragalactic cosmic rays with Milky Way gas. We estimate that this floor level gives an energy flux of $E^2ϕ_ν\simeq 10^{-13^{+0.5}_{-0.5}}~$GeV~cm$^{-2}$~sr$^{-1}$~s$^{-1}$ at $10^{18}~$eV, where uncertainties arise from the modeling of the gas distribution and the experimental determination of the mass composition of ultra-high-energy cosmic rays on Earth. Based on a minimal model of cosmic-ray production to explain the mass-discriminated energy spectra observed on Earth above $5{\times}10^{18}$~eV, we also present generic estimates of the neutrino fluxes expected from extragalactic production that generally exceed the aforementioned guaranteed floor. The prospects for detecting neutrinos above $10^{18}$~eV remain however challenging, unless proton acceleration to the highest energies is at play in a sub-dominant population of cosmic-ray sources or new physical phenomena are at work.

astro-ph.HE↗

arXiv2023

The science of ultra-high energy cosmic rays after more than 15 years of operation of the Pierre Auger Observatory

Ultra-high energy cosmic rays (UHECRs) have been studied with the data of the Pierre Auger Observatory for more than fifteen years. An essential feature of the Observatory is its hybrid design: UHECRs are detected through the observation of the associated extensive air showers (EASs) with different and complementary techniques. The analyses of the multi-detector data have enabled high-statistics and high-precision studies of the UHECR energy spectrum, mass composition and distribution of arrival directions. The resulting science of UHECRs is summarized in this contribution. While no discrete source of UHECRs has been identified so far, the extragalactic origin of the particles has been recently confirmed from the arrival directions above 8~EeV, and the ring is closing around nearby astrophysical sites at higher energies. Also, the established upper limits on fluxes of UHE neutrinos and photons have implications on multi-messenger studies and beyond-the-Standard-Model (BSM) physics.

astro-ph.HE↗

arXiv2022

Observational constraints on cosmic-ray escape from ultra-high energy accelerators

The energy spectrum and mass composition of ultra-high energy cosmic rays inferred at the Pierre Auger Observatory are used to derive a benchmark scenario for the emission mechanisms at play in extragalactic accelerators as well as for their energetics and for the abundances of elements in their environments. Assuming a distribution of sources following the density of stellar mass, the gradual increase of the cosmic ray mass number observed on Earth from $\simeq$2\:EeV up to the highest energies is shown to call for nuclei accelerated up to an energy proportional to their electric charge and emitted with a hard spectral index. In addition, the inferred flux of protons down to $\simeq$0.6\:EeV is shown to require for this population a spectral index significantly softer than that of heavier nuclei. This is consistent with in-source interactions that shape the energy production rate of injected charged nuclei differently from that of the secondary neutrons escaping from the confinement zone. Together with the inferred abundances of nuclei, these results provide constraints on the radiation levels in the source environments. Within this scenario, an additional component that falls off steeply with increasing energy up to the ankle feature is necessary to make up the all-particle flux in the sub-ankle energy range.

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arXiv2022

A search for instanton-induced decay of super-heavy dark matter in the Pierre Auger Observatory data

Using data collected at the Pierre Auger Observatory, we search for signatures of instanton-induced processes that would provide evidence of super-heavy particles decaying in the Galactic halo. Such particles could have been produced sufficiently during the post-inflationary epoch to match the relic abundance of dark matter inferred today. The non-observation of these signatures allows us to probe the instanton strength and to derive a bound, the best ever obtained from instanton-mediated processes, on the reduced coupling constant of gauge interactions in the dark sector: $α_X \lesssim 0.09$, for $10^{9} \lesssim M_X/{\rm GeV} < 10^{19}$.

astro-ph.HE↗

arXiv2022

Theoretical derivation of diffusion-tensor coefficients for the transport of charged particles in magnetic fields

The transport of charged particles in various astrophysical environments permeated by magnetic fields is described in terms of a diffusion process, which relies on diffusion-tensor parameters generally inferred from Monte-Carlo simulations. Based on a red-noise approximation to model the two-point correlation function of the magnetic field experienced by charged particles between two successive times, the diffusion-tensor coefficients were previously derived in the case of pure turbulence. In this contribution to ECRS2022, the derivation is extended to the case of a mean field on top of the turbulence. The results are applicable to a variety of astrophysical environments in regimes where the Larmor radius of the particles is resonant with the power spectrum of the turbulence wavelength (gyro-resonant regime), or where the Larmor radius is greater than the largest turbulence wavelength (high-rigidity regime).

astro-ph.HE↗

arXiv2022

Radiation of ionization electrons: the key role of their 2-pt function of velocities

Several attempts to detect extensive air showers (EAS) induced by ultra-high energy cosmic rays have been conducted in the last decade based on the molecular Bremsstrahlung radiation (MBR) at GHz frequencies from quasi-elastic collisions of ionisation electrons left in the atmosphere after the passage of the cascade of particles. These attempts have led to the detection of a handful of signals only, all of them forward-directed along the shower axis and hence suggestive of originating from geomagnetic and Askaryan emissions extending into GHz frequencies close to the Cherenkov angle. In this contribution to ARENA2022, the lack of detection of events is explained by the coherent suppression of the MBR in frequency ranges below the collision rate due to the destructive interference impacting the emission amplitude of photons between the successive collisions of the electrons. The spectral intensity at the ground level is shown to be several orders of magnitude below the sensitivity of experimental setups. Consequently the MBR cannot be seen as the basis of a new detection technique of EAS for the next decades. The formalism developed to get at this conclusion allowed the key role of the two-point correlation function of the ionisation electron velocities to be highlighted. This can serve to study the intensity of the re-radiation of these ionization electrons subject to the passage of an incoming coherent wave from a radar transmitter. Some hints on this will be presented.

astro-ph.IM↗

arXiv2022

Observational constraints on accelerators of ultra-high energy cosmic rays

We explore two generic hypotheses for tracing the sources of ultra-high energy cosmic rays (UHECRs) in the Universe: star formation rate density or stellar mass density. For each scenario, we infer a set of constraints for the emission mechanisms in the accelerators, for their energetics and for the abundances of elements at escape from their environments. From these constraints, we generate sky maps above 40~EeV expected from a catalog that comprises 410,761 galaxies out to 350 Mpc and provides a near-infrared flux-limited sample to map both stellar mass and star formation rate over the full sky. Considering a scenario of intermittent sources hosted in every galaxy, we show that the main features observed in arrival directions of UHECRs can in turn constrain the burst rate of the sources provided that magnetic-horizon effects are at play in clusters of galaxies.

astro-ph.HE↗

arXiv2022

The science of ultra-high energy cosmic rays after more than 15 years of operation of the Pierre Auger Observatory

The Pierre Auger Observatory has been detecting ultra-high energy cosmic rays (UHECRs) for more than fifteen years. An essential feature of the Observatory is its hybrid design: cosmic rays above $100~$PeV are detected through the observation of the associated air showers with different and complementary techniques, from surface detector arrays and fluorescence telescopes to radio antennas. The analyses of the multi-detector data have enabled high-statistics and high-precision studies of the energy spectrum, mass composition and distribution of arrival directions of UHECRs. The resulting picture is summarized in this contribution. While no discrete source of UHECRs has been identified so far, the extragalactic origin of the particles has been recently determined from the arrival directions above 8~EeV, and the ring is closing around nearby astrophysical sites at higher energies. Also, the established upper limits on fluxes of UHE neutrinos and photons have implications on dark matter and cosmological aspects that are also presented in this contribution.

astro-ph.HE↗

arXiv2022

Observational constraints on cosmic-ray escape from UHE accelerators

Interactions of ultra-high energy cosmic rays (UHECRs) accelerated in specific astrophysical environments have been shown to shape the energy production rate of nuclei differently from that of the secondary neutrons escaping from the confinement zone. Here, we aim at testing a generic scenario of in-source interactions through a phenomenological modeling of the flux and composition of UHECRs. We fit a model in which nucleons and nuclei follow different particle energy distributions to the all-particle energy spectrum, proton spectrum below the ankle energy and distributions of maximum shower depths above this energy, as inferred at the Pierre Auger Observatory. We obtain that the data can be reproduced using a spatial distribution of sources that follows the density of extragalactic matter on both local and large scales, providing hence a realistic set of constraints for the emission mechanisms in cosmic accelerators, for their energetics and for the abundances of elements at escape from their environments. While the quasi mono-elemental increase of the cosmic-ray mass number observed on Earth from ${\simeq}\: 2\:$EeV up to the highest energies calls for nuclei accelerated with a hard spectral index, the inferred flux of protons down to ${\simeq}\: 0.6\:$ EeV is shown to require for this population a spectral index significantly softer than that generally obtained up to now. We demonstrate that modeling UHECR data across the ankle substantiate the conjecture of in-source interactions in a robust statistical framework, although pushing the mechanism to the extreme.

astro-ph.HE↗

arXiv2022

Diffuse flux of ultra-high energy photons from cosmic-ray interactions in the disk of the Galaxy and implications for the search for decaying super-heavy dark matter

An estimate of the expected photon flux above $10^{17}~$eV from the interactions of ultra-high energy cosmic rays with the matter in the Galactic disk is presented. Uncertainties arising from the distribution of the gas in the disk, the absolute level of the cosmic ray flux, and the composition of the cosmic rays are taken into account. Within these uncertainties, the integrated photon flux above $10^{17}~$eV is, averaged out over Galactic latitude less than $5^\circ$, between $\simeq 3.2{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$ and $\simeq 8.7{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$. The all-sky average value amounts to $\simeq 1.1{\times}10^{-2}~$km$^{-2}~$yr$^{-1}~$sr$^{-1}$ above $10^{17}~$eV and decreases roughly as $E^{-2}$, making this diffuse flux the dominant one from cosmic-ray interactions for energy thresholds between $10^{17}~$eV and $10^{18}$~eV. Compared to the current sensitivities of detection techniques, a gain between two and three orders of magnitude in exposure is required for a detection below $\simeq 10^{18}$~eV. The implications for searches for photon fluxes from the Galactic center that would be indicative of the decay of super-heavy dark matter particles are discussed, as the photon flux presented in this study can be considered as a floor below which other signals would be overwhelmed.

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arXiv2021

The UHECR dipole and quadrupole in the latest data from the original Auger and TA surface detectors

The sources of ultra-high-energy cosmic rays are still unknown, but assuming standard physics, they are expected to lie within a few hundred megaparsecs from us. Indeed, over cosmological distances cosmic rays lose energy to interactions with background photons, at a rate depending on their mass number and energy and properties of photonuclear interactions and photon backgrounds. The universe is not homogeneous at such scales, hence the distribution of the arrival directions of cosmic rays is expected to reflect the inhomogeneities in the distribution of galaxies; the shorter the energy loss lengths, the stronger the expected anisotropies. Galactic and intergalactic magnetic fields can blur and distort the picture, but the magnitudes of the largest-scale anisotropies, namely the dipole and quadrupole moments, are the most robust to their effects. Measuring them with no bias regardless of any higher-order multipoles is not possible except with full-sky coverage. In this work, we achieve this in three energy ranges (approximately 8--16 EeV, 16--32 EeV, and 32--$\infty$ EeV) by combining surface-detector data collected at the Pierre Auger Observatory until 2020 and at the Telescope Array (TA) until 2019, before the completion of the upgrades of the arrays with new scintillator detectors. We find that the full-sky coverage achieved by combining Auger and TA data reduces the uncertainties on the north-south components of the dipole and quadrupole in half compared to Auger-only results.

astro-ph.HE↗

arXiv2021

UHECR arrival directions in the latest data from the original Auger and TA surface detectors and nearby galaxies

The distribution of ultra-high-energy cosmic-ray arrival directions appears to be nearly isotropic except for a dipole moment of order $6 \times (E/10~\mathrm{EeV})$ per cent. Nonetheless, at the highest energies, as the number of possible candidate sources within the propagation horizon and the magnetic deflections both shrink, smaller-scale anisotropies might be expected to emerge. On the other hand, the flux suppression reduces the statistics available for searching for such anisotropies. In this work, we consider two different lists of candidate sources: a sample of nearby starburst galaxies and the 2MRS catalog tracing stellar mass within $250~\mathrm{Mpc}$. We combine surface-detector data collected at the Pierre Auger Observatory until 2020 and the Telescope Array until 2019, and use them to test models in which UHECRs comprise an isotropic background and a foreground originating from the candidate sources and randomly deflected by magnetic fields. The free parameters of these models are the energy threshold, the signal fraction, and the search angular scale. We find a correlation between the arrival directions of $11.8\%_{-3.1\%}^{+5.0\%}$ of cosmic rays detected with $E \ge 38~\mathrm{EeV}$ by Auger or with $E \gtrsim 49~\mathrm{EeV}$ by TA and the position of nearby starburst galaxies on a ${15.5^\circ}_{-3.2^\circ}^{+5.3^\circ}$ angular scale, with a $4.2σ$ post-trial significance, as well as a weaker correlation with the overall galaxy distribution.

astro-ph.HE↗

arXiv2021

Transport of charged particles propagating in turbulent magnetic fields as a red-noise process

The transport of charged particles in various astrophysical environments permeated by magnetic fields is described in terms of a diffusion process, which relies on diffusion-tensor parameters generally inferred from Monte-Carlo simulations. In this paper, a theoretical derivation of the diffusion coefficient in the case of a purely turbulent magnetic field is presented. The approach is based on a red-noise approximation to model the 2-pt correlation function of the magnetic field experienced by the particles between two successive times. This approach is shown to describe the regime in which the Larmor radius of the particles is in resonance with the wavelength power spectrum of the turbulence (gyro-resonant regime), extending hence previous results applying to the high-rigidity regime in which the Larmor radius is greater than the larger wavelength of the turbulence. The results are shown to be consistent with those obtained with a Monte-Carlo generator. Although not considered in this study, the presence of a mean field on top of the turbulence is discussed.

astro-ph.HE↗

arXiv2021

Hunting super-heavy dark matter with ultra-high energy photons

At any epoch, particle physics must be open to completely unexpected discoveries, and that is reason enough to extend the reach of searches for ultra-high energy (UHE) photons. The observation of a population of photons with energies $E \gtrsim 100$ EeV would for example imply the existence of either a completely new physical phenomena, or particle acceleration mechanisms heretofore never seen or imagined. But as we outline in this Letter of Interest, there are also good arguments for super-heavy dark matter (SHDM) in a parameter range such that it could be discovered via its decays to, in particular, UHE photons. Only ultra-high energy cosmic ray observatories have capabilities to detect UHE photons. We first investigate how current and future observations can probe and constrain SHDM models in important directions, and then outline some of the scenarios that motivate such searches. We also discuss connections between constraints on SHDM and on the parameter values of cosmological models.

hep-ph↗