arXiv subjects

Giovanni Pierobon

Publications and source records attributed to Giovanni Pierobon.

14 recordsLinked to original sources

arXiv2026

Axion search with telescope for radio astronomy (ASTRA): forecast for observations between 0.5 and 4~GHz

Axion dark matter (DM) is predicted to convert into radio waves in neutron star magnetospheres. We assess the detectability of this signal using a 5 m radio telescope to be installed at the Fan Mountain Observatory, operating in the UHF, L- and S-bands from 0.5 to 4~GHz. We demonstrate that such a telescope can search new parameter space for axion-like particles over a broad range from $2\,μ\text{eV}<m_a<17\,μ\text{eV}$ for axion-photon couplings $g_{aγγ} \gtrsim 2\times 10^{-12}\text{ GeV}^{-1}$ with a three year observing period assuming the standard halo model -- improving neutron star observations by more than an order of magnitude. The search is broadband and is thus complementary to other techniques in the same frequency range. We describe in detail our neutron star population model, noise model, and proposed observing strategy. Most constraining power comes from neutron stars at the Galactic centre, where the smooth DM halo is densest. If a DM spike exists at the Galactic centre, the search is sensitive in the QCD axion model band. UHF and L-band observations (0.5 to 2~GHz) represent the pathfinder phase of a wider program we call ``Axion Search with Telescope for Radio Astronomy'' (ASTRA). Future higher mass searches aimed at discovery potential for the post-inflation axion require further hardware development to cover S, C, X and Ku bands (2 to 18~GHz).

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arXiv2026

The COSMIC WISPers White Paper: The physics case for Weakly Interacting Slim Particles

Axions and other very weakly interacting slim particles (WISPs), with masses below 1 GeV, arise naturally in many extensions of the Standard Model of particle physics. In particular, they could offer a new framework to explain the nature of dark matter and may help address a range of puzzling observations in astrophysics and particle physics. This review provides an overview of ongoing WISP searches and outlines the prospects for the next decade, spanning their theoretical motivation, indirect signatures in astrophysical observations, and dedicated laboratory experiments. It is based on the work carried on by the EU-funded COST Action ``Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments'' (CA21106, https://www.cost.eu/actions/CA21106). This network plays a key role in coordinating and supporting WISP searches across Europe, while also contributing to the development of a roadmap aimed at securing European leadership in this research area. It is emphasized that Europe is currently pursuing a rich, diverse, and cost-effective experimental program, with the potential to deliver one or more transformative discoveries.

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arXiv2026

Prospects for relic neutrino detection using nuclear spin experiments

Direct detection of the cosmic neutrino background (C$ν$B) remains one of the most formidable experimental challenges in modern physics. In this work, we extend recent studies of C$ν$B-induced coherent transitions in polarised nuclear spin ensembles. Adopting an open quantum system framework, we model coherent neutrino effects in large spin ensembles using a Lindblad master equation that also incorporates realistic experimental imperfections such as local dephasing and imperfect polarisation. We solve the Lindblad equation numerically by way of a fast and computationally inexpensive method that can be extended to an arbitrarily large number of spins. Using our numerical solutions, we forecast the sensitivities of future experiments such as CASPEr to the local C$ν$B overdensity parameter $δ_ν$. Our findings indicate that a CASPEr-like experiment, though primarily aimed at axion dark matter search, could also constrain the C$ν$B overdensity to $δ_ν\sim 10^{13}$ in configurations achievable by currently planned experimental efforts, and down to $δ_ν\sim 10^{11}$ in the most optimised scenario. While C$ν$B detection remains out of reach in the foreseeable future, our results highlight the potential of using quantum sensing to probe fundamental physics.

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arXiv2025

Towards a precision calculation of $N_{\rm eff}$ in the Standard Model IV: Estimating the impact of positronium formation

We present a first assessment of how the previously unexplored effect of positronium formation can impact on the value of the effective number of neutrino species in the Standard Model, $N_{\rm eff}^{\rm SM}$. Adopting a Yukawa form for the electrostatic potential, we discuss two possible scenarios that differ primarily in their assumptions about entropy evolution. The first, out-of-equilibrium scenario assumes that thermal corrections to the potential such as Debye screening prevent positronium from appearing until the temperature drops below a threshold. Once the threshold is reached, entropy generated in the QED sector from the equilibration process, if instantaneous, leads to a variation in $N_{\rm eff}^{\rm SM}$ of at most $|ΔN_{\rm eff}| \sim 10^{-4}$, comparable to other uncertainties in the current benchmark value for $N_{\rm eff}^{\rm SM}$. A more gradual formation could however yield a larger change. The second, equilibrium scenario assumes the QED sector to stay in equilibrium at all times. In this case, we show that cancellations between the first, $s$-wave bound- and scattering-states contributions ensure that it is possible to evolve the system across the bound-state formation threshold without generating entropy in the QED sector. The corresponding change in $N_{\rm eff}^{\rm SM}$ then closely matches the $\mathcal{O}(e^2)$ perturbative result derived in previous works and the $\mathcal{O}(e^4)$ contribution is capped at $|ΔN_{\rm eff}| \lesssim 10^{-6}$. We also comment on the impact of deviations from a pure Yukawa potential due to the presence of a thermal width.

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arXiv2025

Fine-grained dark matter substructure and axion haloscopes

The velocity distribution of cold dark matter within galaxies is expected to exhibit ultra-fine-grained substructure as a result of the many foldings of an initially smooth phase-space sheet under gravity. The innumerable folds of this sheet in the inner regions of a galactic halo would appear on solar-system scales like an extremely large number of spatially overlapping streams of dark matter particles. Some of these streams may also receive amplified densities if dark matter undergoes enhanced clustering in the early Universe, as is the case for axions in the post-inflationary scenario. This ultra-fine-grained dark matter substructure is usually considered irrelevant and undetectable for most direct detection experiments, but for axion haloscopes, this may not be the case. We develop and explore several plausible models for the degree of fine-grained axion dark matter substructure so as to evaluate its impact on the detectability of axions. We find that not only is this substructure detectable in haloscope experiments, but that it may enhance the detectability of the axion if data is analysed with a high-enough frequency resolution. This conclusion motivates ongoing high-resolution analyses of axion data by haloscope collaborations, which have the potential to reveal evidence of the QCD axion even if their nominal experimental sensitivity does not reach the required level under the Standard Halo Model assumption.

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arXiv2024

Everything hot everywhere all at once: Neutrinos and hot dark matter as a single effective species

Observational cosmology is rapidly closing in on a measurement of the sum M_nu of neutrino masses, at least in the simplest cosmologies, while opening the door to probes of non-standard hot dark matter (HDM) models. By extending the method of effective distributions, we show that any collection of HDM species, with arbitrary masses, temperatures, and distribution functions, including massive neutrinos, may be represented as a single effective HDM species. Implementing this method in the FlowsForTheMasses non-linear perturbation theory for free-streaming particles, we study non-standard HDM models that contain thermal QCD axions or generic bosons in addition to standard neutrinos, as well as non-standard neutrino models wherein either the distribution function of the neutrinos or their temperature is changed. Along the way, we substantially improve the accuracy of this perturbation theory at low masses, bringing it into agreement with the high-resolution TianNu neutrino N-body simulation to about 2% at k = 0.1 h/Mpc and to within 21% up to k = 1 h/Mpc. We accurately reproduce the results of simulations including axions and neutrinos of multiple masses. Studying the differences between the normal, inverted, and degenerate neutrino mass orderings on their non-linear power, we quantify the error in the common approximation of degenerate masses. We release our code publicly at http://github.com/upadhye/FlowsForTheMassesII .

astro-ph.CO↗

arXiv2024

One trick to treat them all: SuperEasy linear response for any hot dark matter in $N$-body simulations

We generalise the SuperEasy linear response method, originally developed to describe massive neutrinos in cosmological $N$-body simulations, to any hot dark matter (HDM) species with arbitrary momentum distributions. The method uses analytical solutions of the HDM phase space perturbations in various limits and constructs from them a modification factor to the gravitational potential that tricks the cold particles into trajectories as if HDM particles were present in the simulation box. The modification factor is algebraic in the cosmological parameters and requires no fitting. Implementing the method in a Particle-Mesh simulation code and testing it on HDM cosmologies up to the equivalent effect of $\sum m_ν= 0.315$ eV-mass neutrinos, we find that the generalised SuperEasy approach is able to predict the total matter and cold matter power spectra to $\lesssim 0.1\%$ relative to other linear response methods and to $\lesssim 0.25\%$ relative to particle HDM simulations. Applying the method to cosmologies with mixed neutrinos\Plus{}thermal QCD axions and neutrinos\Plus{}generic thermal bosons, we find that non-standard HDM cosmologies have no intrinsically different non-linear signature in the total matter power spectrum from standard neutrino cosmologies. However, because they predict different time dependencies even at the linear level and the differences are augmented by non-linear evolution, it remains a possibility that observations at multiple redshifts may help distinguish between them.

astro-ph.CO↗

arXiv2024

Axion minicluster streams in the solar neighbourhood

A consequence of QCD axion dark matter being born after inflation is the emergence of small-scale substructures known as miniclusters. Although miniclusters merge to form minihalos, this intrinsic granularity is expected to remain imprinted on small scales in our galaxy, leading to potentially damaging consequences for the campaign to detect axions directly on Earth. This picture, however, is modified when one takes into account the fact that encounters with stars will tidally strip mass from the miniclusters, creating pc-long tidal streams that act to refill the dark matter distribution. Here we ask whether or not this stripping rescues experimental prospects from the worst-case scenario in which the majority of axions remain bound up in unobservably small miniclusters. We find that the density sampled by terrestrial experiment on mpc-scales will be, on average, around 70--90\% of the average local DM density, and at a typical point in the solar neighbourhood, we expect most of the dark matter to be comprised of debris from $\mathcal{O}(10^2$--$10^3)$ overlapping streams. If haloscopes can measure the axion signal with high-enough frequency resolution, then these streams are revealed in the form of an intrinsically spiky lineshape, in stark contrast with the standard assumption of a smooth, featureless Maxwellian distribution -- a unique prediction that constitutes a way for experiments to distinguish between pre and post-inflationary axion cosmologies.

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arXiv2023

Miniclusters from axion string simulations

The properties of axion miniclusters and of the voids between them can have very strong implications for the discovery of axions and the dark matter of the Universe. These properties can be strongly affected by axion dynamics in the early Universe, such as the axion string network and the non-linear dynamics around the QCD phase transition. Recently, improvements in numerical simulation techniques have allowed us to calculate the dark matter axion field from axion strings and QCD effects using different methods: directly with low-tension strings but high resolution, directly with effective high-tension strings, or indirectly by extrapolating an attractor solution. In this work, we study the properties of miniclusters in the different approaches used in the literature. We find that, while there are substantial differences in the mass distribution and internal density profiles, globally there is a similar energy distribution between minicluster halos and voids.

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arXiv2023

Axion minivoids and implications for direct detection

In the scenario in which QCD axion dark matter is produced after inflation, the Universe is populated by large inhomogeneities on very small scales. Eventually, these fluctuations will collapse gravitationally to form dense axion miniclusters that trap up to $\sim$75% of the dark matter within asteroid-mass clumps. Axion miniclusters are physically tiny however, so haloscope experiments searching for axions directly on Earth are much more likely to be probing ``minivoids'' -- the space in between miniclusters. This scenario seems like it ought to spell doom for haloscopes, but while these minivoids might be underdense, they are not totally devoid of axions. Using Schrödinger-Poisson and N-body simulations to evolve from realistic initial field configurations, we quantify the extent to which the local ambient dark matter density is suppressed in the post-inflationary scenario. We find that a typical experimental measurement will sample an axion density that is only around 10% of the expected galactic dark matter density. Our results also have implications for experimental campaigns lasting longer than a few years, as well as broadband haloscopes that have sensitivity to transient signatures. We show that for a $\mathcal{O}$(year)-long integration times, the measured dark matter density should be expected to vary by 20--30%.

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arXiv2022

Weaker yet again: mass spectrum-consistent cosmological constraints on the neutrino lifetime

We consider invisible neutrino decay $ν_H \to ν_l + ϕ$ in the ultra-relativistic limit and compute the neutrino anisotropy loss rate relevant for the cosmic microwave background (CMB) anisotropies. Improving on our previous work which assumed massless $ν_l$ and $ϕ$, we reinstate in this work the daughter neutrino mass $m_{νl}$ in a manner consistent with the experimentally determined neutrino mass splittings. We find that a nonzero $m_{νl}$ introduces a new phase space factor in the loss rate $Γ_{\rm T}$ proportional to $(Δm_ν^2/m_{ν_H}^2)^2$ in the limit of a small squared mass gap between the parent and daughter neutrinos, i.e., $Γ_{\rm T} \sim (Δm_ν^2/m_{νH}^2)^2 (m_{νH}/E_ν)^5 (1/τ_0)$, where $τ_0$ is the $ν_H$ rest-frame lifetime. Using a general form of this result, we update the limit on $τ_0$ using the Planck 2018 CMB data. We find that for a parent neutrino of mass $m_{νH} \lesssim 0.1 {\rm eV}$, the new phase space factor weakens the constraint on its lifetime by up to a factor of 50 if $Δm_ν^2$ corresponds to the atmospheric mass gap and up to $10^{5}$ if the solar mass gap, in comparison with naive estimates that assume $m_{νl}=0$. The revised constraints are (i) $τ^0 \gtrsim (6 \to 10) \times 10^5~{\rm s}$ and $τ^0 \gtrsim (400 \to 500)~{\rm s}$ if only one neutrino decays to a daughter neutrino separated by, respectively, the atmospheric and the solar mass gap, and (ii) $τ^0 \gtrsim (2 \to 3) \times 10^7~{\rm s}$ in the case of two decay channels with one near-common atmospheric mass gap. In contrast to previous, naive limits which scale as $m_{νH}^5$, these mass spectrum-consistent $τ_0$ constraints are remarkably independent of the parent mass and open up a swath of parameter space within the projected reach of IceCube and other neutrino telescopes in the next two decades.

hep-ph↗

arXiv2022

Simulations of axion-like particles in the post-inflationary scenario

Axions and axion-like particles (ALPs) are some of the most popular candidates for dark matter, with several viable production scenarios that make different predictions. In the scenario in which the axion is born after inflation, its field develops significant inhomogeneity and evolves in a highly nonlinear fashion. Understanding the eventual abundance and distribution of axionic dark matter in this scenario therefore requires dedicated numerical simulations. So far the community has focused its efforts on simulations of the QCD axion, a model that predicts a specific temperature dependence for the axion mass. Here, we go beyond the QCD axion, and perform a suite of simulations over a range of possible temperature dependencies labelled by a power-law index. We study the complex dynamics of the axion field, including the scaling of cosmic strings and domain walls, the spectrum of non-relativistic axions, the lifetime and internal structure of axitons, and the seeds of miniclusters. In particular, we quantify how much the string-wall network contributes to the dark matter abundance as a function of how quickly the axion mass grows. We find that a temperature-independent model produces 25\% more dark matter than the standard misalignment calculation. In contrast to this generic ALP, QCD axion models are almost six times less efficient at producing dark matter. Given the flourishing experimental campaign to search for ALPs, these results have potentially wide implications for direct and indirect searches.

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arXiv2021

Cosmology of the companion-axion model: dark matter, gravitational waves, and primordial black holes

The companion-axion model introduces a second QCD axion to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. As in single-axion models, the two axions predicted by the companion-axion model are attractive candidates for dark matter. The model is defined by two free parameters, the scales of the two axions' symmetry breaking, so we can classify production scenarios in terms of the relative sizes of these two scales with respect to the scale of inflation. We study the cosmological production of companion-axion dark matter in order to predict windows of preferred axion masses, and calculate the relative abundances of the two particles. Additionally, we show that the presence of a second axion solves the cosmological domain wall problem automatically in the scenarios in which one or both of the axions are post-inflationary. We also suggest unique cosmological signatures of the companion-axion model, such as the production of a $\sim$10 nHz gravitational wave background, and $\sim 100\,M_\odot$ primordial black holes.

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arXiv2021

Phenomenology of the companion-axion model: photon couplings

We study the phenomenology of the 'companion-axion model' consisting of two coupled QCD axions. The second axion is required to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. We investigate here the combined phenomenology of axion-axion and axion-photon interactions, recasting present and future single-axion bounds onto the companion-axion parameter space. Most remarkably, we predict that future axion searches with haloscopes and helioscopes may well discover two QCD axions, perhaps even within the same experiment.

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