arXiv subjects

Piero Ullio

Publications and source records attributed to Piero Ullio.

At least 19 recordsLinked to original sources

arXiv2025

Dark matter mounds from the collapse of supermassive stars: a general-relativistic analysis

Recent work has highlighted the importance of a fully relativistic treatment of the dephasing of gravitational waves induced by dark-matter overdensities in extreme mass-ratio inspirals (EMRIs). However, a general-relativistic description of the dark matter phase-space distribution is currently available only for the case of a dark matter "spike" arising from adiabatic black hole growth. Here we develop a fully general-relativistic formalism for the more realistic scenario in which a supermassive stellar progenitor collapses to a black hole and produces a shallower dark matter overdensity, or "mound". We follow self-consistently the evolution of the supermassive star, its collapse, and the subsequent growth of the resulting black hole, together with the collisionless dark matter orbits. We find that in the regime where the collapse becomes non-adiabatic, the dark matter distribution function is significantly reshaped, with a clear depletion in the low-binding-energy region of phase space. Our results provide a more realistic prediction for the dark matter phase-space distribution around supermassive black holes, which is an essential step in our programme to use future EMRI observations to extract information about both the nature of dark matter and the formation history of the black hole.

gr-qc↗

arXiv2023

The self-confinement of electrons and positrons from dark matter

Radiative emissions from electrons and positrons generated by dark matter (DM) annihilation or decay are one of the most investigated signals in indirect searches of WIMPs. Ideal targets must have large ratio of DM to baryonic matter. However, such ``dark'' systems have a poorly known level of magnetic turbulence, which determines the residence time of the electrons and positrons and therefore also the strength of the expected signal. This typically leads to significant uncertainties in the derived DM bounds. In a novel approach, we compute the self-confinement of the DM-induced electrons and positrons. Indeed, they themselves generate irregularities in the magnetic field, thus setting a lower limit on the presence of the magnetic turbulence. We specifically apply this approach to dwarf spheroidal galaxies. Finally, by comparing the expected synchrotron emission with radio data from the direction of the Draco galaxy collected at the Giant Metre Radio Telescope, we show that the proposed approach can be used to set robust and competitive bounds on WIMP DM.

astro-ph.HE↗

arXiv2022

Neutron Star Heating in Dark Matter Models for Muon g-2 with Scalar Lepton Partners up to the TeV Scale

We investigate the kinetic heating of neutron stars due to dark matter scattering in minimal models with scalar lepton partners mediating interactions which can account for the anomalous magnetic moment of the muon. In particular, such models in which the dark matter is a thermally produced Majorana fermion can be extremely difficult to detect at conventional searches. We demonstrate that future infrared observations of an old neutron star population could definitively probe the entire parameter space of this scenario.

hep-ph↗

arXiv2022

Blazar-Boosted Dark Matter at Super-Kamiokande

Dark matter particles near the center of a blazar, after being accelerated by the elastic collisions with relativistic electrons and protons in the blazar jet, can be energetic enough to trigger detectable signals at terrestrial detectors. In this work, focusing on the blazars TXS 0506+056 and BL Lacertae, we derive novel limits on the cross section of the elastic scattering between dark matter and electrons by means of the available Super-Kamiokande data. Thanks to the large blazar-boosted dark matter flux, the limit on the dark matter-electron scattering cross section for dark matter masses below 100 MeV can be as low as $\sim10^{-38}~\text{cm}^2$, which is orders of magnitude stronger than the analogous results from galactic cosmic rays.

astro-ph.HE↗

arXiv2022

Direct Detection Constraints on Blazar-Boosted Dark Matter

We explore the possibility that relativistic protons in the extremely powerful jets of blazars may boost via elastic collisions the dark matter particles in the surroundings of the source to high energies. We concentrate on two sample blazars, TXS 0506+056 - towards which IceCube recently reported evidence for a high-energy neutrino flux - and BL Lacertae, a representative nearby blazar. We find that the dark matter flux at Earth induced by these sources may be sizable, larger than the flux associated with the analogous process of dark matter boosted by galactic cosmic rays, and relevant to access direct detection for dark matter particle masses lighter than 1 GeV. From the null detection of a signal by XENON1T, MiniBooNE, and Borexino, we derive limits on dark matter-nucleus spin-independent and spin-dependent cross sections which, depending on the modelization of the source, improve on other currently available bounds for light dark matter candidates of 1 up to 5 orders of magnitude.

astro-ph.HE↗

arXiv2021

A Minimal Dark Matter Model for Muon g-2 with Scalar Lepton Partners up to the TeV Scale

The E989 experiment at the Fermi National Laboratory reported a 4.2$σ$ discrepancy between the measured magnetic dipole moment of the muon, and its prediction in the Standard Model (SM). In this study, we address the anomaly by considering a minimal and generic extension to the SM which also provides for a dark matter (DM) candidate. The extra states in this framework are: a SM singlet Majorana fermion, referred to as the Bino, playing the role of DM; and muonic scalars, referred to as sleptons. The couplings between the sleptons, SM muons and the Bino can account for the muon $g-2$ anomaly if the scalar muon partners, or smuons, mix chirality. On the other hand, the DM relic density is satisfied primarily through coannihilation effects involving the Bino and the lighter sleptons. The viable parameter space of our model includes regions with relatively light coannihilating particles, similar to what has been found in previous scans of the Minimal Supersymmetric Standard Model (MSSM). Relaxing the assumption of minimal flavor violation typically assumed in the MSSM, we see that scenarios with sizable smuon mixing and large mass splittings between the smuons can satisfy both the muon $g-2$ anomaly and the DM relic density for coannihilating particle masses up to and beyond the TeV scale. When we specify the origin of the left-right smuon mixing to be trilinear couplings between the smuons and the SM Higgs boson, the constraints on these scenarios arising from perturbative unitarity and electroweak vacuum stability confine the coannihilating particle masses to be $\lesssim$ 1 TeV. We demonstrate that next generation direct detection experiments are only marginally sensitive to the viable parameter space of our model and, thus, a future lepton collider could be the essential probe necessary to distinguish our model from other BSM solutions to the muon $g-2$ anomaly.

hep-ph↗

arXiv2020

Phenomenological consequences of an interacting multicomponent dark sector

We consider a dark sector model containing stable fermions charged under an unbroken $U(1)$ gauge interaction, with a massless dark photon as force carrier, and interacting with ordinary matter via scalar messengers. We study its early Universe evolution by solving a set of coupled Boltzmann equations that track the number density of the different species, as well as entropy and energy exchanges between the dark and visible sectors. Phenomenologically viable realizations include: i) a heavy (order 1 TeV or more) lepton-like dark fermion playing the role of the dark matter candidate, with various production mechanisms active depending on the strength of the dark-visible sector portal; ii) light (few GeV to few tens of GeV) quark-like dark fermions, stable but with suppressed relic densities; iii) an extra radiation component in Universe due to dark photons, with temperature constrained by cosmic microwave background data, and in turn preventing dark fermions to be lighter than about 1 GeV. Extra constraints on our scenario stem from dark matter direct detection searches: the elastic scattering on nuclei is driven by dipole or charge radius interactions mediated by either Standard Model or dark photons, providing long-range effects which, however, are not always dominant, as usually assumed in this context. Projected sensitivities for next-generation detectors cover a significant portion of the viable parameter space and are competitive with respect to the model-dependent constraints derived from the magnetic dipole moments of leptons and cooling of stellar systems.

hep-ph↗

arXiv2020

hammurabi X: Simulating Galactic Synchrotron Emission with Random Magnetic Fields

We present version X of the hammurabi package, the HEALPix-based numeric simulator for Galactic polarized emission. Improving on its earlier design, we have fully renewed the framework with modern C++ standards and features. Multi-threading support has been built in to meet the growing computational workload in future research. For the first time, we present precision profiles of hammurabi line-of-sight integral kernel with multi-layer HEALPix shells. In addition to fundamental improvements, this report focuses on simulating polarized synchrotron emission with Gaussian random magnetic fields. Two fast methods are proposed for realizing divergence-free random magnetic fields either on the Galactic scale where a field alignment and strength modulation are imposed, or on a local scale where more physically motivated models like a parameterized magneto-hydrodynamic (MHD) turbulence can be applied. As an example application, we discuss the phenomenological implications of Gaussian random magnetic fields for high Galactic latitude synchrotron foregrounds. In this, we numerically find B/E polarization mode ratios lower than unity based on Gaussian realizations of either MHD turbulent spectra or in spatially aligned magnetic fields.

astro-ph.IM↗

arXiv2018

On velocity-dependent dark matter annihilations in dwarf satellites

Milky Way dwarf spheroidal satellites are a prime target for Dark Matter (DM) indirect searches. There have been recent reassessments of the expected DM gamma-ray signals in case of long-range interactions, commonly known as Sommerfeld enhancement. Since details of the underlying DM phase-space distribution function become critical, there are potentially large uncertainties in the final result. We provide here a first attempt towards a comprehensive investigation of these systematics, addressing the impact on the expected DM flux from Milky Way dwarfs via Bayesian inference on the available stellar kinematic datasets. After reconsidering the study case of ergodic systems, we investigate for the first time scenarios where DM particle orbits may have a radial or tangential bias. We consider both cuspy and cored parametric DM density profiles, together with the case of a non-parametric halo modelling directly connected to line-of-sight observable quantities. The main findings of our work highlight the relevance of the assumed phase-space distribution: Referring to a generalized J-factor, namely the line-of-sight convolution of the spatial part in case of velocity-dependent annihilation rate, an enhancement (suppression) with respect to the limit of isotropic phase-space distributions is obtained for the case of tangentially (radially) biased DM particle orbits. We provide new estimates for J-factors for the eight brightest Milky Way dwarfs also in the limit of velocity-independent DM annihilation, in good agreement with previous results in literature, and derive data-driven lower-bounds based on the non-parametric modelling of the halo density. The outcome of our broad study stands out as a representative of the state-of-the-art in the field, and falls within the interest of current and future experimental collaborations involved in DM indirect detection programs.

astro-ph.GA↗

arXiv2018

Two-integral distribution functions in axisymmetric galaxies: implications for dark matter searches

We address the problem of reconstructing the phase-space distribution function for an extended collisionless system, with known density profile and in equilibrium within an axisymmetric gravitational potential. Assuming that it depends on only two integrals of motion, namely the energy and the component of the angular momentum along the axis of symmetry $L_z$, there is a one-to-one correspondence between the density profile and the component of the distribution function that is even in $L_z$, as well as between the weighted azimuthal velocity profile and the odd component. This inversion procedure was originally proposed by Lynden-Bell and later refined in its numerical implementation by Hunter & Qian; after overcoming a technical difficulty, we apply it here for the first time in presence of a strongly flattened component, as a novel approach of extracting the phase-space distribution function for dark matter particles in the halo of spiral galaxies. We compare results obtained for realistic axisymmetric models to those in the spherical symmetric limit as assumed in previous analyses, showing the rather severe shortcomings in the latter. We then apply the scheme to the Milky Way and discuss the implications for the direct dark matter searches. In particular, we reinterpret the null results of the Xenon1T experiment for spin-(in)dependent interactions and make predictions for the annual modulation of the signal for a set of axisymmetric models, including a self-consistently defined co-rotating halo.

astro-ph.GA↗

arXiv2018

DarkSUSY 6 : An Advanced Tool to Compute Dark Matter Properties Numerically

The nature of dark matter remains one of the key science questions. Weakly Interacting Massive Particles (WIMPs) are among the best motivated particle physics candidates, allowing to explain the measured dark matter density by employing standard big-bang thermodynamics. We introduce here a radically new version of the widely used DarkSUSY package, which allows to compute the properties of such dark matter particles numerically. With DarkSUSY 6 one can accurately predict a large variety of astrophysical signals from dark matter, such as direct detection rates in low-background counting experiments and indirect detection signals through antiprotons, antideuterons, gamma rays and positrons from the Galactic halo, or high-energy neutrinos from the center of the Earth or of the Sun. For thermally produced dark matter like WIMPs, high-precision tools are provided for the computation of the relic density in the Universe today, as well as for the size of the smallest dark matter protohalos. Furthermore, the code allows to calculate dark matter self-interaction rates, which may affect the distribution of dark matter at small cosmological scales. Compared to earlier versions, DarkSUSY 6 introduces many significant physics improvements and extensions. The most fundamental new feature of this release, however, is that the code has been completely re-organized and brought into a highly modular and flexible shape. Switching between different pre-implemented dark matter candidates has thus become straight-forward, just as adding new - WIMP or non-WIMP - particle models or replacing any given functionality in a fully user-specified way. In this article, we describe the physics behind the computer package, along with the main structure and philosophy of this major revision of DarkSUSY. A detailed manual is provided together with the public release at www.darksusy.org.

hep-ph↗

arXiv2017

Cosmic-ray propagation with DRAGON2: I. numerical solver and astrophysical ingredients

We present version 2 of the DRAGON code designed for computing realistic predictions of the CR densities in the Galaxy. The code numerically solves the interstellar CR transport equation (including inhomogeneous and anisotropic diffusion, either in space and momentum, advective transport and energy losses), under realistic conditions. The new version includes an updated numerical solver and several models for the astrophysical ingredients involved in the transport equation. Improvements in the accuracy of the numerical solution are proved against analytical solutions and in reference diffusion scenarios. The novel features implemented in the code allow to simulate the diverse scenarios proposed to reproduce the most recent measurements of local and diffuse CR fluxes, going beyond the limitations of the homogeneous galactic transport paradigm. To this end, several applications using DRAGON2 are presented as well. The new version facilitates the users to include their own physical models by means of a modular C++ structure.

astro-ph.HE↗

arXiv2016

A critical reassessment of particle Dark Matter limits from dwarf satellites

Dwarf satellite galaxies are ideal laboratories for identifying particle Dark Matter signals. When setting limits on particle Dark Matter properties from null searches, it becomes however crucial the level at which the Dark Matter density profile within these systems is constrained by observations. In the limit in which the spherical Jeans equation is assumed to be valid for a given tracer stellar population, we study the solution of this equation having the Dark Matter mass profile as an output rather than as a trial parametric input. Within our new formulation, we address to what level dwarf spheroidal galaxies feature a reliable mass estimator. We assess then possible extrapolation of the density profiles in the inner regions and -- keeping explicit the dependence on the orbital anisotropy profile of the tracer population -- we derive general trends on the line-of-sight integral of the density profile squared, a quantity commonly dubbed $J$-factor and crucial to estimate fluxes from prompt Dark Matter pair annihilations. Taking Ursa Minor as a study case among Milky Way satellites, we perform Bayesian inference using the available kinematical data for this galaxy. Contrary to all previous studies, we avoid marginalization over quantities poorly constrained by observations or by theoretical arguments. We find minimal $J$-factors to be about 2 to 4 times smaller than commonly quoted estimates, approximately relaxing by the same amount the limit on Dark Matter pair annihilation cross section from gamma-ray surveys of Ursa Minor. At the same time, if one goes back to a fixed trial parametric form for the density, e.g. using a NFW or Burkert profile, we show that the minimal $J$ can hardly be reduced by more than a factor of 1.5.

astro-ph.GA↗

arXiv2015

Towards a realistic astrophysical interpretation of the gamma-ray Galactic center excess

A spherical-symmetric gamma-ray emission from the central region of the Galaxy has been recently identified in Fermi-LAT data, and initially associated to dark matter particle annihilations. Guided by the evidence for a high gas density in the inner kpc of the Galaxy correlated with a very large Supernova rate, and hence with ongoing cosmic-ray acceleration, we investigate instead the possibility of addressing this excess in terms of ordinary cosmic-ray sources and standard steady-state diffusion. We alter the source term, and consistently the correlated gamma-ray emissions, in the context of a template-fitting analysis. We focus on a region of interest (ROI) defined as: $|l|<20^{\circ}; \hspace{5mm} 2^{\circ}<|b|<20^{\circ},$ with $l$ and $b$ the Galactic longitude and latitude coordinates. We analyze in detail the overall goodness of the fit of our framework, and perform a detailed direct comparison against data examining profiles in different directions. Remarkably, the test statistic of the fit related to our scenario turns out to be as good as the Dark Matter one in the ROI here considered.

astro-ph.HE↗

arXiv2015

Gamma-ray sky points to radial gradients in cosmic-ray transport

The standard approach to cosmic-ray (CR) propagation in the Galaxy is based on the assumption that local transport properties can be extrapolated to the whole CR confining volume. Such models tend to underestimate the gamma-ray flux above few GeV measured by the Fermi Large Area Telescope towards the inner Galactic plane. We consider here for the first time a phenomenological scenario allowing for both the rigidity scaling of the diffusion coefficient and convective effects to be position-dependent. We show that within this approach we can reproduce the observed gamma-ray spectra at both low and mid Galactic latitudes - including the Galactic center - without spoiling any local CR observable.

astro-ph.HE↗

arXiv2014

Indirect Detection Analysis: Wino Dark Matter Case Study

We perform a multichannel analysis of the indirect signals for the Wino Dark Matter, including one-loop electroweak and Sommerfeld enhancement corrections. We derive limits from cosmic ray antiprotons and positrons, from continuum galactic and extragalactic diffuse gamma-ray spectra, from the absence of gamma-ray line features at the galactic center above 500 GeV in energy, from gamma-rays toward nearby dwarf spheroidal galaxies and galaxy clusters, and from CMB power-spectra. Additionally, we show the future prospects for neutrino observations toward the inner Galaxy and from antideuteron searches. For each of these indirect detection probes we include and discuss the relevance of the most important astrophysical uncertainties that can impact the strength of the derived limits. We find that the Wino as a dark matter candidate is excluded in the mass range bellow ~800 GeV from antiprotons and between 1.8 and 3.5 TeV from the absence of a gamma-ray line feature toward the galactic center. Limits from other indirect detection probes confirm the main bulk of the excluded mass ranges.

astro-ph.HE↗

arXiv2014

Constraints on dark matter annihilations from diffuse gamma-ray emission in the Galaxy

Recent advances in gamma-ray cosmic ray, infrared and radio astronomy have allowed us to develop a significantly better understanding of the galactic medium properties in the last few years. In this work using the DRAGON code, that numerically solves the CR propagation equation and calculating gamma-ray emissivities in a 2-dimensional grid enclosing the Galaxy, we study in a self consistent manner models for the galactic diffuse gamma-ray emission. Our models are cross-checked to both the available CR and gamma-ray data. We address the extend to which dark matter annihilations in the Galaxy can contribute to the diffuse gamma-ray flux towards different directions on the sky. Moreover we discuss the impact that astrophysical uncertainties of non DM nature, have on the derived gamma-ray limits. Such uncertainties are related to the diffusion properties on the Galaxy, the interstellar gas and the interstellar radiation field energy densities. Light ~10 GeV dark matter annihilating dominantly to hadrons is more strongly constrained by gamma-ray observations towards the inner parts of the Galaxy and influenced the most by assumptions of the gas distribution; while TeV scale DM annihilating dominantly to leptons has its tightest constraints from observations towards the galactic center avoiding the galactic disk plane, with the main astrophysical uncertainty being the radiation field energy density. In addition, we present a method of deriving constraints on the dark matter distribution profile from the diffuse gamma-ray spectra. These results critically depend on the assumed mass of the dark matter particles and the type of its end annihilation products.

astro-ph.HE↗

arXiv2013

The 111 and 129 GeV gamma-ray lines from annihilations in the Milky Way dark matter halo, dark disk and subhalos

Recently a series of indications have been put forward suggesting the presence of two gamma-ray lines at 110-130 GeV (centered at 111 and 129 GeV). Signals of these lines have been observed toward the Galactic center, at some galaxy clusters and among some of the unassociated point sources of the 2 years Fermi catalogue. Such a combination of signals could be generated by dark matter annihilations in the main dark matter halo, its substructures and nearby galaxy clusters. We discuss in this work the consistency between the number of events observed at the line energies in the sky and the predictions using results from the Via Lactea II numerical simulation and extrapolations below its mass resolution, taking into account that the annihilation cross-section to the lines can be estimated from the Galactic center signal. We find that some extrapolations to small substructures can naturally account for the point sources signal, although the hypothesis of background only cannot be rejected. We also study the morphology of the gamma-ray sky at the 2 lines energies, testing different Galactic diffuse background models to account for interstellar medium uncertainties, and different assumptions on the DM diffuse component profile. We find from template fits that within reasonable diffuse background uncertainties the presence of a spherical halo component is preferred with cuspier dark matter halo profiles being preferable even from the full sky fit. We finally check the impact of a dark disk component, suggested by cosmological simulations that include baryons and find that thin dark disks can not be disfavored, thus possibly accounting for the preferentially closer to the Galactic disk distribution of the point sources lines signal.

astro-ph.HE↗