arXiv subjects

Silvia Pascoli

Publications and source records attributed to Silvia Pascoli.

At least 19 recordsLinked to original sources

arXiv2026

DarkAgents

We present DarkAgents: a multi-agent system that leverages the reasoning and code-generation capabilities of large language models (LLMs), together with deterministic tested human-written code, to build orchestrated pipelines for theoretical astroparticle physics research. While related approaches have been proposed in collider physics and cosmology, DarkAgents targets the specific challenges of this domain, such as model building, complex pipeline computations, multiple constraints and assumption auditing. The framework can be powered by different agentic command-line tools, including Mistral's, Anthropic's, OpenAI's and local LLMs via Ollama. As first implementation, we apply DarkAgents to the study of cosmological first order transitions, starting from a classically scale-invariant particle-physics model and ending with the fit to the NANOGrav nanohertz gravitational-waves spectrum. DarkAgent-PT provides as output i) the best-fit values of model parameters, ii) their existing experimental and observational constraints, iii) an audit report of the assumptions and priors entering both i) and ii), of particular relevance for astroparticle physics. Our test runs identify inconsistencies in some fits in the literature and produce novel ones based on the dissipative bulk-flow GW template. The code is publicly available at https://github.com/PhysicsZandi/DarkAgents.

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arXiv2026

Dark Matter Interpretation of the Super-Kamiokande Antineutrino Excess and Predictions for JUNO

Super-Kamiokande has reported a small excess of electron antineutrino events in the 20 MeV energy range, in the search for the diffuse supernova neutrino background. We interpret this signal as a possible indication of dark matter that annihilates dominantly into neutrinos, pointing to a thermal dark matter candidate with $s$-wave annihilation and with mass in the tens of MeV range. This mass scale naturally fits into rich dark sector extensions of the Standard Model. Neutrino experiments, including JUNO, will be able to test this hypothesis in the coming years.

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arXiv2026

Nanohertz gravitational waves from the baryon-dark matter coincidence

The nanohertz gravitational waves (GW) observed by pulsar timing arrays may originate from a cosmological first-order phase transition (PT) at $\sim$ 100 MeV. Taking this possibility seriously motivates the question: why 100 MeV? We point out that a PT at exactly those scales is predicted by the generation of the baryon asymmetry from a dark asymmetry via resonant neutron-dark matter oscillations, and we prove that this PT can induce an observable GW signal compatibly with all experimental constraints. This proposal predicts dark matter self-interactions close to their observational upper limits and lowers the maximal expected mass of neutron stars. Independently of GW, this baryogenesis mechanism is tested by searches for missing-energy at the LHC and for neutron decays. We keep the model consistent with big-bang nucleosynthesis by adding heavy neutral leptons below 100 MeV, which generate neutrino masses and can induce further experimental tests.

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arXiv2025

Boiling away asymmetries: low-scale phase transitions, gravitational waves and leptogenesis

Leptogenesis is one of the most popular mechanisms to account for the observed baryon asymmetry of the Universe. A generic feature of leptogenesis is a large separation of scales between the epoch of baryon asymmetry production (sphaleron freeze-out at temperature $T \sim 130$ GeV) and the one where it affects the big bang nucleosynthesis processes (BBN at $T \sim 1$ MeV). Any entropy release between these two epochs would lead to a dilution of previously produced relics, such as the baryons. Motivated by the recent evidence of a stochastic gravitational waves background (SGWB) in the nHz frequency range, we consider the case of supercooled first-order phase transition and we study the impact of the induced entropy dilution on the leptogenesis parameter space. We employ the Type-I seesaw with 2 right-handed neutrinos as benchmark scenario, and demonstrate that the viable leptogenesis parameter space is significantly reduced. Interestingly, the values of dilution predicted by the SGWB best fit points in several first order phase transition scenarios would completely exclude the leptogenesis parameter space testable by future experiments, thus establishing a phenomenological interconnection between leptogenesis, SGWB, first order phase transition and neutrino mass generation. Our analysis can be generalised to different leptogenesis models and entropy dilution mechanisms.

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arXiv2025

Supercooled Dark Scalar Phase Transitions explanation of NANOGrav data

The evidence of a Stochastic Gravitational Wave Background (SGWB) in the nHz frequency range is posed to open a new window on the Universe. A preferred explanation relies on a supercooled first order phase transition at the 100 MeV - GeV scale. In this article, we address the feasibility going from the particle physics model to the production of the gravitational waves. We take a minimal approach for the dark sector model introducing the fewest ingredients required, namely a new U(1) gauge group and a dark scalar that dynamically breaks the symmetry. Supercooling poses challenges in the analysis that put under question the feasibility of this explanation: we address them, going beyond previous studies by carefully considering the effects of a vacuum domination phase and explicitly tracking the phase transition from its onset to its completion. We find that the proposed model can successfully give origin to the observed PTA SGWB signal. The strong supercooling imposes a correlation between the new gauge coupling and the scalar quartic one, leading to a significant hierarchy between the (heavier) gauge boson and the dark scalar. Ultimately, information on phase transitions from SGWB observations could provide a direct probe of the microphysics of the Early Universe and be used to guide future searches of dark sector in laboratories.

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arXiv2025

ELENA: a software for fast and precise computation of first order phase transitions and gravitational waves production in particle physics models

We present ELENA (EvaLuator of tunnElliNg Actions), an open-source Python package designed to compute the full evolution of first-order phase transitions in the early Universe generated by particle physics models, taking into account several refinements that go beyond commonly assumed simplifications. The core of ELENA is based on a vectorized implementation of the tunnelling potential formalism, which allows for a fast computation of the finite-temperature tunnelling action. This, in turn, enables the sampling of the full range of temperatures where two phases coexist and the use of integral expressions that track the complete evolution of the transition, providing a comprehensive picture of it. In addition, ELENA provides all the tools to compute the resulting stochastic gravitational waves spectrum, allowing for the full chain of computations - from the Lagrangian parameter inputs to the final gravitational waves spectrum - in a fast and self-contained implementation.

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arXiv2025

From oversimplified to overlooked: the case for exploring Rich Dark Sectors

The Standard Model (SM) of particle physics provides a very successful description of fundamental particles and their interactions but it is incomplete, as neutrino masses, dark matter and the baryon asymmetry of the Universe indicate. In addition, the origin of masses and of the approximate fundamental symmetries call out for deeper explanations. The quest for a New SM Theory, that extends the SM to a more general theory, is ongoing. For decades the main focus has been on the TeV scale, but despite an impressive theoretical and experimental effort, no hints of new physics at such scale has been found in experiments. Dark sectors provide an interesting alternative to TeV scale extensions of the SM to explain the open questions in particle and astroparticle physics. Going beyond minimal models, rich dark sectors extend the SM to a complex theory with multiple particles and interactions, in analogy to the SM itself. They have a wealth of theoretical and astrophysical/cosmological consequences and can lead to phenomenological signatures that can be markedly different to that of minimal ones. These include short-lived particles and semi-visible decay signatures, as opposed to minimal models where new states are typically long-lived and purely visible or invisible resonances. Given the experimental configurations and analysis strategies, current dark sector searches might miss such signatures. We advocate a dedicated programme of searches for rich dark sectors that overcomes the assumptions on minimality and on the long lifetime of particles and encompasses a broader range of possibilities. Here, we discuss a prototype model that includes a complex structure akin to the SM: multiple generations of fermions charged under a new spontaneously-broken gauge symmetry.

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arXiv2025

Non-Abelian Domain Walls and Gravitational Waves

We investigate the properties of domain walls arising from non-Abelian discrete symmetries, which we refer to as non-Abelian domain walls. We focus on $S_4$, one of the most commonly used groups in lepton flavour mixing models. The spontaneous breaking of $S_4$ leads to distinct vacua preserving a residual $Z_2$ or $Z_3$ symmetry. Five types of domain walls are found, labelled as SI, SII, TI, TII, and TIII, respectively, the former two separating $Z_2$ vacua and the latter three separating $Z_3$ vacua. We highlight that SI, TI and TIII may be unstable for some regions of the parameter space and decay to stable domain walls. Stable domain walls can collapse and release gravitational radiation for a suitable size of explicit symmetry breaking. A symmetry-breaking scale of order 100 TeV may explain the recent discovery of nanohertz gravitational waves by PTA experiments. For the first time, we investigate the properties of these domain walls, which we obtain numerically with semi-analytical formulas applied to compute the tension and thickness across a wide range of parameter space. We estimate the resulting gravitational wave spectrum and find that, thanks to their rich vacuum structure, non-Abelian domain walls manifest in a very interesting and complex phenomenology.

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arXiv2025

Neutrino Theory in the Precision Era

This document summarises discussions on future directions in theoretical neutrino physics, which are the outcome of a neutrino theory workshop held at CERN in February 2025. The starting point is the realisation that neutrino physics offers unique opportunities to address some of the most fundamental questions in physics. This motivates a vigorous experimental programme which the theory community fully supports. \textbf{A strong effort in theoretical neutrino physics is paramount to optimally take advantage of upcoming neutrino experiments and to explore the synergies with other areas of particle, astroparticle, and nuclear physics, as well as cosmology.} Progress on the theory side has the potential to significantly boost the physics reach of experiments, as well as go well beyond their original scope. Strong collaboration between theory and experiment is essential in the precision era. To foster such collaboration, \textbf{we propose to establish a CERN Neutrino Physics Centre.} Taking inspiration from the highly successful LHC Physics Center at Fermilab, the CERN Neutrino Physics Centre would be the European hub of the neutrino community, covering experimental and theoretical activities.

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arXiv2025

Negative neutrino masses as a mirage of dark energy

The latest cosmological constraints on the sum of the neutrino masses depend on prior physical assumptions about the mass spectrum. To test the accordance of cosmological and laboratory constraints in the absence of such priors, we introduce an effective neutrino mass parameter that extends consistently to negative values. For the $Λ$CDM model, we analyze data from Planck, ACT, and DESI and find a $2.8-3.3σ$ tension with the constraints from oscillation experiments. Motivated by recent hints of evolving dark energy, we analyze the $w_0w_a$ and mirage dark energy models, showing that they favour larger masses consistent with laboratory data, respectively $\sum m_{ν,\mathrm{eff}} = 0.06_{-0.10}^{+0.15}\,\mathrm{eV}$ and $\sum m_{ν,\mathrm{eff}} = 0.04_{-0.11}^{+0.15}\,\mathrm{eV}$ (both at 68%).

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arXiv2025

A panorama of new-physics explanations to the MiniBooNE excess

The MiniBooNE low-energy excess stands as an unexplained anomaly in short-baseline neutrino oscillation experiments. It has been shown that it can be explained in the context of dark sector models. Here, we provide an overview of the possible new-physics solutions based on electron, photon, and dilepton final states. We systematically discuss the various production mechanisms for dark particles in neutrino-nucleus scattering. Our main result is a comprehensive fit to the MiniBooNE energy spectrum in the parameter space of dark neutrino models, where short-lived heavy neutral leptons are produced in neutrino interactions and decay to $e^+e^-$ pairs inside the detector. For the first time, other experiments will be able to directly confirm or rule out dark neutrino interpretations of the MiniBooNE low-energy excess.

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arXiv2025

The FLAMINGO project: the coupling between baryonic feedback and cosmology in light of the $S_8$ tension

Large-scale structure surveys have reported measurements of the density of matter, $Ω_\mathrm{m}$, and the amplitude of clustering, $σ_8$, that are in tension with the values inferred from observations of the cosmic microwave background. While this may be a sign of new physics that slows the growth of structure at late times, strong astrophysical feedback processes could also be responsible. In this work, we argue that astrophysical processes are not independent of cosmology and that their coupling naturally leads to stronger baryonic feedback in cosmological models with suppressed structure formation or when combined with a mechanism that removes dark matter from halos. We illustrate this with two well-motivated extensions of the Standard Model known to suppress structure formation: massive neutrinos and decaying dark matter. Our results, based on the FLAMINGO suite of hydrodynamical simulations, show that the combined effect of baryonic and non-baryonic suppression mechanisms is greater than the sum of its parts, particularly for decaying dark matter. We also show that the dependence of baryonic feedback on cosmology can be modelled as a function of the ratio $f_\mathrm{b}/c^2_\mathrm{v}\sim f_\mathrm{b}/(Ω_\mathrm{m}σ_8)^{1/4}$ of the universal baryon fraction, $f_\mathrm{b}$, to a velocity-based definition of halo concentration, $c^2_\mathrm{v}$, giving an accurate fitting formula for the baryonic suppression of the matter power spectrum. Although the combination of baryonic and non-baryonic suppression mechanisms can resolve the tension, the models with neutrinos and decaying dark matter are challenged by constraints on the expansion history.

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arXiv2024

Testing Realistic $SO(10)$ SUSY GUTs with Proton Decay and Gravitational Waves

We present a comprehensive analysis of a supersymmetric $SO(10)$ Grand Unified Theory, which is broken to the Standard Model via the breaking of two intermediate symmetries. The spontaneous breaking of the first intermediate symmetry, $B-L$, leads to the generation of cosmic strings and right-handed neutrino masses and further to an observable cosmological background of gravitational waves and generation of light neutrino masses via type-I seesaw mechanism. Supersymmetry breaking manifests as sparticle masses below the $B-L$ breaking but far above the electroweak scale due to proton decay limits. This naturally pushes the $B-L$ breaking scale close to the GUT scale, leading to the formation of metastable cosmic strings, which can provide a gravitational wave spectrum consistent with the recent Pulsar Timing Arrays observation. We perform a detailed analysis of this model using two-loop renormalisation group equations, including threshold corrections, to determine the symmetry-breaking scale consistent with the recent Pulsar Timing Arrays signals such as NANOGrav 15-year data and testable by the next-generation limits on proton decay from Hyper-K and JUNO. Simultaneously, we find the regions of the model parameter space that can predict the measured quark and lepton masses and mixing, baryon asymmetry of our Universe, a viable dark matter candidate and can be tested by a combination of neutrinoless double beta decay searches and limits on the sum of neutrinos masses.

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arXiv2024

Baryogenesis and Leptogenesis from Supercooled Confinement

We propose a framework of baryogenesis and leptogenesis that relies on a supercooled confining phase transition (PT) in the early universe. The baryon or lepton asymmetry is sourced by decays of hadrons of the strong dynamics after the PT, and it is enhanced compared to the non-confining case, which was the only one explored so far. This widens the energy range of the PT, where the observed baryon asymmetry can be reproduced, down to the electroweak scale. The framework then becomes testable with gravity waves (GW) at LISA and the Einstein Telescope. We then study two explicit realisations: one of leptogenesis from composite sterile neutrinos that realises inverse see-saw; one of baryogenesis from composite scalars that is partly testable by existing colliders and flavour factories.

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arXiv2023

Where shadows lie: reconstruction of anisotropies in the neutrino sky

The Cosmic Neutrino Background (CNB) encodes a wealth of information, but has not yet been observed directly. To determine the prospects of detection and to study its information content, we reconstruct the phase-space distribution of local relic neutrinos from the three-dimensional distribution of matter within 200 Mpc/h of the Milky Way. Our analysis relies on constrained realization simulations and forward modelling of the 2M++ galaxy catalogue. We find that the angular distribution of neutrinos is anti-correlated with the projected matter density, due to the capture and deflection of neutrinos by massive structures along the line of sight. Of relevance to tritium capture experiments, we find that the gravitational clustering effect of the large-scale structure on the local number density of neutrinos is more important than that of the Milky Way for neutrino masses less than 0.1 eV. Nevertheless, we predict that the density of relic neutrinos is close to the cosmic average, with a suppression or enhancement over the mean of (-0.3%, +7%, +27%) for masses of (0.01, 0.05, 0.1) eV. This implies no more than a marginal increase in the event rate for tritium capture experiments like PTOLEMY. We also predict that the CNB and CMB rest frames coincide for 0.01 eV neutrinos, but that neutrino velocities are significantly perturbed for masses larger than 0.05 eV. Regardless of mass, we find that the angle between the neutrino dipole and the ecliptic plane is small, implying a near-maximal annual modulation in the bulk velocity. Along with this paper, we publicly release our simulation data, comprising more than 100 simulations for six different neutrino masses.

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arXiv2023

Low-Scale Leptogenesis with Low-Energy Dirac CP-Violation

We study the freeze-in scenario of leptogenesis via oscillations within the type-I seesaw model with two quasi-degenerate heavy Majorana neutrinos $N_{1,\,2}$ having masses $M_2 > M_1 \sim (0.1-100)\,\text{GeV}$, $(M_2-M_1)/M_1 \ll 1$, focusing on the role of the CP-violation provided by the Dirac phase $δ$ of the Pontecorvo-Maki-Nakagawa-Sakata lepton mixing matrix. We find that viable leptogenesis can be due solely to CP-violating values of $δ$ and that the $N_{1,\,2}$ total mixing squared $Θ^2=\sum_αΘ^2_α$ needed is within the reach of future experiments, $Θ_α$ parameterising the coupling to the charged lepton $α=e,\,μ,\,τ$. Furthermore, the required parameter space differs from that associated with additional Casas-Ibarra sources of CP-violation. Future determination of $δ$, $Θ^2$ and/or the ratios $Θ_τ^2:Θ^2_μ:Θ^2_e$ would provide a critical test of the considered scenario.

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arXiv2023

Semi-Visible Dark Photon Phenomenology at the GeV Scale

In rich dark sector models, dark photons heavier than tens of MeV can behave as semi-visible particles: their decays contain both visible and invisible final states. We present models containing multiple dark fermions which allow for such decays and inscribe them in the context of inelastic dark matter and heavy neutral leptons scenarios. Our models represent a generalization of the traditional inelastic dark matter model by means of a charge conjugation symmetry. We revisit constraints on dark photons from $e^+e^-$ colliders and fixed target experiments, including the effect of analysis vetoes on semi-visible decays, $A^\prime \to ψ_i (ψ_j \to ψ_k \ell^+\ell^-)$. We find that in some cases, the BaBar and NA64 experiments no longer exclude large kinetic mixing, $\varepsilon \sim 10^{-2}$, and, specifically, the related explanation of the discrepancy in the muon $(g-2)$. This reopens an interesting window in parameter space for dark photons with exciting discovery prospects. We point out that a modified missing-energy search at NA64 can target short-lived $A^\prime$ decays and directly probe the newly-open parameter space.

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arXiv2023

Constraining Light Thermal Inelastic Dark Matter with NA64

A vector portal between the Standard Model and the dark sector is a predictive and compelling framework for thermal dark matter. Through co-annihilations, models of inelastic dark matter (iDM) and inelastic Dirac dark matter (i2DM) can reproduce the observed relic density in the MeV to GeV mass range without violating cosmological limits. In these scenarios, the vector mediator behaves like a semi-visible particle, evading traditional bounds on visible or invisible resonances, and uncovering new parameter space to explain the muon $(g-2)$ anomaly. By means of a more inclusive signal definition at the NA64 experiment, we place new constraints on iDM and i2DM using a missing energy technique. With a recast-based analysis, we contextualize the NA64 exclusion limits in parameter space and estimate the reach of the newly collected and expected future NA64 data. Our results motivate the development of an optimized search program for semi-visible particles, in which fixed-target experiments like NA64 provide a powerful probe in the sub-GeV mass range.

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