arXiv subjects

Giacomo Ferrante

Publications and source records attributed to Giacomo Ferrante.

7 recordsLinked to original sources

arXiv2026

The price for monopole dark matter

We construct an explicit model where dark matter consists of 't Hooft-Polyakov monopoles. The dark sector is in thermal contact with the Standard Model, and dark monopoles are created by a thermal phase transition in the early Universe. Generically, the abundance of monopoles is negligible with respect to that of stable dark elementary particles. We show how to avoid this by taking the lightest stable particle, a dark fermion, sufficiently light for its abundance to be suppressed, yet heavy enough to satisfy constraints on dark radiation. In this specific window of parameters, dark matter is composed of monopoles with a mass of about $10^8$ GeV or larger, depending on the nature of the phase transition. This candidate lies beyond the reach of present, conventional dark-matter detection experiments. However, the model necessarily predicts dark radiation, with $\Delta N_{\rm eff}$ close to present-day bounds. In addition, if the dark phase transition is strongly first order, we find that the corresponding gravitational wave spectrum lies close to the region probed by future interferometers.

hep-ph↗

arXiv2025

Cosmological consequences of spontaneous symmetry breaking

The Standard Model of particle physics and the $\Lambda$CDM model of cosmology provide an incomplete description of our Universe. Both models face challenges, including explaining the nature of dark matter, the origin of the Universe's initial conditions, and the fine-tuning of the Higgs boson mass. This thesis investigates the cosmological implications of spontaneous symmetry breaking to address some of these issues, focusing on theories with a non-trivial vacuum structure. We introduce a novel class of elementary scalars called ''accidents'', which emerge as accidentally flat directions in the vacuum manifold: unlike Nambu-Goldstone boson directions, accident directions are not related to any symmetry. Radiative corrections induce a mass for the accidents that is one-loop suppressed with respect to naive expectations, making them naturally light. We propose that accidents can act as viable dark matter candidates, and as the inflaton driving cosmic inflation. We construct a model of hybrid inflation in which the inflaton potential is an accident direction and is naturally flat. In models of accident inflation where the vacuum manifold has a non-trivial topology, cosmic strings and domain walls form after the end of inflation. Such topological defects generate a stochastic background of gravitational waves. Finally, we investigate the cosmological production of dark magnetic monopoles. Focusing on 't Hooft-Polyakov monopoles from SO(3)$\rightarrow$SO(2) symmetry breaking, we explore both second-order and first-order phase transitions, and we identify the regions of parameter space where the monopole relic density matches the one of dark matter. This model also features stable massive gauge bosons. We find that the relic density of dark gauge bosons is always far larger than the one of monopoles, concluding that dark monopoles cannot constitute a sizeable fraction of dark matter.

hep-ph↗

arXiv2025

No room for minimal monopole dark matter

The magnetic monopole of a dark sector has been advocated as an appealing dark matter candidate. We revisit the computation of the monopole abundance $\Omega_M$, generated by a thermal phase transition in the minimal 't Hooft-Polyakov model. We explore the three regimes where the phase transition is second order, weakly first order, or supercooled, identifying the parameter space regions where $\Omega_M$ can match the observed dark matter abundance. However, the dark sector necessarily contains a stable electrically-charged particle, namely a massive vector boson, with a calculable abundance $\Omega_{W'}$. We show that, under minimal assumptions, $\Omega_{W'}$ is always far larger than $\Omega_M$: dark monopoles cannot constitute a sizeable fraction of dark matter.

hep-ph↗

arXiv2024

Hybrid inflation and gravitational waves from accidentally light scalars

We construct a hybrid-inflation model where the inflaton potential is generated radiatively, as gauge symmetries guarantee it to be accidentally flat at tree level. The model can be regarded as a small-field version of Natural Inflation, with inflation ending when the mass of a second scalar, the waterfall field, turns tachyonic. This provides a minimal, robust realization of hybrid inflation, which predicts specific correlations among Cosmic Microwave Background observables. Tachyonic preheating leads to the production of gravitational waves which, for a low inflationary scale, might be detected by upcoming experiments. Simple variations of the model can give rise to topological defects, such as unstable domain walls. Their dynamics produces a stochastic gravitational wave background, which can be compatible with the recent detection by pulsar timing arrays.

astro-ph.CO↗

arXiv2024

Accidentally light scalars from large representations

In models with spontaneous symmetry breaking by scalar fields in large group representations, we observe that some of the scalar masses can be loop-suppressed with respect to the naive expectation from symmetry selection rules. We present minimal models -- the $\rm{SU(2)}$ five-plet and $\rm{SU(3)}$ ten-plet -- with such accidentally light scalars, featuring compact tree-level flat directions lifted by radiative corrections. We sketch some potential applications, from stable relics and slow roll in cosmology, to hierarchy and fine-tuning problems in particle physics.

hep-ph↗

arXiv2023

Primordial black holes in the curvaton model: possible connections to pulsar timing arrays and dark matter

We revise primordial black holes (PBHs) production in the axion-curvaton model, in light of recent developments in the computation of their abundance accounting for non-gaussianities (NGs) in the curvature perturbation up to all orders. We find that NGs intrinsically generated in such scenarios have a relevant impact on the phenomenology associated to PBHs and, in particular, on the relation between the abundance and the signal of second-order gravitational waves. We show that this model could explain both the totality of dark matter in the asteroid mass range and the tentative signal reported by the NANOGrav and IPTA collaborations in the nano-Hz frequency range. En route, we provide a new, explicit computation of the power spectrum of curvature perturbations going beyond the sudden-decay approximation.

astro-ph.CO↗

arXiv2023

Primordial non-gaussianity up to all orders: theoretical aspects and implications for primordial black hole models

We develop an exact formalism for the computation of the abundance of primordial black holes (PBHs) in the presence of local non-gaussianity (NG) in the curvature perturbation field. For the first time, we include NG going beyond the widely used quadratic and cubic approximations, and consider a completely generic functional form. Adopting threshold statistics of the compaction function, we address the computation of the abundance both for narrow and broad power spectra. While our formulas are generic, we discuss explicit examples of phenomenological relevance considering the physics case of the curvaton field. We carefully assess under which conditions the conventional perturbative approach can be trusted. In the case of a narrow power spectrum, this happens only if the perturbative expansion is pushed beyond the quadratic order (with the optimal order of truncation that depends on the width of the spectrum). Most importantly, we demonstrate that the perturbative approach is intrinsically flawed when considering broad spectra, in which case only the non-perturbative computation captures the correct result. Finally, we describe the phenomenological relevance of our results for the connection between the abundance of PBHs and the stochastic gravitational wave (GW) background related to their formation. As NGs modify the amplitude of perturbations necessary to produce a given PBHs abundance and boost PBHs production at large scales for broad spectra, modelling these effects is crucial to connect the PBH scenario to its signatures at current and future GWs experiments.

astro-ph.CO↗