arXiv subjects

Amedeo M. Favitta

Publications and source records attributed to Amedeo M. Favitta.

5 recordsLinked to original sources

arXiv2025

Axion domain walls and thermal friction

The post-inflationary Peccei-Quinn symmetry-breaking scenario provides a rich theoretical framework to study axion dark matter production through the dynamics oftopological defects. Accurate predictions for the axion abundance require a detailed understanding of the formation and evolution of cosmic strings and domain walls, which are inevitably produced in this scenario. Most existing studies rely on large-scale numerical simulations of the classical equations of motion, which are subject to significant systematic uncertainties. In this contribution, we briefly review the main features of the post-inflationary scenario and the current limitations in the literature, and present preliminary results from a new analytical framework for axion domain wall networks. Our approach is based on nonequilibrium quantum field theory. It employs the two-particle-irreducible effective action to derive effective Fokker-Planck equations for macroscopic quantities such as the average energy density and root-mean-square velocity of the network.We discuss the relevance of thermal, self-interactions, plasma-induced and quantum effects for cosmological axion abundance estimates, with applications to QCD axions and high-mass axion-like particles.

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arXiv2025

Non-equilibrium Quantum Field Theory and Axion Electrodynamics in curved spacetimes

Axions are hypothetical pseudoscalar particles introduced initially as a solution to the Strong CP problem in Quantum Chromodynamics (QCD), and they also arise naturally in a broad class of low-energy compactifications of string theory. Astrophysical, cosmological, and laboratory constraints require axions to be extremely weakly coupled to Standard Model particles, making them viable dark matter candidates .This PhD thesis presents original results developed over three years of research, focusing on axion cosmology and axion electrodynamics. These topics address the production of axions and their associated topological defects in the early Universe, as well as the interactions of axions with Standard Model particles. The analysis combines methods from non-equilibrium quantum field theory and quantum field theory in curved spacetimes. After reviewing the foundations of quantum field theory in curved spacetimes and cosmology, the thesis introduces the Strong CP problem and the Peccei-Quinn mechanism, including the PQWW axion model and its invisible extensions. Ultraviolet completions of QCD axion theories are discussed in both field-theoretic and extra-dimensional frameworks. A significant part of the thesis is devoted to axion electrodynamics, where a classical axion background modifies Maxwell's equations and electromagnetic observables. The non-equilibrium dynamics of self-interacting axion fields coupled to the Standard Model or to a dark sector are also studied using the two-particle-irreducible effective action, and applied to pre- and post-inflationary cosmological scenarios, yielding new insights and preliminary constraints.

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arXiv2024

On the Axion Electrodynamics in a two-dimensional slab and the Casimir effect

We analyze the Axion Electrodynamics in a two-dimensional slab of finite width $L$ containing a homogeneous and isotropic dielectric medium with constant permittivity and permeability. We start from the known decomposition of modes in the nonaxion case and then solve perturbatively the governing equations for the electromagnetic fields to which the axions are also coupled. This is a natural approach, since the finiteness of $L$ destroys the spatial invariance of the theory in the $z$ direction normal to the plates. In this way we derive the value of the axion-generated rotation angle of the electric and magnetic fields after their passage through the slab, and use the obtained results to calculate the Casimir force between the two conducting plates. Our calculations make use of the same method as previously outlined in \cite{hoye20} for the case of Casimir calculations in chiral media and extend former results on the Casimir force in the Axion Electrodynamics.

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arXiv2023

Axion electrodynamics: Green's functions, zero-point energy and optical activity

Starting from the theory of Axion Electrodynamics, we work out the axionic modifications to the electromagnetic Casimir energy using the Green's function, both when the axion field is initially assumed purely time-dependent and when the axion field configuration is a static domain wall. For the first case it means that the oscillating axion background is taken to resemble an axion fluid at rest in a conventional Casimir setup with two infinite parallel conducting plates, while in the second case we evaluate the radiation pressure acting on an axion domain wall. We extend previous theories in order to include finite temperatures. Various applications are discussed. 1. We review the theory of Axion Electrodynamics and particularly the energy-momentum conservation in a linear dielectric and magnetic material. We treat this last aspect by extending former results by Brevik and Chaichian (2022) and Patkos (2022). 2. Adopting the model of the oscillating axion background we discuss the axion-induced modifications to the Casimir force between two parallel plates by using a Green's function approach. 3. We calculate the radiation pressure acting on an axion domain wall at finite temperature T. Our results for an oscillating axion field and a domain wall are also useful for condensed matter physics, where "axionic topological insulators" interact with the electromagnetic field with a Chern-Simons interaction, like the one in Axion Electrodynamics, and there are experimental systems analogous to time-dependent axion fields and domain walls as the ones showed by Jiang, Q. D., \& Wilczek, F. (2019) and Fukushima et al. (2019). 4. We compare our results, where we assume time-dependent or space-dependent axion configurations, with the discussion of the optical activity of Axion Electrodynamics by Sikivie (2021) and Carrol et al. (1990).

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arXiv2023

Axionic and nonaxionic electrodynamics in plane and circular geometry

Various aspects of axion electrodynamics in the presence of a homogeneous and isotropic dielectric medium are discussed. 1. We consider first the "antenna-like" property of a planar dielectric surface in axion electrodynamics, elaborating on the treatment given earlier on this topic by Millar {\it et al.} (2017). We calculate the electromagnetic energy transmission coefficient for a dielectric plate, and compare with the conventional expression in ordinary electrodynamics. 2. We consider the situation where the medium exterior to the plate, assumed elastic, is "bent back" and glued together, so that we obtain a circular dielectric string in which the waves can propagate clockwise or counterclockwise. As will be shown, a stationary wave pattern is permitted by the formalism, and we show how the amplitudes for the two counterpropagating waves can be found. 3. As a special case, by omitting axions for a moment, we analyze the Casimir effect for the string, showing its similarity as well as its difference with the Casimir effect of a scalar field for a piecewise uniform string (Brevik and Nielsen 1990). 4. Finally, including axions again we analyze the enhancement of the surface-generated electromagnetic radiation near the center of a cylindrical haloscope, where the interior region is a vacuum and the exterior region a metal. This enhancement is caused by the curvature of the boundary, and is mathematically a consequence of the behavior of the Hankel function of the second kind for small arguments. A simple estimate shows that enhancement may be quite significant, and can therefore be of experimental interest. This proposal is suggested as an alternative to the reflector arrangement in a similar arrangement recently discussed by Liu {\it et al.} (2022).

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