Advancements in Axion Physics

The QCD axion and axion-like particles are recognized as leading candidates for dark matter. For a full understanding of dark matter, the production mechanism of the axion is crucial. The misalignment mechanism is a prominent candidate. However, considering the quality problem of the Peccei-Quinn symmetry, it seems natural that there is a small breaking of the U(1)PQ symmetry other than QCD. I will show how such a small PQ symmetry breaking can significantly change the QCD axion production mechanism. We call it the trapped misalignment mechanism, and such a trapping phenomenon of the QCD axion was first studied in arXiv:1603.02090 in the context of the clockwork QCD axion model. Interestingly, the resulting axion abundance does not depend on the decay constant. I will also discuss scenarios where the axion mass arises due to a first-order phase transition. Finally, I will touch upon the implications of this scenario for the dark photon production mechanism.

If the dark matter is composed of axions, then axion stars are expected to be abundant in the Universe. This talk is based on 2302.10100, where we demonstrate in fully non-linear (3+1) numerical relativity the instability of compact axion stars due to the electromagnetic Chern-Simons term. We show that above a critical coupling constant, compact axion stars are unstable. The instability is caused by parametric resonance between the axion and the electromagnetic field. Unstable axion stars decay leaving behind a less massive, less compact, remnant. Furthermore, I will show how this phenomenon is relevant to axion star mergers.

In axion-like dark matter cosmologies a very dense axion star forms in the center of every dark matter halo. When such axion stars are massive enough they become unstable and quickly decay via parametric resonance into radio photons. In this talk, based on 2302.10206 and 2301.09769, I will highlight the cosmological consequences of such a decay. In particular, the huge number of radio photons produced by axion stars decays can heat up the intergalactic medium and lead to an early period of reionization. Planck CMB constraints on this early reionization yield the most stringent bounds on the coupling between axion dark matter and photons in the sub neV mass range. I will finally show that upcoming 21cm observations will be able to test couplings that are an order of magnitude smaller that those currently tested by Planck.

We identify a new resonance, axion magnetic resonance (AMR), that can greatly enhance the conversion rate between axions and photons. A series of axion search experiments rely on converting them into photons inside a constant magnetic field background. A common bottleneck of such experiments is the conversion amplitude being suppressed by the axion mass when ma≳10^{-4} eV. We point out that a spatial or temporal variation in the magnetic field can cancel the difference between the photon dispersion relation and that of the axion, hence greatly enhancing the conversion probability. We demonstrate that the enhancement can be achieved by both a helical magnetic field profile and a harmonic oscillation of the magnitude. Our approach can extend the projected ALPS II reach in the axion-photon coupling (gaγ) by two orders of magnitude at ma=10^{-3} eV with moderate assumptions.

In 2009 the highly influential idea of a "string axiverse" was proposed. This talk attempts to review the progress myself and others have made on this idea in the last 14 years. The original probes proposed: matter power spectrum, birefringence, superradiance, and axion decays have all matured in precision both theoretically and observationally. I will then describe recent dramatic progress that has been made in constructing explicit axiverses from Calabi-Yau compactifications across the entire "Kreuzer-Skarke" database in Type IIB string theory, and the similarities and differences between such axiverses and the conjectures in 2009. I will outline the most promising phenomenology of these explicit models, and constraints on string theory, and end with some dreams for the future of this program.

Nearby dwarf spheroidal galaxies are ideal targets in the search for indirect dark matter (DM) signals. In this work, we analyze MUSE spectroscopic observations of a sample of five galaxies, composed of both classical and ultra-faint dwarf spheroidals. The goal is to search for radiative decays of axion-like particles (ALPs) in the mass range of 2.7-5.3 eV. After taking into account the uncertainties associated with the DM spatial distribution in the galaxies, we derive robust bounds on the effective ALP-two-photon coupling. They lie well below the QCD axion band and are significantly more constraining than limits from other probes, in the relevant mass range. We also test the possible presence of a positive signal, concluding that none of the channels selected for this analysis, i.e., not affected by large background contamination, is exhibiting such evidence.

The axion is characterized by a periodic potential leading to degenerate vacua, and thus predicts the presence of domain wall configurations. I plan to discuss the formation of these domain walls in the early Universe, their formation-dependent stability, and methods for their detection. In particular, I will show that the inflationary fluctuation can lead to very stable domain wall, which can be a domain wall production mechanism in string axiverse.  I will also discuss the production of gravitaional waves by domain wall network decay, and cosmic birefringence.

The properties of axion miniclusters and 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 the dynamics in the early Universe, such as the axion string network and the non-linear dynamics around the QCD phase transition. In this talk we briefly discuss the phenomenology of miniclusters and their potential impact on direct detection experiments. 

Ultra-light axion-like particles may form axion-strings that persist to temperatures below that of big bang nucleosynthesis. Such strings have been considered previously as sources of gravitational waves and cosmic microwave background (CMB) polarization rotation. In this talk I show that axion strings deposit a sub-dominant fraction of their energy into high-energy Standard Model (SM) final states, for example, by the direct production of heavy radial modes that subsequently decay to SM particles. This high-energy SM radiation is absorbed by the primordial plasma, leading to novel signatures in precision big bang nucleosynthesis, the CMB power spectrum, and gamma-ray surveys. Future CMB surveys could find striking evidence of axion strings with lower decay constants.

The presence of interference, in particular extended fringes in cosmic filaments, is a striking consequence of wave/fuzzy dark matter models that is clearly visible in numerical simulations of the Schrödinger-Poisson equations. In this work we study the phenomenological consequences of such interference. Motivated by ellipsoidal collapse considerations and recent simulation results, we develop a toy model of virialised filaments at turn-around and show how one can construct a self-consistent wave dark matter field that displays the characteristics of interference observed in cosmological simulations. We derive suitable population statistics for the subpopulation of filaments satisfying our modul assumptions and analyse their properties in both two- and three-point statistics. Interestingly, our results indicate that phase correlations as measured by the isotropic line correlation function, are particularly sensitive to the presence of interference in a random configuration of such idealised filaments.

Cold dark matter axions produced in the post-inflationary Peccei-Quinn symmetry breaking scenario serve as clear targets for their experimental detection, since it is in principle possible to give a sharp prediction for their mass once we understand precisely how they are produced from the decay of global strings in the early Universe. In this work, we perform a dedicated analysis of the spectrum of axions radiated from strings based on large scale numerical simulations of cosmological evolution of the Peccei-Quinn field on the static lattice. It is found that there are several systematic effects that have been overlooked in the previous works, which could explain the discrepancy in the literature. We confirmed the trend that the spectral index of the axion emission spectrum increases with the string tension, but did not find a clear evidence of whether it continues to increase or saturates to a constant at larger values of the string tension due to the severe discretization effects. Taking this uncertainty and performing the extrapolation with a simple power law assumption on the spectrum, we quantify the predicted mass range of dark matter axions.

In an axiverse with numerous axions, the cosmological moduli problem poses a significant challenge because the abundance of axions can easily exceed that of dark matter. The well-established stochastic axion scenario offers a simple solution, relying on relatively low-scale inflation. However, axions are typically subject to mixing due to mass and kinetic terms, which can influence the solution using stochastic dynamics. Focusing on the fact that the QCD axion has a temperature-dependent mass, unlike other axions, we investigate the dynamics of the QCD axion and another axion with mixing. We find that the QCD axion abundance is significantly enhanced and becomes larger than that of the other axion for a certain range of parameters. This enhancement widens the parameter regions accounting for dark matter. In addition, we also find a parameter region in which both axions have enhanced abundances of the same order, which result in multi-component dark matter.

I will review our recent findings in axion production by considering a non-standard expansion before Big Bang nucleosynthesis onset. I will start with cold dark matter production through the misalignment mechanism, firstly assuming the energy density of the universe is dominated by a particle field described by a general equation of state. I will show the parameter space where the QCD axion is a dark matter candidate. Finally, I will refer to our latest work, revisiting axion thermal production during early matter domination or a late reheating era.

The creation of axions from a scaling network of axionic strings at the physical string tension has been analysed using the Moore-Klaer model. Improvements on previous work has been achieved through a careful extrapolation of the lattice-spacing → 0 and heavy-scale → ∞ limits, including QCD corrections to a purely radiation equation of state, and using the latest results for the temperature-dependent QCD topological susceptibility. The power spectrum of axion fluctuations from the scaling string epoch through the collapse of the string network through to final small-amplitude axion fluctuations has been determined.

In this talk, I will discuss some subtleties of the strong CP problem that might have been overlooked. QCD theta vacuum and instantons induce in the Lagrangian a CP-odd topological term which is naively thought to incur CP violation. However, whether or not such a term really incurs CP violation depends on how instantons affect the fermion correlation functions. After integrating out the gluons, there are two potential sources of CP violation, namely complex chiral phases in the quark mass matrix and the so-called 't Hooft vertex, which arises from instanton effects. Only one of them can be removed by field redefinitions, meaning that QCD should violate CP unless the chiral phases in these two terms happen to be aligned. In this talk, we identify the origin of the commonly believed misalignment: it comes from the coherence between different topological sectors in the path integral. However, if one takes the infinity spacetime volume limit before the summation over different topological sectors, which is enforced by the path integral formulation because the spacetime volume appears inside the integral over the field configurations, the coherence disappears. We thus obtain alignment between the aforementioned chiral phases and CP is conserved in strong interactions.

TBA

We study the dynamics of global axion string loop decays with a series of dedicated simulations, providing valuable insights on the early universe dynamics of post-inflationary axions.

In the scenario where the Peccei-Quinn symmetry is broken after inflation, a network of topological defects, i.e. strings and domain walls, forms, posing significant computational challenges due to the presence of large hierarchies, rendering numerical simulations highly intricate. Nevertheless, this so-called post-inflationary scenario is indeed very promising, as it allows us to give a precise prediction of the axion dark matter mass, serving as valuable input for experimental searches. 

We present a comprehensive study of the radiation emitted by global axion string loops, revealing critical insights important for the full network dynamics. 

In contrast to similar works, we investigate the continuum limit of these simulations in a threefold approach, with the aim of resolving some long-standing discrepancies in the literature. 

Additionally, we employ state-of-the art simulations with adaptive mesh refinement (AMR), allowing us to reach higher dynamical ranges by focussing the computational resources around the string cores.