arXiv subjects

José Correia

Publications and source records attributed to José Correia.

5 recordsLinked to original sources

arXiv2026

The spectrum of axions in a scaling string network

We report on measurements of axion spectra and their unequal time correlators in simulations on $12288^3$ fixed-grids, the largest such measurements to date. We use a novel combination of the fields that allow us to better distinguish the contributions from propagating axions from the contributions from strings, showing that previous measurements of the axion energy spectrum are approximately 30\% derived from the string fields. We conclude that axion emission from a scaling string network is close to scale-invariant ($q \gtrsim 1$ in the standard parametrisation), and that the energy spectrum of sub-horizon modes behaves as $p_\text{ax}\ln( k τ)$, where $k$ is the comoving wavenumber, $τ$ the conformal time and $p_\text{ax} \simeq 10$. The number density spectrum evolves towards a single curve for $kτ\lesssim 10^2$, with higher wavenumber deviations arising from initial conditions and resonant axion production at the string width scale. Our results are consistent with scaling framework, in which the total number density of axions produced from strings is $n_\text{ax}=1.66(17) f_\text{a}^2 H$, where $f_\text{a}$ is the axion decay constant and $H$ the Hubble rate. To interpret these results, we develop a simple analytic model based on an ensemble of string segments. The model reproduces the main features of the measured spectra and unequal-time correlators, and predicts $q \le 1$, leaving $q=1$ as the only value consistent with theory and measurement. We report on axion production from the final collapse of the network in a future work.

hep-ph↗

arXiv2026

The axion mass in the post-inflationary scenario in a minimal scaling model

We calculate the dark matter axion mass in the post-inflationary scenario in a minimal model with a set of classical field theory simulations on $12\,288^3$ grids. In the model, the Peccei-Quinn U(1) symmetry is broken by a complex scalar field at a temperature $f_{\rm a} \sim 10^{11}$ GeV, creating a string network. The strings become the boundaries of domain walls at the QCD transition, which draw the strings together and annihilate them. The decay of the strings and domain walls leaves behind axion radiation, with a comoving number density estimated to be $9.48(23)f_{\rm a}^2 H_*$, normalised to unit scale factor when the axion mass is equal to the Hubble rate $H_*$. Using the most commonly quoted lattice results for the topological susceptibility, the dark matter axion mass in this minimal model is $17.46(84)\,μ\text{eV}$. This corresponds to a symmetry-breaking scale of $3.27(15) \times 10^{11}\, \text{GeV}$, and a haloscope resonant frequency of $4.22(20)\,\text{GHz}$. We argue that field theory simulations of non-minimal models where the strings have a Higgs condensate or gauge flux, and are annihilated by domain walls, will give axion mass predictions below about $40\,μ\text{eV}$.

hep-ph↗

arXiv2026

Gravitational waves from strong first order phase transitions

We study gravitational wave production at strong first order phase transitions, with large-scale, long-running simulations of a system with a scalar order parameter and a relativistic fluid. One transition proceeds by detonations with asymptotic wall speed $v_\text{w}=0.92$ and transition strength $α_n=0.67$, and the other by deflagrations, with a nominal asymptotic wall speed $v_\text{w}=0.44$ and transition strength $α_n=0.5$. We investigate in detail the power spectra of velocity and shear stress and - for the first time in a phase transition simulation - their time decorrelation, which is essential for the understanding of gravitational wave production. In the detonation, the decorrelation speed is larger than the sound speed over a wide range of wavenumbers in the inertial range, supporting a visual impression of a flow dominated by supersonic shocks. Vortical modes do not contribute greatly to the produced gravitational wave power spectra even in the deflagration, where they dominate over a range of wavenumbers. In both cases, we observe dissipation of kinetic energy by acoustic turbulence, and in the case of the detonation an accompanying growth in the integral scale of the flow. The gravitational wave power approaches a constant with a power law in time, from which can be derived a gravitational wave production efficiency. For both cases this is approximately $\tildeΩ^\infty_\text{gw} \simeq 0.017$, even though they have quite different kinetic energy densities. The corresponding fractional density in gravitational radiation today, normalised by the square of the mean bubble spacing in Hubble units, for flows which decay in much less than a Hubble time, is $Ω_{\text{gw},0}/(H_\text{n} R_*)^2=(4.8\pm1.1)\times 10^{-8}$ for the detonation, and $Ω_{\text{gw},0}/(H_\text{n} R_*)^2=(1.3\pm0.2)\times 10^{-8}$ for the deflagration.

astro-ph.CO↗

arXiv2026

The COSMIC WISPers White Paper: The physics case for Weakly Interacting Slim Particles

Axions and other very weakly interacting slim particles (WISPs), with masses below 1 GeV, arise naturally in many extensions of the Standard Model of particle physics. In particular, they could offer a new framework to explain the nature of dark matter and may help address a range of puzzling observations in astrophysics and particle physics. This review provides an overview of ongoing WISP searches and outlines the prospects for the next decade, spanning their theoretical motivation, indirect signatures in astrophysical observations, and dedicated laboratory experiments. It is based on the work carried on by the EU-funded COST Action ``Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments'' (CA21106, https://www.cost.eu/actions/CA21106). This network plays a key role in coordinating and supporting WISP searches across Europe, while also contributing to the development of a roadmap aimed at securing European leadership in this research area. It is emphasized that Europe is currently pursuing a rich, diverse, and cost-effective experimental program, with the potential to deliver one or more transformative discoveries.

hep-ph↗

arXiv2025

Scaling density of axion strings in terasite simulations

We report on a study of axion string networks using fixed-grid simulations of up to $16384$ points per side. The length of string can be characterised in terms of standard dimensionless parameters $ζ_\text{w}$ and $ζ_\text{r}$, the length density measured in the cosmic rest frame and the string rest frame, scaled with the cosmic time. The motion of the string can be characterised by the root-mean-square (RMS) velocity of the string. Starting from a range of initial length densities and velocities, we analyse the string network in the standard scaling framework and find evolution towards a fixed point with estimated values $\hatζ_{\text{w},*} = 1.220(57)$ and $\hatζ_{\text{r},*} = 1.491(93)$. The two measures are related by the RMS velocity, which we estimate to be $\hat{v}_{*} = 0.5705(93)$. The length density is consistent with previous measurements, while the velocity is about 5% lower. For simulations starting from low enough density, the length density parameters $ζ_\text{w}$ and $ζ_\text{r}$ remain below their fixed point values throughout, while growing slowly, giving rise to an impression of approximately logarithmic increase with time. This has been proposed as the true long-term behaviour. We find that the growth tends to slow down as the values of $ζ_\text{w}$ and $ζ_\text{r}$ identified as fixed points are approached. In the case of $ζ_\text{r}$, the growth stops for simulations which started close to the fixed point length density. The difference between $ζ_\text{w}$ and $ζ_\text{r}$ can be understood to result from the continuing velocity evolution. Our results indicate that the growth of $ζ_\text{w}$ is a transient appearing at low densities and while the velocity is converging. This highlights the importance of studying the string density and the velocity together, and the preparation of initial conditions.

hep-ph↗