Multimodal axion emissions from Abelian-Higgs cosmic strings

Abstract

We show that axions can be produced from Abelian-Higgs cosmic strings due to the axion-gauge coupling. The strong magnetic field is confined in the string, and the electric field is induced around the moving string, allowing axion productions from the dynamics of cosmic strings. Our numerical analysis on the string collision shows that a sizable number of axions can be produced at the reconnection, and further emissions occur from moving kinks afterward. Furthermore, the simulation on the string network shows multimodal axion emissions in the sense that axions are produced in both the low-energy and high-energy regimes. The former can contribute to the cold dark matter and the latter can be regarded as dark radiation. We found that the axion with sub-GeV mass can explain the current relic dark matter abundance and simultaneously predicts a sizable amount of dark radiation which can be probed by future observations.

Figures (5)

• Figure 1: Snapshots of the collision and reconnection of two straight strings (red) and the emitted axions (cyan). Time evolves from left to right.
• Figure 2: Time evolution of the number density spectrum of the axion emitted from the two string collision. Time evolves from bottom to top with the interval $\Delta t = 0.63v^{-1}$.
• Figure 3: Spectrum of the comoving number density of emitted axions in terms of the comoving wave number, $k$. Red, blue, magenta, black curves correspond respectively to the spectrum at $v\tau = 15,25,35,64$. The solid line and shaded region show the mean value and 1-$\sigma$ error from 8 independent simulations with different initial conditions. The dashed and dash-dotted lines show the power-law fitting proportional $k^{3}$ and $k^{-0.5}$ respectively. The vertical dotted line corresponds to $k/a = 10H$ at $v\tau = 64$.
• Figure 4: Contour of $\Omega^{\rm (DM)}_a = \Omega_{\rm DM}$ are shown in red for $g_{a\gamma'} v=1$ (solid), $g_{a\gamma'} v=10^{-1}$ (dashed) and $g_{a\gamma'} v=10^{-2}$ (dot-dashed), respectively. The green horizontal lines show the corresponding DR constraints $\Delta N_{\rm eff}\leq 1$ at $z=1100$ with $A = 0.06$, $\alpha = -0.5$ and $k_*= 10v$. The gray-shaded region denotes the constraint from the GW observation.
• Figure 5: Dependence of $\Delta N_{\rm eff}$ on the symmetry-breaking scale $v$ at the CMB epoch ($z=1100$). The cases $g_{a\gamma'} v=1, 10^{-2}, 10^{-4}$ are shown in red, green, and blue, respectively. The black dashed line indicates $\Delta N_{\rm eff}=1$ for reference. The yellow shaded region, bounded by the dot-dashed line, is excluded by GW observations. $A = 0.06$, $\alpha = -0.5$, and $k_{*} = 10v$ are adopted with shaded bands illustrating $10\,\%$ uncertainty in $\alpha$.