Radiation from Global Topological Strings using Adaptive Mesh Refinement: Massive Modes

TL;DR

This study uses adaptive mesh refinement simulations to analyze massive radiation from global topological strings, revealing strong suppression of massive modes compared to massless channels and their exponential dependence on particle mass.

Contribution

It provides a detailed quantitative analysis of massive radiation emission from global strings using AMR, highlighting suppression effects and the potential for exploring higher string tension regimes efficiently.

Findings

Massive radiation is strongly suppressed relative to massless radiation.

Massive radiation is emitted in high harmonics of the fundamental frequency.

Exponential suppression of massive radiation with increasing particle mass.

Abstract

We implement adaptive mesh refinement (AMR) simulations of global topological strings using the public code, GRChombo. We perform a quantitative investigation of massive radiation from single sinusoidally displaced string configurations, studying a range of string widths defined by the coupling parameter $\lambda$ over two orders of magnitude, effectively varying the mass of radiated particles $m_H \sim \sqrt{\lambda}$. We perform an in-depth investigation into the effects of AMR on massive radiation emission, including radiation trapping and the refinement required to resolve high frequency modes. We use quantitative diagnostic tools to determine the eigenmode decomposition, showing a complex superposition of high frequency propagating modes with different phase and group velocities. We conclude that massive radiation is generally strongly suppressed relative to the preferred massless channel, with suppression increasing at lower amplitudes and higher $\lambda$. Only in extreme nonlinear regimes (e.g.\ with relative amplitude $\varepsilon \sim 1.5$ and $\lambda < 1$) do we observe massive and massless radiation to be emitted at comparable magnitude. We find that massive radiation is emitted in distinct high harmonics of the fundamental frequency of the string, and we demonstrate that, for the sinusoidal configurations studied, massive radiation is exponentially suppressed with $\sqrt{\lambda}$ (i.e. the particle mass). Finally, we place these results in the context of axions and gravitational waves produced by cosmological cosmic string networks, and note that AMR provides a significant opportunity to explore higher $\lambda$ (thin string) regimes whilst using fewer computational resources.