Axion Dark Matter from Global Cosmic Strings: Simulations and Effective Description

Abstract
In the post-inflationary scenario for the QCD axion, the dark matter abundance is set by the decay of a network of global cosmic strings. Understanding the dynamics of the string network is crucial for a precise prediction of the axion mass, which can inform experimental searches. This prediction is strongly tied to the exact shape of the spectrum of radiated axions, which needs to be studied with dedicated numerical simulations. Due to the large separation of scales in the system these simulations have a limited dynamical range and results need to be extrapolated over a large range in string tension. Previous works disagree on this extrapolation, leaving a mass window in the μeV to meV range open. In this talk, I will explain which parameters quantify the dynamics of axion strings, which techniques can be used to improve the dynamical range, and which systematics need to be taken into account. I will then discuss simulations of a much more controlled setup: the collapse of an initially static circular loop. We compare different field theory simulations with the Nambu-Goto-Kalb-Ramond (NGKR) effective theory over a large range of tensions. Both approach the Nambu-Goto limit as ~1/tension, but the coefficient of the leading correction to the collapse dynamics is 20-35% larger in NGKR compared to the field theory results. Studying the loop collapse and the subsequent rebound, we identify additional events that modify the ultraviolet part of the axion spectrum, while the infrared agrees with NGKR.

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