Dark matter from axion strings with adaptive mesh refinement

Axions are hypothetical particles that may explain the observed dark matter density and the non-observation of a neutron electric dipole moment. An increasing number of axion laboratory searches are underway worldwide, but these efforts are made difficult by the fact that the axion mass is largely unconstrained. If the axion is generated after inflation there is a unique mass that gives rise to the observed dark matter abundance; due to nonlinearities and topological defects known as strings, computing this mass accurately has been a challenge for four decades. Recent works, making use of large static lattice simulations, have led to largely disparate predictions for the axion mass, spanning the range from 25 microelectronvolts to over 500 microelectronvolts. In this work we show that adaptive mesh refinement simulations are better suited for axion cosmology than the previously-used static lattice simulations because only the string cores require high spatial resolution. Using dedicated adaptive mesh refinement simulations we obtain an over three order of magnitude leap in dynamic range and provide evidence that axion strings radiate their energy with a scale-invariant spectrum, to within similar to 5% precision, leading to a mass prediction in the range (40,180) microelectronvolts.

Automated summary

Axions are hypothetical particles that may explain dark matter density and the absence of a neutron electric dipole moment. Current laboratory searches for axions face challenges due to the lack of precise knowledge about their mass. Previous static lattice simulations have provided widely varying predictions for the axion mass, but recent works using adaptive mesh refinement simulations have shown improved accuracy and suggest a mass prediction range of (40,180) microelectronvolts.