Axion mass prediction from adaptive mesh refinement cosmological lattice simulations

TL;DR

This paper uses advanced adaptive mesh refinement simulations to precisely calculate the axion radiation spectrum from cosmic strings, leading to a refined prediction of the axion mass relevant for dark matter.

Contribution

It introduces the most accurate large-scale simulations of axion-string networks using adaptive mesh refinement, improving the precision of axion mass predictions.

Findings

Spectral index of axion radiation is scale-invariant within 1%

Predicted axion mass range is approximately 45-65 μeV

Potential increase in axion mass prediction up to 300 μeV due to string-domain-wall collapse

Abstract

The quantum chromodynamics (QCD) axion arises as the pseudo-Goldstone mode of a spontaneously broken abelian Peccei-Quinn (PQ) symmetry. If the scale of PQ symmetry breaking occurs below the inflationary reheat temperature and the domain wall number is unity, then there is a unique axion mass that gives the observed dark matter (DM) abundance. Computing this mass has been the subject of intensive numerical simulations for decades since the mass prediction informs laboratory experiments. Axion strings develop below the PQ symmetry-breaking temperature, and as the string network evolves it emits axions that go on to become the DM. A key ingredient in the axion mass prediction is the spectral index of axion radiation emitted by the axion strings. We compute this index in this work using the most precise and accurate large-scale simulations to date of the axion-string network leveraging adaptive mesh refinement to achieve the precision that would otherwise require a static lattice with 262,144$^3$ lattice sites. We find a scale-invariant axion radiation spectrum to within 1% precision. Accounting for axion production from strings prior to the QCD phase transition leads us to predict that the axion mass should be approximately $m_a\in(45,65)$ $\mu \mathrm{eV}$. However, we provide preliminary evidence that axions are produced in greater quantities from the string-domain-wall network collapse during the QCD phase transition, potentially increasing the mass prediction to as much as 300 $\mu$eV.