Abstract
In the QCD axion dark matter scenario with postinflationary Peccei-Quinn symmetry breaking, the number density of axions, and hence the dark matter density, depends on the length of string per unit volume at cosmic time , by convention written . The expectation has been that the dimensionless parameter tends to a constant , a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations shows a logarithmic increase with time, while theoretical modeling suggests an inverse logarithmic correction. Either case would result in a large enhancement of the string density at the QCD transition, and a substantial revision to the axion mass required for the axion to constitute all of the dark matter. With a set of new simulations of global strings, we compare the standard scaling (constant-) model to the logarithmic growth and inverse-logarithmic correction models. In the standard scaling model, by fitting to linear growth in the mean string separation , we find . We conclude that the apparent corrections to are artifacts of the initial conditions, rather than a property of the scaling network. The residuals from the constant- (linear ) fit also show no evidence for logarithmic growth, restoring confidence that numerical simulations can be simply extrapolated from the Peccei-Quinn symmetry-breaking scale to the QCD scale. Reanalysis of previous work on the axion number density suggests that recent estimates of the axion dark matter mass in the postinflationary symmetry-breaking scenario we study should be increased by about 50%.
- Received 7 October 2019
DOI:https://doi.org/10.1103/PhysRevLett.124.021301
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