Scaling density of axion strings

Hindmarsh, Mark, Lizarraga, Joanes, Lopez-Eiguren, Asier and Urrestilla, Jon (2020) Scaling density of axion strings. Physical Review Letters, 124 (2). a021301. ISSN 0031-9007

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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 t, by convention written ζ/t2. The expectation has been that the dimensionless parameter ζ tends to a constant ζ0, a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations ζ 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-ζ) 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 ζ=t/ζ, we find ζ0=1.19±0.20. We conclude that the apparent corrections to ζ are artifacts of the initial conditions, rather than a property of the scaling network. The residuals from the constant-ζ (linear ζ) 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%.

Item Type: Article
Schools and Departments: School of Mathematical and Physical Sciences > Physics and Astronomy
SWORD Depositor: Mx Elements Account
Depositing User: Mx Elements Account
Date Deposited: 05 Aug 2020 08:38
Last Modified: 06 Aug 2020 08:12

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