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Scaling density of axion strings

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posted on 2023-06-07, 07:41 authored by Mark HindmarshMark Hindmarsh, Joanes Lizarraga, Asier Lopez-Eiguren, Jon Urrestilla
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 ?/t2. The expectation has been that the dimensionless parameter ? tends to a constant ?0, 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 ?=t/?, we find ?0=1.19±0.20. 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%.

History

Publication status

  • Published

File Version

  • Accepted version

Journal

Physical Review Letters

ISSN

0031-9007

Publisher

American Physical Society

Issue

2

Volume

124

Article number

a021301

Event location

United States

Department affiliated with

  • Physics and Astronomy Publications

Full text available

  • Yes

Peer reviewed?

  • Yes

Legacy Posted Date

2020-08-05

First Open Access (FOA) Date

2020-08-05

First Compliant Deposit (FCD) Date

2020-08-05

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