More axions from strings

We study the contribution to the QCD axion dark matter abundance that is produced by string defects during the so-called scaling regime. Clear evidence of scaling violations is found, the most conservative extrapolation of which strongly suggests a large number of axions from strings. In this regime, nonlinearities at around the QCD scale are shown to play an important role in determining the final abundance. The overall result is a lower bound on the QCD axion mass in the post-inflationary scenario that is substantially stronger than the naive one from misalignment.

Automated summary

Clear evidence of scaling violations in the contribution to QCD axion dark matter abundance from string defects is found, suggesting a large number of axions from strings. Nonlinearities around the QCD scale are shown to play a crucial role in determining the final abundance, resulting in a substantially stronger lower bound on the QCD axion mass in the post-inflationary scenario compared to the naive estimate from misalignment.