More Axions from Strings

TL;DR

This paper investigates the contribution of string defects to QCD axion dark matter abundance, revealing scaling violations and nonlinear effects that imply a higher axion production and a stronger lower mass bound than previously estimated.

Contribution

It provides the first detailed analysis of string defect contributions during the scaling regime, highlighting the impact of nonlinearities on axion abundance calculations.

Findings

Evidence of scaling violations in string defect evolution

Nonlinear effects significantly influence axion abundance

Stronger lower bounds on QCD axion mass in the post-inflationary scenario

Abstract

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.