The dark-matter axion mass

TL;DR

This paper presents advanced numerical simulations of axion production from string networks, refining the estimated axion mass necessary for dark matter, with implications for axion cosmology.

Contribution

First comprehensive simulation accounting for large short-distance string tension effects in axion production, leading to a refined axion mass estimate for dark matter.

Findings

Axion mass estimate: 26.2 ± 3.4 micro-eV

Axion production less efficient than angle-averaged misalignment

Refined understanding of axion string network dynamics

Abstract

We evaluate the efficiency of axion production from spatially random initial conditions in the axion field, so a network of axionic strings is present. For the first time, we perform numerical simulations which fully account for the large short-distance contributions to the axionic string tension, and the resulting dense network of high-tension axionic strings. We find nevertheless that the total axion production is somewhat less efficient than in the angle-averaged misalignment case. Combining our results with a recent determination of the hot QCD topological susceptibility (Borsanyi et al 2016), we find that if the axion makes up all of the dark matter, then the axion mass is m_a = 26.2 +-3.4 micro-electron volts.