The QCD Axion: Some Like It Hot

TL;DR

This paper refines the bounds on the QCD axion mass by comparing theoretical phase-space distributions with observational data, leading to a tighter constraint and future forecast prospects.

Contribution

It provides a novel comparison of the QCD axion phase-space distribution from chiral perturbation theory with experimental data, deriving a stronger mass bound and forecasting future constraints.

Findings

Upper bound on axion mass: 0.16 eV at 95% credible interval

Tighter constraint than previous bounds by ~30%

Forecasts for future cosmological survey constraints

Abstract

We compare the QCD axion phase-space distribution from unitarized next-to-leading order chiral perturbation theory with the one extracted from pion-scattering data. We derive a robust bound by confronting momentum-dependent Boltzmann equations against up-to-date observations of the Cosmic Microwave Background, of the Baryonic Acoustic Oscillations and of primordial abundances. These datasets imply $m_{a} \leq \, 0.16 $ eV for the 95% credible interval, i.e. $\sim$30% stronger bound than what previously found. We present forecasts using dedicated likelihoods for future cosmological surveys and the sphaleron rate from unquenched lattice QCD.