Nonperturbative structure in coupled axion sectors and implications for direct detection

TL;DR

This paper explores the nonlinear dynamics of coupled axion fields, revealing how nonperturbative effects influence dark matter signatures, gravitational waves, and the formation of oscillons, with implications for experimental detection.

Contribution

It provides the first detailed simulation-based analysis of nonperturbative effects in coupled axion sectors and their impact on observables and dark matter phenomenology.

Findings

Nonperturbative dynamics can equilibrate axion abundances, aiding detection.

Gravitational wave signals are likely undetectable above 10^{-22} eV.

Oscillons formed are longer-lived due to nonlinear effects.

Abstract

Pairs of misalignment-produced axions with nearby masses can experience a nonlinear resonance that leads to enhanced direct and astrophysical signatures of axion dark matter. In much of the relevant parameter space, self-interactions cause axion fluctuations to become nonperturbative and to collapse in the early Universe. We investigate the observational consequences of such nonperturbative structure in this "friendly axion" scenario with $3+1$ dimensional simulations. Critically, in a substantial fraction of parameter space we find that nonlinear dynamics work to equilibrate the abundance of the two axions, making it easier than previously expected to experimentally confirm the existence of a resonant pair. We also compute the gravitational wave emission from friendly axion dark matter; while the resulting stochastic background is likely undetectable for axion masses above $10^{-22} \, \text{eV}$, the polarization of the cosmic microwave background does constrain possible hyperlight, friendly subcomponents. Finally, we demonstrate that dense, self-interaction--bound oscillons formed during the period of strong nonlinearity are driven by the homogeneous axion background, enhancing their lifetime beyond the in-vacuum expectation.