Resonant nonlinear pairs in the axiverse and their late-time direct and astrophysical signatures

A generic low-energy prediction of string theory is the existence of a large collection of axions, commonly known as a string axiverse. Axions also have a natural cosmological production mechanism, vacuum misalignment, making them well-motivated dark matter (DM) candidates. Much work on axion production has considered the case of a single free axion, but in a realistic axiverse, string axions are expected to be distributed densely over many orders of magnitude in mass and to interact with one another through their joint potential. In this paper, we show that nonlinearities in this potential lead to a new type of resonant energy transfer between axions with nearby masses. This resonance generically transfers energy from axions with larger decay constants to those with smaller decay constants and leads to a multitude of signatures. These include enhanced direct detection prospects for a resonant pair comprising even a small subcomponent of dark matter, and boosted small-scale structure if the pair is the majority of DM. Near future iterations of experiments such as ADMX and DM Radio will be sensitive to this scenario, as will astrophysical probes of DM substructure.

Automated summary

This paper investigates the resonant energy transfer phenomenon between axions in a string axiverse, showing that nonlinearities in the axion potential result in energy transfer from axions with larger decay constants to those with smaller decay constants. This resonance has important implications for the direct detection prospects and small-scale structure of axions.