Flavor Violating Axions in the Early Universe

TL;DR

This paper investigates how flavor-violating axions could have influenced the early universe's radiation content, with current and future cosmological data providing constraints comparable to laboratory experiments.

Contribution

It introduces a model-independent effective field theory framework to quantify axion production and derives cosmological bounds on flavor-violating axions.

Findings

Current data exclude part of the parameter space.

Future CMB-S4 surveys will tighten constraints.

Cosmological bounds will rival laboratory limits.

Abstract

Flavor violating axion couplings can be in action before recombination, and they can fill the early universe with an additional radiation component. Working within a model-independent framework, we consider an effective field theory for the axion field and quantify axion production. Current cosmological data exclude already a fraction of the available parameter space, and the bounds will improve significantly with future CMB-S4 surveys. Remarkably, we find that future cosmological bounds will be comparable or even stronger than the ones obtained in our terrestrial laboratories.