Axion Isocurvature Collider

TL;DR

This paper explores how extremely light axions can serve as isocurvature colliders during inflation, producing observable clock signals in the bispectrum that could be detected in future cosmological experiments.

Contribution

It demonstrates that axion-induced isocurvature perturbations can generate measurable clock signals in the bispectrum, providing a new way to probe high-energy physics through cosmological observations.

Findings

Significant clock signals can arise in the squeezed limit of the bispectrum.

Characteristic angular dependence of the clock signals is identified.

Potential observability of these signals in future experiments is discussed.

Abstract

Cosmological colliders can preserve information from interactions at very high energy scale, and imprint them on cosmological observables. Taking the squeezed limit of cosmological perturbation bispectrum, information of the intermediate particle can be directly extracted from observations such as cosmological microwave background (CMB). Thus cosmological colliders can be powerful and promising tools to test theoretical models. In this paper, we study extremely light axions (including QCD axions and axion-like-particles), and consider them constituting cold dark matter (CDM) at late times. We are interested in inflationary isocurvature modes by such axions, and try to figure out how axion perturbations can behave as isocurvature colliders. We work out an example where the intermediate particle is a boson, and show that, in the squeezed limit, it is possible to provide a clock signal of significant amplitudes, with a characteristic angular dependence. This provides a channel to contribute and analyze clock signals of isocurvature bispectrum, which we may hopefully see in future experiments.