Axion Isocurvature Collider
  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.
