A Bump in the Blue Axion Isocurvature Spectrum

TL;DR

This paper numerically computes the blue axion isocurvature spectrum in a supersymmetric model, revealing a bump that enhances the signal, and provides a fitting function for data analysis and experimental forecasts.

Contribution

It introduces a numerical method to accurately compute the spectrum, uncovering a bump, and offers a practical fitting function for future data analysis.

Findings

Discovery of a bump in the spectrum that enhances the isocurvature signal

Development of a three-parameter fitting function for the spectrum

Numerical computation fills the gap left by previous analytic methods

Abstract

Blue axion isocurvature perturbations are both theoretically well-motivated and interesting from a detectability perspective. These power spectra generically have a break from the blue region to a flat region. Previous investigations of the power spectra were analytic, which left a gap in the predicted spectrum in the break region due to the non-applicability of the used analytic techniques. We therefore compute the isocurvature spectrum numerically for an explicit supersymmetric axion model. We find a bump that enhances the isocurvature signal for this class of scenarios. A fitting function of three parameters is constructed that fits the spectrum well for the particular axion model we study. This fitting function should be useful for blue isocurvature signal hunting in data and making experimental sensitivity forecasts.