Black Hole Spin Constraints on the Mass Spectrum and Number of Axion-like Fields

TL;DR

This paper uses astrophysical black hole spin measurements to constrain the number and mass distribution of axion-like particles, providing new limits on theories with multiple bosonic fields such as string theory and M-theory.

Contribution

It introduces a novel methodology to constrain the number and mass distribution of multiple axion-like fields using black hole superradiance data.

Findings

Excluded more than 30 axion-like fields in certain models.

Placed upper bounds on the number of axions for specific mass distributions.

Demonstrated constraints for scenarios including QCD axion, GUTs, and fuzzy dark matter.

Abstract

Astrophysical observations of spinning BHs, which span $ 5M_\odot\lesssim M_{\rm BH}\lesssim 5\times 10^8 M_\odot$, can be used to exclude the existence of certain massive bosons via the superradiance phenomenon. In this work, we explore for the first time how these measurements can be used to constrain properties of statistical distributions for the masses of multiple bosonic fields. Quite generally, our methodology excludes $N_{\rm ax}\gtrsim 30$ scalar fields with a range of mass distribution widths and central values spanning many orders of magnitude. We demonstrate this for the specific example of axions in string theory and M-theory, where the mass distributions in certain cases take universal forms. We place upper bounds on $N_{\rm ax}$ for certain scenarios of interest realised approximately as mass distributions in M-theory, including the QCD axion, grand unified theories, and fuzzy dark matter.