Observational prospects of self-interacting scalar superradiance with next-generation gravitational-wave detectors

TL;DR

This paper explores how next-generation gravitational-wave detectors can detect or constrain ultralight scalar particles with self-interactions through superradiance around black holes, extending current search capabilities.

Contribution

It provides the most complete modeling of self-interacting scalar superradiance signatures and assesses the potential of future detectors to probe new parameter space.

Findings

Current detectors are insufficient to detect self-interactions that halt superradiance.

Next-generation detectors can explore scalar masses around 10^{-13} to 10^{-12} eV/c^2.

Projected sensitivity extends to very high interaction scales, surpassing current constraints.

Abstract

Current- and next-generation gravitational-wave observatories may reveal new, ultralight bosons. Through the superradiance process, these theoretical particle candidates can form clouds around astrophysical black holes and result in detectable gravitational-wave radiation. In the absence of detections, constraints$-$contingent on astrophysical assumptions$-$have been derived using LIGO-Virgo-KAGRA data on boson masses. However, the searches for ultralight scalars to date have not adequately considered self-interactions between particles. Self-interactions that significantly alter superradiance dynamics are generically present for many scalar models, including axion-like dark matter candidates and string axions. We implement the most complete treatment of particle self-interactions available to determine the gravitational-wave signatures expected from superradiant scalar clouds and revisit the constraints obtained in a past gravitational-wave search targeting the black hole in Cygnus X-1. We also project the reach of next-generation gravitational-wave observatories to scalar particle parameter space in the mass-coupling plane. We find that while proposed observatories have insufficient reach to self-interactions that can halt black hole spin-down, next-generation observatories are essential for expanding the search beyond gravitational parameter space and can reach a mass and interaction scale of $\sim 10^{-13}-10^{-12}$ eV/c$^2$ and $\gtrsim 10^{17}$ GeV, respectively.