Black hole superradiance of self-interacting scalar fields
  Black hole superradiance is a powerful probe of light, weakly-coupled hidden
sector particles. Many candidate particles, such as axions, generically have
self-interactions that can influence the evolution of the superradiant
instability. As pointed out in arXiv:1604.06422 in the context of a toy model,
much of the existing literature on spin-0 superradiance does not take into
account the most important self-interaction-induced processes. These processes
lead to energy exchange between quasi-bound levels and particle emission to
infinity; for large self-couplings, superradiant growth is saturated at a
quasi-equilibrium configuration of reduced level occupation numbers. In this
paper, we perform a detailed analysis of the rich dynamics of spin-0
superradiance with self-interactions, and the resulting observational
signatures. We focus on quartic self-interactions, which dominate the evolution
for most models of interest. We explore multiple distinct regimes of parameter
space introduced by a non-zero self-interaction, including the simultaneous
population of two or more bound levels; at large coupling, we confirm the basic
picture of quasi-equilibrium saturation and provide evidence that the
"bosenova" collapse does not occur in most of the astrophysical parameter
space. Compared to gravitational superradiance, we find that gravitational wave
"annihilation" signals and black hole spin-down are parametrically suppressed
with increasing interactions, while new gravitational wave "transition" signals
can take place for moderate interactions. The novel phenomenon of scalar wave
emission is less suppressed at large couplings, and if the particle has
Standard Model interactions, then coherent, monochromatic axion wave signals
from black hole superradiance may be detectable in proposed axion dark matter
experiments.
