Black holes as fermion factories

Abstract Ultralight bosons near rotating black holes can undergo significant growth through superradiant energy extraction, potentially reaching field values close to the Planck scale and transforming black holes into effective transducers for these fields. The interaction between boson fields and fermions may lead to parametric production or Schwinger pair production of fermions, with efficiencies significantly exceeding those of perturbative decay processes. Additionally, the spatial gradients of scalar clouds and the electric components of vector clouds can accelerate fermions, resulting in observable fluxes. This study considers both Standard Model neutrinos and dark sector fermions, which could contribute to boosted dark matter. Energy loss due to fermion emissions can potentially quench the exponential growth of the cloud, leading to a saturated state. This dynamic provides a framework for establishing limits on boson-neutrino interactions, previously constrained by neutrino self-interaction considerations. In the saturation phase, boson clouds have the capacity to accelerate fermions to TeV energies, producing fluxes that surpass those from atmospheric neutrinos near black holes. These fluxes open new avenues for observations through high-energy neutrino detectors like IceCube, as well as through dark matter direct detection efforts focused on targeted black holes.