Neutrino Signals from Superradiant Clouds around Primordial Black Holes ✦
Primordial black holes (PBHs) are a dark matter candidate, especially in the asteroid-mass window (10¹⁷–10²³ g). We explore a scenario with spinning PBHs, where superradiance builds boson clouds that emit a nearly monochromatic stream of neutrinos with energies of a few MeV. Both galactic and extragalactic contributions are calculated and compared with antineutrino data from Borexino, KamLAND, and Super-Kamiokande. It is found that for scalar bosons, constraints on the PBH fraction of dark matter cover a large part of the asteroid range and extend to higher masses. For instance, with spin a≈0.9, α_g=0.25, and Yukawa coupling g_{νφ}=10⁻⁴, the tightest limit f_PBH≈10⁻⁷ is reached at M_PBH≈2×10²² g, which is significantly stricter than lensing bounds. This approach complements neutrino searches for dark matter, differing from limits based on Hawking radiation.
Context
The nature of dark matter, first identified in the work of Vera Rubin, remains a central mystery. Among the candidates, primordial black holes (PBHs), formed from primordial inhomogeneities under gravity, attract with their simplicity. The asteroid mass window (10^17–10^23 g) is especially intriguing since PBHs within it could account for all dark matter, evading constraints from Hawking evaporation and microlensing. However, the lack of reliable probes in this range calls for new approaches. This work proposes using neutrino signals from superradiant clouds around PBHs, offering an additional search channel.
Methods
The authors consider a scenario where a cloud of a light scalar boson forms around a rotating black hole through superradiance—a wave analogue of the Penrose process. If this boson has a Yukawa coupling to neutrinos, the cloud enters a saturated phase, emitting a steady, nearly monochromatic neutrino flux. From the theory, fluxes from a single PBH are computed and then integrated over populations in the Galaxy (NFW profile) and beyond, accounting for cosmological redshift. The extragalactic flux is calculated using standard ΛCDM cosmology, with the expansion of the universe described by Hubble's law and accelerating due to dark energy. The resulting predictions are compared with experimental data from Borexino, KamLAND, and Super-Kamiokande for antineutrinos in the 2–31 MeV range.
Results
For a scalar boson with a gravitational fine-structure constant α_g = 0.25–0.3 and a Yukawa coupling g_{νϕ} ~ 10^{-3}–10^{-4}, strong constraints on the PBH fraction of dark matter, f_PBH, are obtained. Specifically, for α_g=0.3 and g_{νϕ}=10^{-4}, values f_PBH > 10^{-6} are excluded for PBH masses around 4×10^23 g, while for α_g=0.25 and g_{νϕ}=10^{-4}, f_PBH ~ 10^{-7} is reached for M_PBH ~ 2×10^22 g. These limits surpass constraints from microlensing (Subaru-HSC) by orders of magnitude in the upper part of the asteroid window. The vector case yields neutrino energies too low to compete with existing data. Interestingly, the galactic contribution is about 36% larger than the extragalactic one, thanks to the concentration of dark matter in the center of the Galaxy.
Implications
The results demonstrate that low-energy neutrino observatories can serve as powerful tools for probing dark matter in the form of PBHs, complementing traditional methods. This is especially important for the asteroid window, where direct astronomical observations are challenging. The connection to new light bosons also opens avenues for exploring physics beyond the Standard Model.
Future development
In the future, the analysis could be extended to other boson types (e.g., axions) and emission channels, including gamma rays. Using directional information from next-generation neutrino telescopes like JUNO or Hyper-Kamiokande will help separate the signal from backgrounds. Moreover, combining with data on the cosmic microwave background and large-scale structure could yield a more complete picture of the PBH population.
Impact
The method touches several areas at once: dark matter physics, neutrino astrophysics, early universe cosmology, and gravitational theory.
Next steps
Immediate next steps include more detailed modeling of cloud formation taking into account PBH accretion and mergers, as well as sensitivity analyses for future detectors like JUNO and Hyper-Kamiokande.
Key open problems
The work connects unresolved problems: the nature of dark matter, the formation mechanism of primordial black holes in inflationary models, the existence of light scalars, and the mass of neutrinos.
🎯 If an asteroid-mass PBH of 10^20 g were in the Solar System, its size would be comparable to a proton (radius ~0.1 fm), and its rotation could generate a boson cloud emitting millions of neutrinos per second.
Key numbers
- Asteroid mass window for PBHs: 10^{17} – 10^{23} g
- Maximum constraint on f_PBH: ~10^{-7}
- Typical neutrino energy: up to ~10 MeV
- Hubble constant: 67.66 km/s/Mpc
- Age of the Universe: 1.4×10^{10} years