Can LIGO Detect Nonannihilating Dark Matter?

Dark matter (DM) from the galactic halo can accumulate in neutron stars and transmute them into sub-2.5M_{⊙} black holes if the dark matter particles are heavy, stable, and have interactions with nucleons. We show that nondetection of gravitational waves from mergers of such low-mass black holes can constrain the interactions of nonannihilating dark matter particles with nucleons. We find benchmark constraints with LIGO O3 data, viz., σ_{χn}≥O(10^{-47}) cm^{2} for bosonic DM with m_{χ}∼PeV (or m_{χ}∼GeV, if they can Bose-condense) and ≥O(10^{-46}) cm^{2} for fermionic DM with m_{χ}∼10^{3} PeV. These bounds depend on the priors on DM parameters and on the currently uncertain binary neutron star merger rate density. However, with increased exposure by the end of this decade, LIGO will probe cross sections that are many orders of magnitude below the neutrino floor and completely test the dark matter solution to missing pulsars in the Galactic center, demonstrating a windfall science case for gravitational wave detectors as probes of particle dark matter.

Low Mass Black Holes from Dark Core Collapse.

Unusual masses of black holes being discovered by gravitational wave experiments pose fundamental questions about the origin of these black holes. Black holes with masses smaller than the Chandrasekhar limit ≈1.4 M_{⊙} are essentially impossible to produce through stellar evolution. We propose a new channel for production of low mass black holes: stellar objects catastrophically accrete nonannihilating dark matter, and the small dark core subsequently collapses, eating up the host star and transmuting it into a black hole. The wide range of allowed dark matter masses allows a smaller effective Chandrasekhar limit and thus smaller mass black holes. We point out several avenues to test our proposal, focusing on the redshift dependence of the merger rate. We show that redshift dependence of the merger rate can be used as a probe of the transmuted origin of low mass black holes.

Neutrino and Positron Constraints on Spinning Primordial Black Hole Dark Matter.

Primordial black holes can have substantial spin-a fundamental property that has a strong effect on its evaporation rate. We conduct a comprehensive study of the detectability of primordial black holes with non-negligible spin, via the searches for the neutrinos and positrons in the MeV energy range. Diffuse supernova neutrino background searches and observation of the 511 keV gamma-ray line from positrons in the Galactic center set competitive constraints. Spinning primordial black holes are probed up to a slightly higher mass range compared to nonspinning ones. Our constraint using neutrinos is slightly weaker than that due to the diffuse gamma-ray background, but complementary and robust. Our positron constraints are typically weaker in the lower mass range and stronger in the higher mass range for the spinning primordial black holes compared to the nonspinning ones. They are generally stronger than those derived from the diffuse gamma-ray measurements for primordial black holes having masses greater than a few ×10^{16} g.

Primordial Black Holes as a Dark Matter Candidate Are Severely Constrained by the Galactic Center 511 keV γ-Ray Line.

We derive the strongest constraint on the fraction of dark matter that can be composed of low mass primordial black holes by using the observation of the Galactic Center 511 keV γ-ray line. Primordial black holes of masses ≲10^{15} kg will evaporate to produce e^{±} pairs. The positrons will lose energy in the Galactic Center, become nonrelativistic, then annihilate with the ambient electrons. We derive robust and conservative bounds by assuming that the rate of positron injection via primordial black hole evaporation is less than what is required to explain the SPI/INTEGRAL observation of the Galactic Center 511 keV γ-ray line. Depending on the primordial black hole mass function and other astrophysical uncertainties, these constraints are the most stringent in the literature and show that primordial black holes contribute to less than 1% of the dark matter density. Our technique also probes part of the mass range which was completely unconstrained by previous studies.

Multi-PeV Signals from a New Astrophysical Neutrino Flux beyond the Glashow Resonance.

The IceCube neutrino discovery was punctuated by three showers with E_{ν}≈1-2 PeV. Interest is intense in possible fluxes at higher energies, though a deficit of E_{ν}≈6 PeV Glashow resonance events implies a spectrum that is soft and/or cutoff below ∼few PeV. However, IceCube recently reported a through-going track depositing 2.6±0.3 PeV. A muon depositing so much energy can imply E_{ν_{µ}}≳10 PeV. Alternatively, we find a tau can deposit this much energy, requiring E_{ν_{τ}}∼10× higher. We show that extending soft spectral fits from TeV-PeV data is unlikely to yield such an event, while an ∼E_{ν}^{-2} flux predicts excessive Glashow events. These instead hint at a new flux, with the hierarchy of ν_{µ} and ν_{τ} energies implying astrophysical neutrinos at E_{ν}∼100 PeV if a tau. We address implications for ultrahigh-energy cosmic-ray and neutrino origins.

Dark Matter Velocity Spectroscopy.

Dark matter decays or annihilations that produce linelike spectra may be smoking-gun signals. However, even such distinctive signatures can be mimicked by astrophysical or instrumental causes. We show that velocity spectroscopy-the measurement of energy shifts induced by relative motion of source and observer-can separate these three causes with minimal theoretical uncertainties. The principal obstacle has been energy resolution, but upcoming experiments will have the precision needed. As an example, we show that the imminent Astro-H mission can use Milky Way observations to separate possible causes of the 3.5-keV line. We discuss other applications.

Testing the Dark Matter Scenario for PeV Neutrinos Observed in IceCube.

Late time decay of very heavy dark matter is considered as one of the possible explanations for diffuse PeV neutrinos observed in IceCube. We consider implications of multimessenger constraints, and show that proposed models are marginally consistent with the diffuse γ-ray background data. Critical tests are possible by a detailed analysis and identification of the sub-TeV isotropic diffuse γ-ray data observed by Fermi and future observations of sub-PeV γ rays by observatories like HAWC or Tibet AS+MD. In addition, with several-year observations by next-generation telescopes such as IceCube-Gen2, muon neutrino searches for nearby dark matter halos such as the Virgo cluster should allow us to rule out or support the dark matter models, independently of γ-ray and anisotropy tests.