Primordial black holes as cosmic accelerators of light dark matter: Novel direct detection constraints

Current multiton dark matter (DM) detectors are largely incapable of detecting light dark matter from the Galactic halo due to the energy threshold limitations of their recoil measurements. However, primordial black holes (PBHs) can evaporate via Hawking radiation to particles whose energies are set by the black hole temperature. Consequently, weakly interacting light dark matter (or dark radiation) particles produced in this manner can reach the Earth with sufficient flux and kinetic energy above the experimental thresholds. This opens up a novel avenue to probe the light dark sector in terrestrial experiments. In this work, we explore this possibility by considering fermionic DM produced through PBH evaporation and investigate its electron recoil signatures in direct detection experiments. We analyze both energy independent (constant) and energy dependent (scalar and vector mediated) DM-electron interactions, highlighting the strong dependence of the recoil spectra on the underlying Lorentz structure of the interaction. In addition, we also account for the attenuation effects due to loss of kinetic energy while DM traverses through Earth’s crust, which can significantly modify the incoming DM flux. Incorporating these effects carefully, we place constraints on light DM using the electron recoil data from XENONnT, LZ, and PandaX-4T. Finally, we also discuss the detection prospects of such dark matter in current and future generation neutrino detectors, such as Super Kamiokande and Hyper Kamiokande.

💡 Research Summary

The authors investigate a novel pathway to probe sub‑GeV dark matter (DM) by exploiting the high‑energy particles emitted from evaporating primordial black holes (PBHs) via Hawking radiation. Light fermionic DM produced in this way can acquire kinetic energies of tens to hundreds of MeV, far exceeding the typical ∼keV recoil thresholds of current multi‑ton liquid‑xenon direct‑detection experiments (XENONnT, LZ, PandaX‑4T). The paper proceeds through several key steps.

First, the Hawking temperature (T_{\rm PBH}= \hbar c^{3}/(8\pi G M_{\rm PBH}k_{B})) is used to compute the thermal spectrum of emitted particles. Using the public BlackHawk v2.3 code, the authors obtain the differential emission rate for a Dirac fermion DM candidate, including the grey‑body factor (\Gamma). They then integrate contributions from PBHs residing in the Milky Way halo (modeled with an NFW density profile) and from the extragalactic PBH population (including redshift effects) to obtain the total DM flux at Earth as a function of kinetic energy. Figure 1 shows that for benchmark PBH masses (M_{\rm PBH}=10^{15}) g and (10^{16}) g, and a DM mass of 1 MeV, the flux peaks at kinetic energies of order 10–100 MeV, with Galactic and extragalactic components comparable.

Second, the authors calculate the expected electron‑recoil spectra in xenon detectors. They consider three interaction structures: (i) an energy‑independent contact cross‑section (\bar\sigma_{e}) (the heavy‑mediator limit), (ii) a scalar‑mediated interaction with a propagator ((q^{2}+m_{\phi}^{2})^{-2}), and (iii) a vector‑mediated interaction with a similar propagator ((q^{2}+m_{V}^{2})^{-2}). Here (q) is the momentum transfer. Because the DM is semi‑relativistic, the momentum‑dependent terms can dramatically modify the scattering rate compared with the non‑relativistic approximation. The authors fold the differential cross‑section with the xenon atomic response function (including ionization form factors) to obtain the recoil rate as a function of observed electron energy.

Third, they address the often‑neglected attenuation of DM as it traverses the Earth’s atmosphere and crust before reaching underground detectors. Using the mean energy‑loss per unit length (\langle dE/dx\rangle) derived from DM‑electron scattering, they numerically integrate the energy loss along realistic trajectories (accounting for the incident angle and the layered composition of the overburden). This treatment goes beyond the simple scaling approximations used in earlier works and is essential when the DM kinetic energy is comparable to the electron mass, where the stopping power changes rapidly. The resulting attenuated flux (\Phi_{\rm att}(E)) is then used in the recoil‑rate calculation.

Fourth, the authors perform a statistical analysis of the latest electron‑recoil data from XENONnT (1.16 t·yr exposure), LZ (5.5 t·day), and PandaX‑4T (363.3 t·day). Using a Poisson likelihood and profiling over background uncertainties, they derive 90 % confidence level upper limits on (\bar\sigma_{e}) and on the scalar/vector couplings for a range of DM masses (0.1 MeV–10 MeV) and PBH masses (10^{15}–10^{17} g). The limits are markedly stronger for scalar and vector mediators when the mediator mass is light (≲ MeV), reflecting the enhancement of the momentum‑dependent cross‑section at the high DM velocities. Including Earth attenuation weakens the constraints for the lighter PBH masses (higher Hawking temperature) because a larger fraction of the DM loses energy below detection thresholds, while for (M_{\rm PBH}\sim10^{16}) g the attenuation effect is modest and the limits remain robust.

Fifth, the derived limits are translated into constraints on the PBH abundance fraction (f_{\rm PBH} = \Omega_{\rm PBH}/\Omega_{\rm DM}). For fixed DM mass and interaction strength, the non‑observation of excess electron recoils yields upper bounds on (f_{\rm PBH}) that are competitive with, and in some mass windows stronger than, existing bounds from extragalactic gamma‑ray and neutrino observations.

Finally, the paper explores the detection prospects in large‑volume neutrino detectors such as Super‑Kamiokande (22.5 kton fiducial volume) and the upcoming Hyper‑Kamiokande (187 kton). Despite their higher energy thresholds, the enormous target mass compensates, especially for light mediators where the scattering cross‑section rises at low momentum transfer. Simple rate estimates suggest that these detectors could provide complementary or even superior constraints in certain regions of parameter space, particularly for PBH masses around (10^{15}) g and DM masses below a few MeV.

In summary, the study demonstrates that Hawking‑radiated, semi‑relativistic DM from PBHs acts as a natural “cosmic accelerator,” enabling existing direct‑detection experiments to probe light dark sectors that would otherwise be invisible. By incorporating full relativistic scattering formulas and a realistic treatment of Earth attenuation, the authors deliver the most comprehensive set of limits to date on the combined PBH‑DM parameter space, and they highlight the promising role of future large‑scale neutrino observatories in extending these searches.