Primordial Black Hole Hawking Radiation Could Reveal Dark Photon Tridents in Gamma Rays

Primordial black holes evaporating in the halo of the Milky Way may offer physicists an unexpected window onto hidden particles that no collider on Earth can reach. A new theoretical study published in The European Physical Journal C by Kingman Cheung, C. J. Ouseph, Po-Yan Tseng and Sin Kyu Kang argues that the gamma rays streaming from these microscopic black holes could carry the fingerprint of dark photons — hypothetical massive cousins of the ordinary photon that are thought to mediate interactions in a hidden sector beyond the Standard Model. The key, according to the authors, lies in a rare and exotic decay process known as the dark photon trident, in which a single dark photon splits into three ordinary photons.

Primordial black holes, unlike the stellar collapsed objects familiar to astronomers, are hypothesized to have formed in the very early universe through several possible channels. They may have condensed from overdense regions generated by primordial fluctuations after inflation, or emerged from first-order phase transitions in which collisions of bubble walls concentrated enough energy density inside a Schwarzschild radius, or even from the collapse of macroscopic intermediate states called fermi-balls involving dark sector particles. If these objects make up all or part of the dark matter, they would be distributed throughout the Galactic halo, and those below roughly 10 to the minus 15 solar masses would be actively evaporating today through Hawking radiation, emitting a steady stream of particles in all directions.

The appeal of primordial black holes as factories of new physics stems from a remarkable feature of Hawking radiation: the spectrum of emitted particles depends only on the black hole’s temperature and on the masses and spins of the particles themselves, not on how strongly they interact with anything else. This makes evaporating black holes natural sources of feebly interacting particles that would be nearly impossible to produce or detect in collider experiments, where feeble interaction cross-sections translate into vanishingly small production rates. Among the candidates for such particles is the dark photon, a hypothetical massive spin-1 boson that could arise from extending the Standard Model gauge group with an additional U(1) symmetry.

The dark photon connects to the visible world through a mechanism called kinetic mixing, a renormalizable dimension-4 operator that induces a weak coupling between the dark photon and the Standard Model photon, typically via loops of heavy particles charged under both gauge groups. This mixing is quantified by a parameter epsilon, and it is what makes the dark photon a potential portal to hidden sectors — possibly mediating self-interacting or inelastic dark matter, facilitating freeze-in production processes, and potentially explaining anomalies such as the white dwarf cooling excess and the electron recoil excess observed at XENON1T. Experiments such as NA64, LDMX, HPS, FASER and Belle II are already probing wide swathes of dark photon parameter space.

What happens to a dark photon after it is radiated away depends sharply on its mass. Above twice the electron mass, it decays cleanly into an electron-positron pair. But below that threshold, the usual two-photon decay is forbidden by the Landau-Yang theorem, and the dominant channel becomes a loop-suppressed process in which the dark photon transforms into three photons — the trident decay. This three-photon final state produces an energy distribution that is fundamentally different from the two-body spectra of conventional sources. The researchers derived an explicit formula for this spectrum, showing that the photon energy fraction x ranges from zero to one with a characteristic polynomial shape, and they carefully accounted for how the spectrum is boosted when the dark photon is produced relativistically by the black hole.

Focusing on asteroid-mass primordial black holes with masses between 10 to the 15 and 10 to the 18 grams, the team computed the total gamma-ray flux expected from a region within five degrees of the Galactic Center, modeling the dark matter distribution with a Navarro-Frenk-White profile and computing the Hawking radiation spectra with the publicly available BlackHawk v2.0 package. In their baseline scenario, only Standard Model particles emerge from the black holes, with photons arriving directly, from neutral pion decays, and from final-state radiation off electrons and muons. In the dark-photon scenario, a third contribution appears: dark photons radiated as primary particles, decaying into photon tridents on their way to Earth. For the signal to be visible, the dark photons must decay before arrival, and the team required an average decay probability greater than 99 percent, showing that for the relevant kinetic mixing values the decays are effectively prompt at the source, with lab-frame decay lengths of only hundredths of a kiloparsec compared to the roughly 8 kiloparsec distance to the Galactic Center.

The resulting spectral signature is striking. Because the energy of each trident photon is typically lower than the energy of the parent dark photon, and because the production of dark photons heavier than the primary photon peak is suppressed, the dark-photon contribution always appears as a displaced, softer peak to the left of the primary Hawking photon peak. The researchers emphasize that this shape — a primary peak plus a secondary decay peak — cannot be replicated by smooth diffuse emission or by power-law sources such as pulsars or active galactic nuclei, and its alignment with the Galactic dark matter halo’s spatial profile and temporal stability further distinguish it from transient or isotropic backgrounds. The team also verified that inverse Compton scattering of evaporated electrons off ambient photon fields is negligible here, since the upscattered photons would fall far below the MeV range, and that the extragalactic contribution amounts to only about 0.8 percent of the Galactic Center flux.

Turning to detection, the authors performed a likelihood analysis using the projected sensitivity of e-ASTROGAM, a proposed gamma-ray mission covering the 0.1 to 10 MeV range where this signal lives. For primordial black holes with masses of 10 to the 15 to 10 to the 17 grams, the analysis finds that the dark-photon signal could reach a test statistic above 9, corresponding to approximately 3 sigma significance, enough to detect the signal and to distinguish it from conventional Hawking radiation. Notably, for fixed black hole mass and dark photon mass, the constraints in the dark-photon scenario are at least as stringent as those in the Standard Model scenario, because each dark photon adds three degrees of freedom and brightens the overall signal. However, for heavier black holes or heavier dark photons, the two scenarios converge: once the Hawking temperature drops well below the dark photon mass, its production is exponentially suppressed and the spectra become nearly indistinguishable. The COSI mission, soon to launch as a successor to COMPTEL, is projected to be one to two orders of magnitude less sensitive than e-ASTROGAM with a narrower energy range, but it may still provide a complementary near-term probe of part of the parameter space.

Perhaps the most compelling result is the reach into territory that no existing experiment has covered. The researchers show that the primordial black hole scenario can probe unexplored parameter space in the range of kinetic mixing between 10 to the minus 7 and 10 to the minus 3 and dark photon masses of at least 0.5 MeV — a window in which terrestrial experiments fail because the dark photon decay length is too long for practical detection, while supernova 1987A bounds lose grip because the mean free path is too short for the particles to escape. The authors caution that supernova modeling uncertainties allow order-of-magnitude variations in those bounds, keeping the region marginally open, and that recent X-ray constraints are complementary but do not fully close it. They also estimate that a factor-of-few uncertainty in the astrophysical background normalization would shift the sensitivity by at most order one in epsilon, which does not qualitatively close the accessible window, though a full template-level analysis with real data remains future work. The dark matter profile itself matters: switching from the Navarro-Frenk-White assumption to an isothermal profile would weaken the projected reach by a factor of about 2.3, while a cuspier Moore profile would strengthen it by a factor of about 0.72 in the required black hole abundance.

The study is deliberately framed as a theoretical forecast rather than a detection claim. No observational evidence for primordial black holes as dark matter currently exists, and the entire analysis rests on assumed black hole abundances, a monochromatic mass distribution, and projected instrument performance. Yet the logic is powerful: if evaporating primordial black holes exist, they would emit Standard Model and beyond-the-Standard-Model particles alike, indifferent to interaction strength, and the distinctive three-photon spectrum of a sub-megaelectronvolt dark photon would leave a characteristic distortion in the MeV gamma-ray sky. As missions like COSI and a possible e-ASTROGAM begin to survey this poorly explored band with modern sensitivity, the cosmic evaporation of ancient black holes could become one of the few practical laboratories for the dark sector — a search in which the message is written not in accelerator collisions, but in the faint gamma-ray glow of black holes that formed before the stars.

Subject of Research: Dark photon trident decays in Hawking radiation from asteroid-mass primordial black holes as a gamma-ray probe of dark sectors

Article Title: Searching for dark photon tridents through primordial black hole signatures

Article References: Cheung, K., Ouseph, C. J., Tseng, P.-Y., & Kang, S. K. (2026). Searching for dark photon tridents through primordial black hole signatures. The European Physical Journal C, 86(10), Article 1220. https://doi.org/10.1140/epjc/s10052-026-16317-4

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16317-4

Keywords: primordial black holes, dark photons, Hawking radiation, gamma rays, dark matter, trident decay, e-ASTROGAM, COSI, kinetic mixing, Galactic Center, beyond the Standard Model, MeV astronomy