⚛️ phenomenology

Searching for Dark Photon Tridents Through Primordial Black Hole Signatures

This paper proposes that future gamma-ray observatories can detect and distinguish asteroid-mass primordial black holes as dark matter candidates by identifying unique spectral signatures from the trident decay of dark photons (with mass $\leq$ 1 MeV) produced via Hawking radiation.

Original authors: Kingman Cheung, C. J. Ouseph, Po-Yan Tseng, Sin Kyu Kang

Published 2026-09-02

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CC BY 4.0

Original authors: Kingman Cheung, C. J. Ouseph, Po-Yan Tseng, Sin Kyu Kang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is filled with invisible matter that does not glow, reflect light, or interact with the world in ways our eyes can see. Scientists call this dark matter, and while they know it exists because of how it pulls on stars and galaxies, they have never directly caught a single particle of it. One leading idea suggests that some of this dark matter might be made of tiny, ancient black holes formed in the very first moments after the Big Bang. These are not the massive black holes found at the centers of galaxies, but rather small, asteroid-sized remnants that have been drifting through space for billions of years. According to a famous theory proposed by Stephen Hawking, even these tiny black holes are not truly black; they slowly leak energy and particles as they evaporate. If these primordial black holes exist, they should be constantly spewing out a stream of radiation, including light and other exotic particles, which could eventually reach Earth.

A team of researchers has now explored a new way to look for these elusive objects by focusing on a hypothetical particle called a dark photon. This particle is thought to be a heavy cousin of the ordinary photon, which carries light. Unlike the dark photon, which is invisible to us, the ordinary photon is what allows us to see the world. The researchers imagined a scenario where a primordial black hole emits a dark photon, which then travels a short distance before transforming into three ordinary photons. This specific transformation, known as a trident decay, creates a unique pattern of light that is different from the standard glow of a black hole. By calculating exactly what this light would look like and comparing it to the expected background noise of the universe, the team determined that future telescopes could spot this signature. Their work suggests that if these asteroid-mass black holes exist and are producing dark photons, the next generation of space observatories will be able to find them, opening a new window into the hidden sectors of our universe.

The study focuses on a specific mass range for these black holes, between $10^{15}$ and $10^{18}$ grams, which is roughly the weight of a large asteroid. When such a black hole evaporates, it heats up and emits particles. In the standard view of physics, it would only emit known particles like electrons and photons. However, the researchers considered a more complex possibility where the black hole also emits dark photons. Because these dark photons are unstable, they decay quickly into three regular photons. This process creates a distinct energy spectrum, or a specific distribution of light energies, that stands out against the smooth, predictable glow of ordinary Hawking radiation. The researchers found that this three-photon signal has a unique shape that cannot be easily mimicked by other cosmic sources, such as pulsars or active galaxies, making it a powerful tool for identification.

To test this idea, the team modeled the gamma-ray signals coming from the center of our Milky Way galaxy, where the density of dark matter is expected to be highest. They compared two scenarios: one where the black holes only emit standard particles, and another where they also emit dark photons. Using the projected sensitivity of a future mission called e-ASTROGAM, they analyzed whether a telescope could tell the difference between these two scenarios. The results showed that for certain masses of dark photons, specifically those lighter than 1 MeV, the signal from the dark photon scenario would be bright enough to be distinguished from the standard background. The presence of the dark photons actually makes the signal easier to detect because it adds extra brightness and a unique spectral shape that differs from the standard model.

The researchers also examined the conditions required for this signal to be visible. They calculated that the dark photons must decay into three photons before traveling all the way from the center of the galaxy to Earth. Their analysis showed that for the range of parameters they studied, the dark photons decay almost immediately after being created, ensuring that the three-photon signal arrives at our detectors as a concentrated burst rather than a diffuse glow. This prompt decay means the signal retains its unique shape, allowing scientists to identify it clearly against the backdrop of other cosmic radiation. The study confirms that if these asteroid-mass black holes exist and produce dark photons, the resulting gamma-ray signature would be strong enough to be detected by upcoming instruments.

This work does not claim to have found dark matter or proven the existence of primordial black holes. Instead, it provides a detailed roadmap for how future experiments could find them. The researchers identified specific regions of the parameter space, defined by the mass of the dark photon and how strongly it interacts with ordinary matter, where a discovery is possible. They found that this approach could explore areas of physics that are currently inaccessible to particle accelerators on Earth. By focusing on the unique trident decay of dark photons, the study offers a viable path to uncovering the properties of the dark sector and potentially confirming the existence of asteroid-mass primordial black holes. The findings suggest that the next generation of gamma-ray telescopes will be able to test these ideas, potentially revealing a new layer of reality hidden within the dark matter that surrounds us.

Technical Summary: Searching for Dark Photon Tridents Through Primordial Black Hole Signatures

Problem Statement
Primordial Black Holes (PBHs) are viable candidates for dark matter, particularly in the asteroid-mass range ( $10^{15}$ – $10^{18}$ g), where they emit significant Hawking radiation. While PBHs are predicted to emit Standard Model (SM) particles, they also serve as potential sources for feebly-interacting Beyond-the-Standard-Model (BSM) particles, such as dark photons ( $A'$ ). A specific challenge arises for dark photons with masses $m_{A'} \leq 2m_e$ (below the electron-positron pair production threshold). In this regime, the tree-level decay into two photons is forbidden by the Landau-Yang theorem, forcing the dominant decay channel to be the loop-suppressed trident process ( $A' \to 3\gamma$ ). Detecting these particles is difficult in terrestrial colliders due to small kinetic mixing parameters ( $\epsilon$ ) and the challenge of resolving three low-energy photons. This paper investigates whether the unique gamma-ray signatures from the decay of dark photons produced by PBH Hawking radiation can be distinguished from conventional SM Hawking radiation by future gamma-ray observatories.

Methodology
The authors model the gamma-ray spectrum originating from the Galactic Center (within $|R| < 5^\circ$ ) under two distinct scenarios:

SM-scenario: PBHs emit only Standard Model particles. The resulting photon spectrum includes primary photons and secondary photons from neutral pion decays, electron/muon final state radiation (FSR), and other SM processes.
Dark-photon-scenario: PBHs emit both SM particles and dark photons via Hawking radiation. The dark photons subsequently decay into three photons ( $A' \to 3\gamma$ ) before reaching Earth.

The study utilizes the BlackHawk v2.0 package to compute the exact graybody factors and emission spectra for non-rotating PBHs, incorporating massive particle effects and energy cutoffs. The total photon flux is calculated by integrating the primary emission and the decay products of dark photons, assuming a Navarro–Frenk–White (NFW) dark matter halo profile.

Key assumptions and constraints include:

Decay Probability: The analysis requires that dark photons decay with a probability $>99\%$ before reaching Earth. This constraint defines the viable parameter space for the kinetic mixing $\epsilon$ and mass $m_{A'}$ .
Spectral Modeling: The energy spectrum of the trident decay ( $dN_\gamma/dE_\gamma$ ) is derived from the Dalitz plot distribution, which differs significantly from the two-body decays typical of SM processes (e.g., $\pi^0 \to 2\gamma$ ).
Statistical Analysis: A likelihood analysis is performed using the projected sensitivity of the e-ASTROGAM mission (and to a lesser extent, COSI). The Test Statistic (TS) is used to quantify the significance of distinguishing the dark-photon-scenario from the SM-scenario and from the astrophysical background. A significance threshold of $\Sigma = 3$ (TS > 9) is adopted.

Key Contributions

Trident Decay Signature: The paper establishes that the $A' \to 3\gamma$ decay produces a distinct energy spectrum in the MeV range (0.1–10 MeV). This spectrum exhibits a characteristic shape and peak location that is qualitatively different from the smooth power-law backgrounds of pulsars or active galactic nuclei, and distinct from the SM Hawking radiation spectrum.
Parameter Space Exploration: The authors identify a previously inaccessible region of the dark photon parameter space ( $10^{-7} \leq \epsilon \leq 10^{-3}$ and $m_{A'} \geq 0.5$ MeV). This region is difficult to probe with terrestrial experiments due to long decay lengths (preventing detection in detectors) but short enough to be constrained by supernova cooling (SN1987A), though uncertainties in supernova modeling leave this window open.
Differentiation Capability: The study demonstrates that the presence of dark photons enhances the total gamma-ray flux (due to additional degrees of freedom) and alters the spectral shape, allowing future missions to potentially distinguish the dark-photon-scenario from the pure SM-scenario.

Results

Spectral Distinction: For PBH masses in the range $[10^{15}, 10^{18}]$ g, the inclusion of dark photons introduces a secondary peak in the gamma-ray spectrum at energies lower than the primary Hawking peak. This deviation is most pronounced when the dark photon mass is lighter than the PBH temperature ( $m_{A'} < T_H$ ).
Detectability: Using e-ASTROGAM sensitivity, the authors find that for specific combinations of PBH mass ( $M_{PBH}$ ) and abundance fraction ( $f_{PBH}$ ), the dark-photon-scenario can be distinguished from the SM-scenario with a significance of $\sim 3\sigma$ .
Constraints: The analysis shows that constraints on PBH abundance derived from the dark-photon-scenario are generally stronger (i.e., allow for lower $f_{PBH}$ ) than those from the SM-scenario alone, due to the enhanced signal brightness. However, for heavy PBHs or heavy dark photons ( $m_{A'} \gg T_H$ ), the dark photon contribution is exponentially suppressed, causing the constraints to converge with the SM-scenario limits.
Extragalactic Background: The authors estimate that extragalactic gamma-ray contributions within the chosen Region of Interest (ROI) are negligible ( $\sim 0.8\%$ of the Galactic Center flux), ensuring that the signal is dominated by Galactic sources.

Significance and Claims
The paper claims that searching for dark photon tridents via PBH signatures offers a viable and complementary avenue to explore the dark sector, specifically targeting the "white trapezoidal" parameter space that is currently unconstrained by terrestrial experiments. The authors assert that future gamma-ray missions, particularly e-ASTROGAM, have the potential to detect these unique three-photon signatures and differentiate them from conventional Hawking radiation. This approach not only tests the existence of dark photons but also provides a method to probe the nature of asteroid-mass PBHs as dark matter candidates. The work concludes that the interplay between PBH evaporation and dark sector physics can lead to observable modifications in gamma-ray signals, offering a robust probe for new physics in the MeV energy range.

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