⚛️ phenomenology

PBR sensitivity to boosted dark matter explanation for the KM3-230213A event

This paper evaluates the sensitivity of the upcoming PBR balloon-borne telescope to detect boosted dark matter as an explanation for the high-energy KM3NeT muon event, demonstrating its potential to discover or constrain such scenarios for dark matter energies ranging from $1.5\times 10^8$ to over $10^9$ GeV.

Original authors: M. H. Reno, T. Venters, M. Abrate, J. Adams, J. Alfaro, D. Allard, P. Alldredge, R. Aloisio, R. Ammendola, A. Anastasio, L. Anchordoqui, D. Badoni, J. Baláž, B. Baret, M. Battisti, R. Bellotti, M. Ber

Published 2026-10-22

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: M. H. Reno, T. Venters, M. Abrate, J. Adams, J. Alfaro, D. Allard, P. Alldredge, R. Aloisio, R. Ammendola, A. Anastasio, L. Anchordoqui, D. Badoni, J. Baláž, B. Baret, M. Battisti, R. Bellotti, M. Bertaina, M. Betts, S. Blin, M. Boezio, P. Bořil, I. Buckland, J. Burton Heibges, F. Cafagna, R. Caruso, M. Casolino, K. Černý, B. Chamanbahar, N. Cordrey, A. Creusot, A. Cummings, C. De Santis, P. Degarate, N. DeWitt Pierrat, A. Di Giovanni, A. Di Salvo, B. J. DiLella, J. Eser, S. Ferrarese, G. Filippatos, J. Ford, N. Gagalis, P. Gálvez Molina, S. Garbolino, D. Garg, R. Gargiulo, B. Gockel, A. Grelle, C. Guépin, A. Haungs, T. Heibges, J. Hinkel, A. K. Ikehara, J. Krizmanic, L. Kupari, E. H. Lenzing, F. Liberatori, S. Mackovjak, D. Mandát, M. Manfrin, A. Marcelli, L. Marcelli, G. Masciantonio, V. Masone, E. Mayotte, A. Meli, E. Mentzell, M. Mese, S. Meyer, M. Mignone, M. Miller, M. Mongelli, J. Moses, E. Msihid, R. Munini, M. Murdock, A. Novikov, A. V. Olinto, Y. Onel, G. Osteria, B. Panico, E. Parizot, G. Passeggio, T. Paul, M. Pech, K. Peñaló Castillo, F. Perfetto, C. Petta, P. Picozza, L. Piotrowski, Z. Plebaniak, H. Qureshi, E. Reali, F. Reynaud, E. Ricci, M. Ricci, A. Rivetti, A. Roy, F. Sarazin, V. Scherini, P. Schovánek, F. Schroeder, V. Scotti, A. Sotgiu, R. Sparvoli, B. Stillwell, I. Strhárský, J. Szabelski, Y. Takizawa, R. Torres, R. Triggiani, C. Trimarelli, J. Tutt, M. Unger, M. Venugopal, P. von Ballmoos, D. Washington, A. Weindl, L. Wiencke, S. Wissel, A. Yuan

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

Deep in the ocean, far beneath the waves, a network of sensors listens for the faintest ripples of light. This is the realm of neutrino astronomy, a field dedicated to tracking ghostly particles that zip through the universe almost entirely unimpeded. Unlike light, which bounces off dust or gets absorbed by gas, these particles can travel billions of years across the cosmos and pass straight through planets without stopping. Because they carry information from the most violent and energetic events in the universe—like exploding stars or colliding black holes—they are considered the ultimate messengers of the cosmos. For years, scientists have been trying to catch these particles to understand the extreme engines that power the universe. Recently, a detector in the Mediterranean Sea, known as KM3NeT, recorded a muon, a heavy cousin of the electron, with an energy so high it defied current expectations. This single event, recorded in early 2023, has sparked a global conversation about whether our understanding of physics is complete or if something entirely new is at play.

The mystery centers on a specific particle track detected by the KM3NeT observatory. The muon was moving almost horizontally, skimming just above the horizon, with a reconstructed energy of 120 PeV, a number so large it is difficult to comprehend. While the detector is designed to catch neutrinos from space, such a high-energy particle could theoretically be a muon that originated in Earth's atmosphere, created when cosmic rays hit the air. However, the researchers behind a new study have shown that this explanation is effectively impossible. They calculated that for an atmospheric muon to survive the journey through rock and water to reach the detector with that much energy, it would have needed to start with an impossibly high energy at the surface. Even with the most generous assumptions about how these particles lose energy, the odds of an atmospheric muon making the trip are so vanishingly small that they can be ruled out. This leaves the door open for the particle to be something far more exotic, perhaps originating from a source beyond our solar system.

With the ordinary explanation discarded, the focus shifts to the possibility of new physics. One compelling theory suggests that the event was not caused by a standard neutrino, but by a stream of "boosted dark matter." In this scenario, invisible particles from a distant astrophysical source travel toward Earth and interact with the planet's core. Instead of passing through or stopping, these dark matter particles collide with atomic nuclei inside the Earth, creating a chain reaction that produces a pair of heavy particles called taus. These taus then exit the Earth and decay in the atmosphere, creating a shower of particles that emits a flash of light. This specific chain of events, proposed by other theorists, offers a way to explain why the KM3NeT detector saw such a massive signal while other observatories, like IceCube in Antarctica, saw nothing. The difference lies in the path the particles took; the Earth itself acts as a filter, allowing only specific types of interactions to emerge on the other side.

To test whether this exotic explanation could be true, a team of scientists analyzed the capabilities of a future instrument called PBR, or the POEMMA-Balloon with Radio. Scheduled to launch in 2028, this instrument will float on a high-altitude balloon over the Southern Ocean, carrying a specialized telescope designed to look just below the horizon. Its job is to catch the faint, ultra-fast flashes of light produced when particles emerging from the Earth decay in the atmosphere. The researchers used computer simulations to see if PBR could detect the specific type of signal that would be produced if the KM3NeT event were indeed caused by boosted dark matter. They modeled the journey of these hypothetical particles as they traveled through the Earth, interacted, and emerged to create a detectable flash of light in the sky.

The results of this simulation are promising. The study finds that if the KM3NeT event was caused by this specific type of dark matter, the PBR instrument would be sensitive enough to detect similar events, provided the dark matter particles have energies above a certain threshold. For the most likely scenarios, the instrument could discover these signals at energies as low as 150 million GeV, and it would be highly effective at energies above one billion GeV. The researchers also noted that the sensitivity depends on the specific properties of the dark matter, such as how strongly it interacts with normal matter. In some cases, the instrument could be even more powerful, capable of seeing signals across a wide range of energies that overlap with the mysterious event seen by KM3NeT. Crucially, the study shows that the location of the original event in the sky is well within the range that PBR will be able to monitor during its flight.

This work does not prove that the KM3NeT event was caused by dark matter, nor does it confirm that the new physics model is correct. Instead, it establishes a clear path forward for verification. By calculating exactly what the PBR instrument would see if the theory were true, the researchers have provided a concrete target for the upcoming mission. If PBR detects these specific flashes of light from the same direction, it would be a monumental discovery, confirming the existence of a new type of particle and opening a new window into the dark sector of the universe. If it sees nothing, it will place strict limits on how such particles can behave, forcing physicists to rethink their theories. Either way, the combination of the KM3NeT observation and the PBR simulation represents a significant step in the ongoing effort to understand the most energetic processes in the cosmos, moving from a single puzzling data point to a testable scientific hypothesis.

Technical Summary: PBR Sensitivity to Boosted Dark Matter Explanation for the KM3-230213A Event

Problem Statement
The KM3NeT/ARCA detector recently reported a nearly horizontal muon track (KM3-230213A) with a reconstructed energy of $E_\mu = 120^{+110}_{-60}$ PeV. While this event represents the highest energy neutrino candidate observed in a neutrino telescope, its origin is ambiguous. Standard astrophysical interpretations face significant challenges: a diffuse cosmogenic neutrino flux at this level conflicts with limits from IceCube and the Pierre Auger Observatory, and transient source associations lack definitive gamma-ray counterparts. Furthermore, atmospheric muons are ruled out as the source due to the extreme energy loss required for a muon to penetrate the Earth's crust and water to reach the detector with the observed final energy. These tensions have motivated the exploration of Beyond the Standard Model (BSM) physics, specifically scenarios involving "boosted" dark matter (DM) that could produce high-energy leptons via interactions within the Earth.

Methodology
This paper evaluates the capability of the upcoming POEMMA-Balloon with Radio (PBR) mission to detect or constrain such BSM scenarios. PBR is a stratospheric balloon-borne pathfinder instrument scheduled for a 2028 flight, equipped with a hybrid focal surface including a Cherenkov Camera (CC) designed to detect optical Cherenkov light from upward-going extensive air showers (EAS) initiated by Earth-skimming tau neutrinos.

The authors adopt the specific BSM framework proposed by Farzan and Hostert, where an astrophysical flux of high-energy dark sector particles ( $\chi$ ) interacts coherently with Earth nuclei to produce a meta-stable particle ( $\phi$ ), which subsequently decays into a pair of leptons ( $\ell\ell$ , where $\ell = \mu$ or $\tau$ ). The study focuses on the $\chi \to \phi \to \tau\tau$ channel, as the resulting tau leptons can exit the Earth and decay in the atmosphere to generate detectable EASs.

The methodology involves:

Kinematic Modeling: Calculating the probability of a $\chi$ particle interacting in the Earth and producing an emerging tau lepton ( $P_{exit}$ ). This accounts for the $\chi$ interaction length, the $\phi$ decay length, and the tau's electromagnetic energy loss and decay probability as it traverses the Earth.
Instrument Simulation: Using Monte Carlo simulations to model the PBR Cherenkov Camera's acceptance. This includes the geometric field of view (FoV), the Earth's limb geometry relative to the balloon's 33 km altitude, and observational constraints such as solar and lunar illumination (dark sky requirements).
Sensitivity Calculation: Deriving the projected sensitivity to the $\chi$ fluence ( $E_\chi^2 F_\chi$ ) for two scenarios: a short transient burst (1000 s) and a long-duration source (20 days). The sensitivity is compared against the fluence required to explain the KM3-230213A event, assuming the KM3NeT observation corresponds to a $\chi \to \phi \to \mu\mu$ process.
Parameter Variation: The analysis explores different values for the $\chi$ -nucleon cross-section ( $\sigma_0 = 10^{-33}$ to $10^{-32}$ cm $^2$ ) and branching fractions for the $\phi$ decay ( $B_{\tau\tau}$ vs. $B_{\mu\mu}$ ), including a scenario where the branching fraction scales with the square of the lepton mass ( $B_{\tau\tau} = B_{\mu\mu} \times (m_\tau/m_\mu)^2$ ).

Key Contributions and Results

Discovery Reach: The paper establishes that PBR has discovery potential for boosted dark matter with energies $E_\chi > 10^9$ GeV. Depending on BSM input parameters, this reach extends down to $E_\chi \approx 1.5 \times 10^8$ GeV.
Comparison with KM3NeT:
- For a short burst (1000 s) and assuming equal branching fractions ( $B_{\tau\tau} = B_{\mu\mu}$ ) with $\sigma_0 = 10^{-32}$ cm $^2$ , PBR's sensitivity crosses the required KM3NeT fluence at $E_\chi \approx 4 \times 10^8$ GeV.
- For a 20-day observation period, the crossover occurs at higher energies ( $E_\chi \sim 8 \times 10^8$ GeV for $\sigma_0 = 10^{-32}$ cm $^2$ ).
- If the branching fraction scales with the lepton mass squared ( $B_{\tau\tau} = B_{\mu\mu} \times (m_\tau/m_\mu)^2$ ), the sensitivity improves by a factor of $\sim 20$ , allowing PBR to probe the entire energy range associated with the KM3NeT event ( $E_\chi \approx 70 - 760$ PeV) for both short and long bursts.
Sky Coverage: The study confirms that the sky location of KM3-230213A is near-optimal for a 20-day flight launching from New Zealand in April 2028.
Environmental Factors: The authors provide a conservative estimate of cloud impact, noting that while clouds below 2 km altitude could raise the energy threshold from $\sim 2 \times 10^8$ GeV to $\sim 3 \times 10^8$ GeV and reduce acceptance at lower energies, the effect is marginal at higher energies where taus are more likely to decay above the cloud layer.

Significance
The paper concludes that while the analysis focuses on a single BSM example (the Farzan-Hostert scenario), the methodology is broadly applicable to any BSM physics that modifies tau production and exit probabilities in the Earth. PBR's unique ability to point below the Earth's limb from a stratospheric balloon allows it to probe particle transit properties that ground-based detectors cannot. If the KM3-230213A event is indeed a signature of boosted dark matter, PBR is well-positioned to either discover such sources or place stringent constraints on the fluence and interaction cross-sections of these particles, thereby extending the science reach of neutrino astronomy into the realm of BSM physics.

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