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The Sensitivity of PUEO to Cosmogenic Neutrinos and Exotic Physics Scenarios

This paper evaluates the scientific reach of the upcoming PUEO balloon experiment, demonstrating its unique capability to constrain the proton fraction of ultrahigh-energy cosmic rays and set leading neutrino-based limits on ultraheavy dark matter decay and cosmic string models in the 1–1000 EeV energy range.

Original authors: Angelina Sherman, Ke Fang, Dan Hooper

Published 2026-04-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Angelina Sherman, Ke Fang, Dan Hooper

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

Imagine the universe as a giant, chaotic highway. On this highway, there are particles zooming around at speeds so fast they are almost impossible to comprehend. These are Ultra-High-Energy Cosmic Rays (UHECRs). They are like cosmic bullets, mostly made of protons (hydrogen nuclei) or heavier atomic nuclei, shot out from violent events like exploding stars or black holes.

For decades, scientists have been trying to figure out exactly what these "bullets" are made of and where they come from. The problem is, these bullets get knocked off course by invisible magnetic fields in space, so by the time they hit Earth, we can't tell where they started.

Enter PUEO (Payload for Ultrahigh Energy Observations). Think of PUEO as a giant, high-tech "net" floating high above Antarctica on a long-duration balloon. Its job isn't to catch the bullets themselves, but to catch the ghosts they leave behind.

The Ghosts: Cosmogenic Neutrinos

When those cosmic bullets crash into the "fog" of the universe (specifically, the leftover light from the Big Bang), they create a shower of new particles. One of the most important byproducts of this crash is the neutrino.

Neutrinos are like ghosts. They have no electric charge and almost no mass. They don't care about magnetic fields; they don't get deflected. They pass through planets, stars, and even the entire Earth without stopping. Because they travel in a straight line from their birth, if we catch a neutrino, we can trace it back to its source.

The Big Question:
The paper asks: How many of these cosmic bullets are protons?

If the bullets are mostly protons, they crash hard and create a lot of neutrino ghosts.
If the bullets are mostly heavy nuclei (like iron), they break apart differently and create fewer ghosts.

PUEO is designed to be the most sensitive ghost-hunter ever built. If it sees a lot of ghosts, we know the cosmic rays are mostly protons. If it sees nothing, we know they are likely heavy, complex atoms.

The "What-If" Scenarios: Exotic Physics

The paper also uses PUEO as a telescope to look for things that shouldn't exist according to our current rules of physics. It's like checking the universe for "glitches in the matrix."

Superheavy Dark Matter:
We know 85% of the universe is "Dark Matter," an invisible substance that holds galaxies together. Usually, we think of it as light, slow particles. But what if some of it is made of Superheavy Dark Matter (SHDM)? Imagine a particle so heavy it weighs as much as a small asteroid, but it's invisible. If these monsters decay (fall apart) over billions of years, they would spit out high-energy neutrinos. PUEO is looking for the specific "signature" of these decays.
Cosmic Strings:
Imagine the fabric of space-time is like a sheet of fabric. If the universe had a phase transition early on (like water freezing into ice), it might have created wrinkles or tears in that fabric. These are called Cosmic Strings. They are incredibly thin but infinitely long and heavy. As they wiggle and snap, they could shoot out energy in the form of neutrinos. PUEO is checking if the sky is being "rained on" by these cosmic strings.

The Verdict: What Will PUEO Find?

The authors ran simulations (computer models) to see what PUEO will likely find during its 30-day flight over Antarctica.

The Cosmic Ray Mystery: PUEO will likely be able to tell us if the most energetic particles in the universe are protons, but only if those protons come from sources that have been getting more active over time (like a galaxy that has been "waking up" and firing more bullets as the universe aged). If the sources are quiet or the particles are heavy, PUEO might not see anything.
The Dark Matter Hunt: While PUEO is the best neutrino detector for finding decaying dark matter, the paper notes that gamma-ray telescopes (which look for light) are actually better at spotting these heavy particles. However, PUEO will set the strictest neutrino limits, telling us exactly how long these dark matter particles can live before they decay.
The Cosmic Strings: For some specific models of cosmic strings, PUEO might actually catch a few neutrinos. If it does, it would be a massive discovery, proving that these theoretical "wrinkles" in space-time are real.

The Bottom Line

Think of PUEO as a very sensitive microphone in a silent room.

If it hears a whisper, it tells us the "cosmic bullets" are mostly protons.
If it hears a specific, strange hum, it might be the sound of dark matter falling apart.
If it hears a rhythmic tapping, it might be the vibration of cosmic strings.

Even if PUEO hears silence (detects no neutrinos), that is still a huge victory. It tells scientists, "Okay, our theories about protons and dark matter are wrong in this specific way," forcing us to rewrite the rulebook of the universe. It's a mission to listen to the faintest whispers of the most energetic events in the cosmos.

1. Problem Statement

Ultrahigh-energy cosmic rays (UHECRs) with energies exceeding $10^{18}$ eV (1 EeV) remain one of the most significant mysteries in astroparticle physics. While their existence is confirmed, their chemical composition (proton vs. heavy nuclei) and sources are debated.

The Gap: UHECRs interact with cosmic photon backgrounds (CMB and EBL) via the Greisen-Zatsepin-Kuzmin (GZK) process, producing "cosmogenic" neutrinos. The flux of these neutrinos is highly sensitive to the proton fraction of UHECRs. However, no neutrinos above 1 EeV have been observed to date.
The Opportunity: The Payload for Ultrahigh Energy Observations (PUEO), a long-duration balloon experiment launching in late 2025, aims to detect Askaryan radiation from UHE neutrinos in the 1–1000 EeV range.
Objective: This paper assesses PUEO's scientific reach, specifically its ability to:
1. Constrain the proton fraction ( $f_p$ ) of the UHECR spectrum.
2. Probe exotic physics scenarios, including the decay of superheavy dark matter (SHDM) and radiation from cosmic strings.

2. Methodology

The authors employed a multi-faceted simulation and forecasting approach:

Cosmogenic Neutrino Modeling:
- Used the CRPropa 3.2 simulation framework to propagate UHECR protons through the universe.
- Modeled interactions with the CMB and EBL to generate charged pions, which decay into neutrinos.
- Parameterized the UHECR source evolution using a redshift-dependent injection rate $dN/dt \propto (1+z)^m$ (where $m$ represents the evolution strength, e.g., $m \sim 3.4$ for star formation rate).
- Adopted a power-law injection spectrum with an exponential cutoff: $dN/dE \propto E^{-\gamma} e^{-E/E_{max}}$ .
- Calculated the expected number of events ( $N_{exp}$ ) by integrating the predicted neutrino flux against PUEO's effective area ( $A\Omega_{eff}$ ) over a 30-day flight.
Exotic Physics Modeling:
- Superheavy Dark Matter (SHDM): Modeled the decay of unstable particles ( $m_{DM} > 10^8$ GeV) into neutrinos ( $\chi \to \nu\nu$ ). Calculated fluxes from both Galactic (NFW profile) and extragalactic dark matter distributions using the HDMSpectra code.
- Cosmic Strings: Modeled neutrino emission from cosmic string cusps and kinks, radiating via moduli decay. Used spectral formulas dependent on the coupling strength ( $\alpha$ ) and modulus mass ( $m$ ).
Sensitivity Analysis:
- Defined a non-detection limit as a 90% confidence level upper limit corresponding to an expected 2.3 events (Poisson statistics).
- Compared PUEO's projected limits against existing constraints from IceCube, gamma-ray observations, and current UHECR data (Pierre Auger Observatory and Telescope Array).

3. Key Contributions

Quantitative Proton Fraction Constraints: The paper provides the first detailed forecast of how PUEO can constrain the proton fraction ( $f_p$ ) of UHECRs at $10^{19.55}$ eV. It demonstrates that PUEO can distinguish between models with strong source evolution and high proton fractions.
Exotic Physics Reach: It establishes PUEO as the leading neutrino detector for constraining SHDM lifetimes above $10^{10}$ GeV and specific cosmic string models, even if gamma-ray constraints remain tighter for SHDM.
Benchmarking: The study benchmarks PUEO's sensitivity against the "best-fit" models from the Pierre Auger Observatory and Telescope Array, showing that current best-fit models predict very low event rates for PUEO, making a detection challenging but a non-detection highly informative.

4. Key Results

A. Cosmogenic Neutrinos & UHECR Composition

Source Evolution Dependence: PUEO's sensitivity to the proton fraction is strongly degenerate with the source evolution index ( $m$ $m$ ).
- For weak evolution ( $m=0$ , Auger best-fit) or moderate evolution ( $m=3$ ), PUEO is unlikely to detect cosmogenic neutrinos unless the proton fraction is unrealistically high (exceeding the total observed cosmic ray flux).
- For strong evolution ( $m=7$ ), PUEO could detect neutrinos if a significant fraction of the highest-energy cosmic rays are protons.
Constraints: If PUEO detects zero events, it will place stringent upper limits on $f_p$ for scenarios involving strong source evolution and high maximum proton energies ( $E_{max} \sim 100$ EeV).

B. Superheavy Dark Matter (SHDM)

Sensitivity: PUEO is projected to be the most sensitive neutrino experiment for SHDM masses $m_{DM} \gtrsim 10^{10}$ GeV.
Comparison with Gamma Rays: While PUEO will set the strongest neutrino-based limits on SHDM lifetimes (saturating current constraints for $\tau_{DM} \sim 10^{30}$ s), gamma-ray observations currently provide tighter overall constraints on SHDM decay.
Event Prediction: For a benchmark model ( $m_{DM} = 10^{11}$ GeV, $\tau_{DM} = 10^{30}$ s), PUEO expects only $\sim 0.02$ events, indicating that a detection would require either a higher flux or a different decay channel.

C. Cosmic Strings

Detectability: PUEO has the potential to detect neutrinos from cosmic string cusps for specific parameter spaces (high coupling $\alpha$ and specific modulus masses).
Benchmark Results:
- For $\alpha = 10^7, m = 10^5$ GeV: $N_{exp} \approx 0.13$ .
- For $\alpha = 2 \times 10^7, m = 10^4$ GeV: $N_{exp} \approx 1.1$ .
This suggests that while not guaranteed, PUEO could provide the first observational constraints on specific cosmic string models if the coupling is sufficiently strong.

5. Significance

Probing the "Top-Down" vs. "Bottom-Up" Debate: PUEO serves as a critical test for whether UHECRs are accelerated in astrophysical sources (bottom-up) or produced by the decay of exotic heavy particles (top-down). A null result in the 1–1000 EeV range would severely constrain top-down models.
UHECR Composition: By constraining the proton fraction, PUEO will help resolve the long-standing discrepancy between the Pierre Auger Observatory (suggesting heavier nuclei) and the Telescope Array (suggesting protons) regarding the composition of the highest-energy cosmic rays.
New Physics Frontier: The paper highlights that PUEO will operate in an energy regime ( $>10^{19}$ eV) where neutrino observations are uniquely sensitive to new fundamental physics, potentially offering constraints on dark matter and topological defects that are complementary to, and in some cases superior to, gamma-ray limits.

In summary, the paper establishes PUEO as a pivotal instrument for the next decade of high-energy astrophysics, capable of either detecting the first ultrahigh-energy neutrinos or placing the most stringent limits to date on the composition of cosmic rays and the nature of exotic physics.