The Sensitivity of PUEO to Cosmogenic Neutrinos and Exotic Physics Scenarios

TL;DR

The paper investigates how ultrahigh-energy neutrinos can illuminate the composition and origin of ultrahigh-energy cosmic rays (UHECRs) and test exotic high-energy physics. It forecasts PUEO's sensitivity using CRPropa 3.2 to model cosmogenic neutrinos and translates fluxes into expected detections, while also evaluating two exotic channels: superheavy dark matter (SHDM) decay and cosmic-string moduli radiation, using HDMSpectra and a modulus-cusp framework respectively. The authors find that PUEO can constrain the UHECR proton fraction $f_p$ in scenarios with strong source evolution and high-energy protons, and will place competitive or leading neutrino-based limits on SHDM for $m_{\rm DM} \gtrsim 10^{10}$ GeV and on certain cosmic-string models depending on $\alpha$ and $m$. Overall, PUEO offers a crucial probe of UHECR sources and physics beyond the Standard Model at the highest energies, complementing gamma-ray constraints and other next-generation detectors.

Abstract

Several observatories designed to detect ultrahigh-energy neutrinos are planned for the next decade. The most imminent of these is the Payload for Ultrahigh Energy Observations (PUEO), a long-duration balloon-based experiment that will provide unprecedented sensitivity to neutrinos with energies in the range of ~ 1 - 1000 EeV. In this work, we assess the scientific reach of PUEO. In particular, we evaluate the sensitivity of this observatory to cosmogenic neutrinos and, in turn, to the proton fraction of the ultrahigh-energy cosmic-ray spectrum. We also consider the potential of PUEO to probe scenarios in which neutrinos are produced through the decays of ultraheavy dark matter particles or are radiated from cosmic strings. We find that PUEO will be able to constrain the proton composition of ultrahigh-energy cosmic rays in scenarios that feature very strong source evolution and in which protons are accelerated to extremely high energies. Although gamma-ray observations are generally more sensitive to decaying particles than neutrino observations, PUEO is expected to set the strongest neutrino-detector constraints above 10^19 eV. PUEO will also provide the strongest constraints on some models of cosmic strings.

Figures (5)

• Figure 1: The spectra of UHECR protons (brown) and their corresponding cosmogenic neutrinos (teal). The colored spectra are normalized such that they produce (on average) 2.3 events in PUEO's upcoming flight. The varying linestyles correspond to different source evolution models ($m=3,5,7$). The PUEO 30-day sensitivity is shown by the solid dark gray line, while the black scatter points indicate the cosmic-ray spectrum as measured by the Pierre Auger Observatory Auger_spec_2017. The light gray curves represent the neutrino spectra predicted from the Auger and Telescope Array best-fit cosmic-ray models. In this figure, we have adopted an injected spectrum with an index of $\gamma = 2.0$ and an energy cutoff of $E_{\text{max}} = 100$ EeV.
• Figure 2: The spectrum of ultrahigh-energy neutrinos from the decay of superheavy dark matter particles with $m_{\text{DM}} = 10^{11}$ GeV and $\tau_{\rm DM} = 10^{30}$ s. This value of the lifetime approximately saturates the current constraints from gamma-ray observations Das:2023wtk. The solid line indicates the total neutrino spectrum, while the dashed and dotted lines show the contribution from the Galactic and cosmological dark matter distributions, respectively. The PUEO 30-day sensitivity is shown by the dark gray curve.
• Figure 3: The spectra of ultrahigh-energy neutrinos from cosmic string radiation, as calculated in the model of Ref. Berezinsky:2011cp. The three curves correspond to different values of the coupling strength of moduli to cosmic strings $\alpha$, and the modulus mass $m$. The PUEO 30-day sensitivity is shown by the gray curve.
• Figure 4: The regions of the UHECR parameter space that would be ruled out by PUEO if it does not detect any ultrahigh-energy neutrino events in its 30-day flight. Such a nondetection would exclude some models with both very strong source evolution, $m$, and a large proton fraction, $f_p$. Results are shown for $\gamma=1.0$, 2.0, and 3.0 and $E_{\rm max}=40$ and $1000 \, {\rm EeV}$. The proton fraction is defined at $10^{19.55}$ eV. The dashed gray line shows the most conservative limit placed by Ref. IceCubeCollaborationSS:2025jbi. The vertical lines indicate the source evolution of gamma-ray bursts (GRB), sources that follow the star formation rate (SFR), and medium-high-luminosity active galactic nuclei (MHL-AGN).
• Figure 5: The projected limit from PUEO (assuming no events are observed) on the lifetime of superheavy dark matter (orange hatched). The shaded region indicates the parameter space that is already disfavored by existing neutrino observations, while the dashed line denotes the parameter space that is disfavored by existing gamma-ray observations. For $m_{\text{DM}} \gtrsim 10^{10}$ GeV, PUEO is expected to place the strongest neutrino-based limits on SHDM, although gamma-ray constraints will remain more restrictive.