The highest-energy cosmic rays include a substantial proton fraction.

The verdict rests on three lines of evidence read from the primary literature, and the recent first assessment of the Auger subclaim confirms rather than shifts the weighting already applied.

First, the case against a substantial proton fraction at the extreme end. Auger's shower-maximum measurements indicating a heavier composition above about 2×10^18 eV↗︎ now carry a recorded assessment of supported at credence 0.92: the elongation-rate break and the decreasing Xmax fluctuations are robust across current hadronic models (arxiv.org/pdf/1307.3956), and the Auger deep-learning analysis of surface-detector data (Phys. Rev. Lett. 134, 021001 (2025), doi.org/10.1103/PhysRevLett.134.021001) finds the composition increasingly heavy and pure at the highest energies, incompatible with a large light-nuclei fraction between 50 and 100 EeV. That assessment also finds the Telescope Array tension limited and compatible within systematics, matching the reading here. This remains the strongest single consideration and weighs directly against the claim above roughly 5×10^19 eV.

Second, the case that the question remains open. Telescope Array hybrid Xmax data are compatible with a pure-proton composition below 10^19 eV↗︎ (arxiv.org/pdf/1810.00431), and the joint composition working group finds the Auger and Telescope Array datasets consistent within systematic uncertainties. A credible experiment whose data remain compatible with protons blocks a contradicted verdict.

Third, the shared limitation. The claim's decomposition assumes hadronic interaction models reliably predict the shower observables↗︎ at center-of-mass energies beyond the LHC's reach; the muon deficit observed in simulations shows the models are incomplete, which caps confidence in composition inferences on both sides.

The claim's truth also turns on two soft terms. "Highest-energy": a substantial proton fraction is well supported near 10^18 eV and plausible to about 10^19 eV, but disfavored above ~5×10^19 eV on Auger's reading. "Substantial": a proton fraction of order tens of percent at the top of the spectrum is disfavored, but a minority component is not excluded. Credence 0.4 reflects the strict reading (substantial fraction at the top of the observed spectrum). The prior pass searched for post-2024 developments and found nothing that moves the verdict: the 2025 PRL result is already weighed, and the joint working-group picture is unchanged as of the latest spectrum comparison (arxiv.org/html/2509.05530v1). What would change the conclusion: AugerPrime mass-sensitive measurements settling the fraction event by event, resolution of the muon puzzle, or a joint Auger–TA analysis attributing the difference to systematics in one dataset.

[Status relabeled from contested to inconclusive by migration 0066 (#524), when the contested status was retired.]