The mass composition of UHECRs remains a major astrophysical puzzle. For historical consistency, I’ll review the "dip" and "disappointing" models, alongside the energy losses of CRs, source distributions, and large-scale structures. I’ll discuss how the "muon puzzle" in air showers challenges standard composition interpretations derived from X_max and R_max, revealing deep limitations in current interaction models. Finally, I evaluate the scenario where the highest-energy CRs are protons originating from hypothetical mirror neutrons, oscillating to our protons predominantly in cosmic voids.
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Askhat Gazizov
DESY Potsdam
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