The Pierre Auger Observatory measures cosmic rays at the highest energies known in nature. For many years, these particles were often assumed to be predominantly protons. A key question is therefore their mass composition: the relative fractions of different atomic nuclei in the arriving cosmic-ray flux. The Pierre Auger Collaboration has now presented a new measurement of the depth of shower maximum, Xmax, using 17 years of fluorescence-detector data and an exposure 2.4 times larger than in the previous analysis.
The mean depth, ⟨Xmax⟩, shows a pronounced change in its evolution at about 1018.4 eV. Below this energy, the composition becomes lighter with increasing energy, while above it the trend reverses. At the same time, the event-to-event fluctuations, quantified by σ(Xmax), decrease from about 60 to 30 g/cm2, indicating a transition toward a heavier and less diverse composition.
Figure 1: Mean depth of shower maximum (left) and its fluctuations (right) as a function of energy. The lines show predictions from the EPOS-LHC-R and Sibyll2.3e hadronic-interaction models for proton and iron primaries. Together, the measurements indicate a change toward a heavier composition above 1018.4 eV.
A more detailed, model-dependent interpretation is obtained by fitting the measured Xmax distributions with mixtures of proton, helium, nitrogen and iron primaries. While the inferred fractions depend on the hadronic-interaction model, both models show the proton contribution peaking around 1018.1 eV and becoming very small above 1018.7 eV, beyond the feature in the cosmic-ray flux commonly referred to as the ankle. Intermediate-mass nuclei increasingly dominate toward higher energies. This evolution of the composition is found to be consistent across the northern and southern regions of the Observatory’s sky coverage.
Figure 2: Fractions of proton, helium, nitrogen and iron primaries inferred from the measured Xmax distributions for two hadronic-interaction models.
So, what are the highest-energy particles of the Universe made of?
The measurements indicate that they are not predominantly protons: toward the highest energies, the cosmic-ray flux is increasingly dominated by intermediate-mass and heavier nuclei.
Related Paper:
Depth of Maximum of Air-Shower Profiles above 10^17.7 eV Measured with the Fluorescence Detector of the Pierre Auger Observatory
The Pierre Auger Collaboration, Phys. Rev. D 114 (2026) 043016
[arxiv.org/abs/2605.12598] [doi: 10.1103/n616-15v5] [doi: 10.5281/zenodo.18024023]
