It used to be natural to expect [111] that indeed the highest energy cosmic rays are protons. However, this is changing
rather rapidly in light of recent dedicated studies using Auger data [7, 242
, 509
], which favor a significant contribution from
heavy nuclei. The implications for the Lorentz symmetry analysis of the differences between protons and heavy nuclei, while
significant in the detail (see, e.g., Ref. [488]), are not as large as one might naively expect. This is due to the fact that it just
happens to be the case that the photodisintegration threshold is reached when the energy of typical heavy nuclei, such as Fe,
is
, i.e., just about the value of the photopion-production threshold, expected for cosmic-ray
protons.