It would certainly be convenient for me if it is a proton, given the experiments I work on (protons at the highest energies -> more ultrahigh energy neutrinos -

It would certainly be convenient for me if it is a proton, given the experiments I work on (protons at the highest energies -> more ultrahigh energy neutrinos -> maybe my experiments will see something!), but, unfortunately for me, I don't think we know that. Nuclear species composition can typically only be done statistically, and it looks like this wasn't even measured in the fluorescence detector, which would give the best measurement of the penetration depth of the particle in the atmosphere (the best handle on particle id). In,the journal article, which admittedly I've just skimmed, they give direction reconstructions giving different species assumptions (and Milky Way models, which are important for heavier nuclei) and say that while they can exclude a photon (which... would be weird!), without data from the fluorescence detector, they can't tell the difference between a proton and a heavier nucleus. Though given how TA mass-composition measurements turn light at the highest energies, they may perhaps privately argue it's likely a proton. But, alas, Pierre Auger Observatory would argue otherwise. I'll ask Toshihiro what he thinks next time I see him... It is intriguing that the smaller TA has seen more high-energy events than the bigger PAO. Maybe there really are big differences between the northern and southern hemisphere at play here...