Nimrod Shaham, Tsvi Piran
The simple interpretation of the penetration depth measurements by PAO of UHECRs at the energy range 1.2\times10^18 - 3.5\times10^19 eV suggests a composition change from protons to heavier nuclei at this energy range. However, we show that a detailed comparison of these data with air shower simulations poses serious problems. First, we show that combinations of protons and Fe; protons, He and Fe and even protons, He, N and Fe are inconsistent with both the mean and RMS penetration depth data. Then, we derive a robust upper bound for the proton fraction of the UHECRs flux, and show that it implies extremely high metallicities. To demonstrate the problem, we present a simple model for the UHECRs spectrum and composition taking into account acceleration and propagation effects and estimate the source's spectral index and composition. We show that the observations requires a Fe to protons number ratio of 1:50 at the source, as well as a very hard spectrum. The lack of natural sources with such a metallicity combined with the hard spectral index and the overall incompatibility of the full data set with the simulations reveal a serious problem. Assuming that the observations and simulations are correct we conclude that the input physics is wrong and that the results points towards new physics that modifies the baryonic interactions at CM energy of a few dozens TeV, at which UHECRs collisions take place.
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http://arxiv.org/abs/1204.1488
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