Origin of the ankle in the ultra-high energy cosmic ray spectrum and of the extragalactic protons below it

Abstract

The sharp change in slope of the ultra-high energy cosmic ray (UHECR) spectrum around 10^{18.6} eV (the ankle), combined with evidence of a light but extragalactic component near and below the ankle which evolves to intermediate composition above, has proved exceedingly challenging to understand theoretically. We show that for a range of source conditions, photo-disintegration of ultra-high energy nuclei in the region surrounding a UHECR accelerator naturally accounts for the observed spectrum and composition of the entire extragalactic component, which dominates above about 10^{17.5} eV. The mechanism has a clear signature in the spectrum and flavors of neutrinos.

Contribution to ICRC 2015. Full exposition (Unger, Farrar and Anchordoqui, accepted for Phys. Rev. D) can be found in arXiv:1505.02153