Up against the Andes in the high plains of Argentina, the 3000-km2 Pierre Auger Observatory has, since 2004, been recording the particle and fluorescence showers initiated by ultrahigh-energy cosmic-ray (UHECR) particles—mostly protons and fully ionized nuclei — hitting the top of the atmosphere. Such an enormous expanse, studded with 1600 surface detectors watched over by four fluorescence-telescope complexes, is appropriate because UHECRs become ever more scarce with increasing energy. Above 1019 eV, one can expect to find only a few dozen CRs per square kilometer per century.

A lot has been learned in the past decade from Auger and similar, albeit smaller, facilities worldwide. But a newly reported Auger result challenges much of what had become conventional wisdom. 1 And it has sent astrophysicists and particle theorists scrambling for explanations.

Seeking to determine the nuclear identities of the highest-energy CRs, the Auger collaboration examined the development of their...

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