The case for antiparticles in the extragalactic cosmic radiation

The presence of an excess of low-energy antiprotons in the primary cosmic radiation1–3 has given rise to several possible explanations4, some of which involve exotic processes such as mini-black holes and extragalactic antiparticles. Here we consider the latter possibility5 and show that there are interesting implications for the cosmic radiation at higher energies. Indeed, it may be possible to account for a previously puzzling feature of the cosmic ray spectrum—a ‘bump’ in the range 1014–1015 eV—by hypothesizing a primary extragalactic origin for the bulk of the observed cosmic ray antiprotons, although such an explanation is not unique. In this model most of the cosmic rays above 1015 eV are extragalactic. We prescribe a method of testing this hypothesis experimentally.