Neutron oscillation as a source of excess sub-GeV antiprotons in galactic cosmic rays

Abstract

The recent detection^1,2 of an antiproton/proton (p̄/p) ratio at energies much less than 1 GeV is several orders of magnitude above that predicted for p̄ production from primary cosmic ray collisions. It is well recognized that there is no mechanism to explain this sub-GeV (130–320 MeV) p̄ excess although many suggestions have been made. For example, Eichler³ considers secondary production in high-energy collisions while Kiraly⁴ et al. suggest black hole evaporation as a p̄ source. Most of these suggestions have inherent difficulties such as the imposition of severe constraints on the sources to suppress excessive γ-ray production. We postulate here that the phenomenon of neutron– antineutron (nn̄) oscillations as a signature of grand unified theories may operate in neutron-rich astrophysical sources such as supernovae to produce the required low-energy galactic antiproton background.