Antiprotons in the cosmic radiation

Abstract

Golden et al.¹ have demonstrated that the measured ratio of the flux of antiprotons to protons (p̄/p ratio) is significantly greater than expected from the commonly adopted ‘leaky box’ model of cosmic ray propagation, a model in which the p̄'s are secondaries generated by the interaction of cosmic rays with the interstellar medium (ISM) in the Galaxy. At the energy at which measurements were made (kinetic energy≃8 GeV), disturbing effects due to interplanetary modulation are negligible and there is now general agreement^2–5 that the excess is a factor of 3 at least (an earlier calculation⁶ of the expected p̄/p which gave a result near to observation has been shown to be incorrect^2–5). Bogolomov⁷ found a similar but statistically less compelling enhancement at somewhat lower energies. A prominent characteristic of the p̄/p ratio for galactic secondaries is a rapid fall with decreasing energy, below≃5 GeV, due to kinematic factors. It was therefore remarkable that Buffington et al.⁸ recently reported a ratio only a factor of 2.5 smaller at a very low kinetic energy (average≃0.2 GeV). This result, if correct, necessitates serious consideration of more exotic explanations. An important feature at these very low energies is the modulation of the flux by the interplanetary field and we have now examined this facet. We describe an improved calculation of the energy spectrum of p̄ expected for the standard leaky box model and show that even the enhanced secondary p̄ production in the ‘closed Galaxy’ model of Peters and Westergaard⁹ fails by a wide margin to reproduce the high observed flux at low energies. A more radical departure from conventional thinking is to regard the observed p̄s as primaries; a universal baryon-symmetric model and a black hole evaporation model are considered.