Observation of Cosmic-Ray Antiprotons at Energies below 500 MeV

A search for cosmic-ray antiprotons was performed using a balloon-borne experiment with a superconducting magnetic rigidity spectrometer (BESS). We have detected four antiprotons in the kinetic energy range between 300 and 500 MeV, the first antiprotons to be identified by a direct mass measurement. The corresponding $\frac{\overline{p}}{p}$ flux ratio at the top of the atmosphere is found to be ${1.2}_{-0.65\mathrm{}}^{+1.0\mathrm{}}$ × ${10}^{-5\mathrm{}}$. No antiprotons were observed in the range between 175 and 300 MeV, resulting in a 90% C.L. upper limit on the flux ratio of 2.9 × ${10}^{-5\mathrm{}}$.