Search for Primary Antiparticles and Cosmological Antimatter with BESS

The Balloon-borne Experiment with a Superconducting Spectrometer (BESS) has performed precise measurements of low-energy cosmic-ray antiproton spectra to investigate signatures of exotic origins such as dark matter candidates or primordial black holes, and has searched for antimatter in cosmic rays that might reach Earth from antimatter domains formed in the early Universe. BESS carried out its second Antarctic flight, BESS-Polar II, during the 2007–2008 Austral Summer, and obtained over 4.7 billion cosmic-ray events from 24.5 days of observation during a solar-minimum period, when the sensitivity of the low-energy antiproton measurements to a possible primary source would be greatest. Here, we report the scientific results.