Search for Antideuterons of Cosmic Origin Using the BESS-Polar II Magnetic-Rigidity Spectrometer

We searched for antideuterons (d's) in the 4. 710^9 cosmic-ray events observed during the BESS-Polar II flight at solar minimum in 2007--2008 but found no candidates. The resulting 95% C. L. upper limit on the d flux is 6. 710^-5 (m^2 s sr GeV/n) ^-1 in an energy range from 0. 163 to 1. 100 GeV/n. The result has improved by more than a factor of 14 from the upper limit of BESS97, which had a potential comparable to that of BESS-Polar II in the search for cosmic-origin d's and was conducted during the former solar minimum. The upper limit of d flux from BESS-Polar II is the first result achieving the sensitivity to constrain the latest theoretical predictions.