Search for Antihelium with the BESS-Polar Spectrometer
  In two long-duration balloon flights over Antarctica, the BESS-Polar
collaboration has searched for antihelium in the cosmic radiation with higher
sensitivity than any reported investigation. BESS- Polar I flew in 2004,
observing for 8.5 days. BESS-Polar II flew in 2007-2008, observing for 24.5
days. No antihelium candidate was found in BESS-Polar I data among 8.4\times
10^6 |Z| = 2 nuclei from 1.0 to 20 GV or in BESS-Polar II data among 4.0\times
10^7 |Z| = 2 nuclei from 1.0 to 14 GV. Assuming antihelium to have the same
spectral shape as helium, a 95% confidence upper limit of 6.9 \times 10^-8 was
determined by combining all the BESS data, including the two BESS-Polar
flights. With no assumed antihelium spectrum and a weighted average of the
lowest antihelium efficiencies from 1.6 to 14 GV, an upper limit of 1.0 \times
10^-7 was determined for the combined BESS-Polar data. These are the most
stringent limits obtained to date.
