* Astronomy

Title: Search for Antihelium with the BESS-Polar Spectrometer
Authors: K. Abe, H. Fuke, S. Haino, T. Hams, M. Hasegawa, A. Horikoshi, A. Itazaki, K.C. Kim, T. Kumazawa, A. Kusumoto, M.H. Lee, Y. Makida, S. Matsuda, Y. Matsukawa, K. Matsumoto, J.W. Mitchell, Z. Myers, J. Nishimura, M. Nozaki, R. Orito, J.F. Ormes, K. Sakai, M. Sasaki, E.S. Seo, Y. Shikaze, R. Shinoda, R.E. Streitmatter, J. Suzuki, Y. Takasugi, K. Takeuchi, K. Tanaka, N. Thakur, T. Yamagami, A. Yamamoto, T. Yoshida, K. Yoshimura

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 x 10^6 |Z| = 2 nuclei from 1.0 to 20 GV or in BESS-Polar II data among 4.0 x 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 x 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 x 10^-7 was determined for the combined BESS-Polar data. These are the most stringent limits obtained to date.

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