ANTIHELIUM IN COSMIC-RAYS - A NEW UPPER LIMIT AND ITS SIGNIFICANCE

We have searched for antihelium in the galactic cosmic rays using data obtained on the flight of the Balloon-borne Experiment with a Superco nducting Spectrometer (BESS) launched from Lynn Lake, Manitoba, in nor thern Canada on 1995 July 25. The balloon reached an altitude of 36.5 km with a residual overburden of 5 g cm(-2). The total observation tim e was 12.2 kr with a live-time fraction of 0.61. No antihelium was obs erved after selections based on dE/dx and event quality cuts at rigidi ties between 1.6 and 16 GV/c; we infer that 536,420 helium nuclei surv ived the same cuts. This result leads to an upper limit to the <(He)ov er bar>/He abundance ratio of 8.1 x 10(-6) (95% confidence level), a f actor of 2.7 over the lowest previous limit. Because this limit is for the first time at the level where predicted limits become astrophysic ally interesting (Ahlen et al.), we have reexamined the transport of c osmic rays in intergalactic space. We show that little can be learned about distant (> 10 Mpc) domains of antimatter from a null result.