To make an antihelium near Earth, cosmic-ray collisions would have to create four particles--two antiprotons and two antineutrons--at nearly the same time.
One antihelium nucleus would be created for every million trillion or so cosmic-ray protons, which themselves are rare.
If they did see an antihelium, it would be very dramatic evidence" that distant antimatter galaxies do exist, Stecker says.
NEEDLE IN A HAYSTACK The search for cosmic antihelium began in 1979.
Scientists knew that stumbling upon an antihelium was a long shot.
With stronger magnets, larger collecting areas, longer flights, and improved electronics, these experiments had steadily increasing sensitivity and detected more and more antiprotons, but none detected antihelium.
They calculated its sensitivity at one antihelium among roughly 100 million nuclei of normal helium.
If it ever flies, AMS-02 will improve the sensitivity of the search to about one antihelium in a billion helium nuclei.