Synopsis

Still No Signs of Antihelium in Cosmic Rays

The dominance of matter over antimatter is further extended with a balloon experiment ruling out the presence of antihelium in cosmic rays at the lowest level to date.

NASA

Most of our universe appears to be matter rather than antimatter, but isolated pockets of antimatter could exist in the cosmos. However, new data from a balloon-borne experiment make this possibility seem less and less likely. The BESS collaboration has been looking for the antimatter equivalent of helium nuclei in cosmic rays. But as the collaboration reports in Physical Review Letters, such nuclei haven’t been detected, putting the most stringent limits yet on the antihelium abundance in Earth’s cosmological neighborhood.

Limits on the amount of antimatter that could exist near Earth have been set by satellite experiments that look for evidence of matter-antimatter annihilation in diffuse gamma rays (see 8 December 2011 Synopsis). However, we would have a hard time knowing if one of our neighboring galaxies were entirely made out of antimatter, since the light coming from such an antigalaxy would presumably be the same as from a matter-dominated galaxy. The only direct signal we might have is in cosmic rays, some of which likely originate from outside our galaxy. Previous studies have found anti-electrons and antiprotons in the cosmic-ray flux, but these can be produced in high-energy collisions of normal matter. If, however, a single nucleus made of antimatter could be detected, that would be strong evidence for antigalaxies.