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.
The Balloon-borne Experiment with Superconducting Spectrometer (BESS) is a joint Japan-U.S. project that has been carrying out antimatter searches since 1993 (as well as measurements of cosmic-ray antiprotons and normal matter nuclei). Its last two balloon flights in 2003 and 2007–2008 flew over the Antarctic and collected over billion cosmic-ray events in total. The team has now finished sifting through the data and has found no signs of antihelium. The implication is that helium outnumbers antihelium by at least million to one. – Michael Schirber