The Universe seems to be made of matter rather than antimatter, but our current understanding of cosmology does not exclude the possibility of there being small regions of antimatter out there. However, new observations reported by a Japan–US collaboration using the Balloon-borne Experiment with a Superconducting Spectrometer (BESS) show no sign of antihelium in cosmic rays, setting an even lower limit on the possible abundance of antimatter (K. Abe et al. Phys. Rev. Lett. 108, 131301; 2012).

Credit: © BESS-POLAR COLLABORATION

The asymmetry between matter and antimatter is one of the fundamental puzzles in modern physics. In the Big Bang, equal amounts of matter and antimatter should have been created, and would have annihilated each other except that some kind of symmetry breaking between particles and antiparticles seems to have led to the disappearance of antiparticles at an early stage in the history of the Universe. Pockets of primordial antimatter could still exist, but finding them isn't easy. Matter and antimatter emit photons of the same wavelength, so light from distant galaxies does not provide much of a clue, although annihilation occurring at the boundaries with normal matter regions would show clear gamma-ray signatures — signatures that have not yet been seen.

Simple antiparticles such as antiprotons can, however, be created in high-energy collisions of normal matter; BESS and other spectrometers have recorded many thousands of them in cosmic rays and large numbers of them can be created for use in accelerators (such as in CERN's Antiproton Decelerator). But more-complex antiparticles such as antinuclei must originate from antimatter regions of space: the discovery then of even a single antinucleus heavier than hydrogen would have significant impact in cosmology. But how do you look for an atom-sized needle in a haystack the size of the Universe?

Since 1993, BESS (pictured) has been hunting for signs of antihelium in cosmic rays, carrying aloft a magnetic spectrometer with time-of-flight and Čerenkov-radiation detectors to identify helium and possible antihelium nuclei through determination of their mass and charge. The BESS Polar I and Polar II missions flew over Antarctica in 2004 and 2007–2008, collecting more than a month's worth of data. Analysing the large data set has, however, revealed no antihelium in the cosmic rays, implying that antihelium is at least ten million times less abundant than its normal matter twin.