Posted on behalf of Zeeya Merali.
April’s turning out to be a good month for antimatter aficionados. Nature reports online today the production of 18 particles of antihelium-4, the heaviest antiparticle yet produced, at the Relativistic Heavy Ion Collider (RHIC) on Long Island, New York. Four more of the hefty antiparticles have also graced CERN’s Large Hadron Collider, in Geneva, Switzerland, with their presence, according to an announcement last week. If that’s not enough, on 29 April, NASA plans to send the antihelium-hunting AMS-02 spectrometer to the International Space Station on the space shuttle Endeavour.
We blogged about RHIC’s anti-achievement earlier this year, when the collider’s STAR collaboration posted a preprint describing the production of 6.6 x10-27 kilograms (3.73 gigaelectronvolts/c2) of the antiparticle – made up of two antiprotons and two antineutrons – when they smashed together gold nuclei. Playing catch-up, the LHC’s ALICE collaboration announced that they had created four antihelium nuclei after sieving through data acquired in November 2010 from lead-atom collisions. The ALICE results will discussed in detail at the Quark Matter Conference in Annecy, France, in May.
Matter and antimatter particles should have been produced at the same rate in the early universe, and by rights they should have completely annihilated with each other, leaving nothing behind to form stars, galaxies and people. So both colliders are looking for any slight differences in the behaviour of particles and their antimatter counterparts, to provide clues for why so much more matter hung around after the big bang, while so little antimatter survived.
It’s tough to generate and examine heavy antiparticle nuclei in accelerators, however, since antiprotons and antineutrons usually annihilate with matter before clubbing together. The low antihelium production rate (about one nucleus for every billion collisions at RHIC) puts a damper on prospects for creating yet heavier nuclei to fill out the anti-periodic table, says Michael Doser, an expert on antimatter at CERN. Harder still will be attempting to catch and hold heavy antinuclei. Last year, the ALPHA collaboration at CERN reported trapping 38 antihydrogen atoms, which was a struggle.
But the meagre numbers of antihelium nuclei seen will help set a benchmark for the rate of antihelium production that scientists studying the AMS-02 spectrometer data should expect to see in space. “If they see an excess of antihelium that could indicate that our galaxy contains antimatter stored away somewhere – in antimatter stars,” says Doser. “That result will certainly spark a lot of discussion.”