By Geoff Brumfiel
Physicists have rooted through a morass of collisions to find the heaviest antimatter nucleus yet inside one of their particle accelerators.
Collisions between gold nuclei at the Relativistic Heavy Ion Collider (RHIC) on Long Island, New York, have yielded heavy isotopes of antihydrogen that include a subatomic particle known as an antistrange quark, which is heavier than less unusual up or down quarks. The extra mass of the exotic antiquark is enough to make this antihydrogen isotope heavier than the previous record-holder, antihelium. Further studies of the new antinuclei may provide information about the cores of neutron stars, or even insight into the earliest days of the Universe. The work appears online today in the journal Science1.
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Few pieces of science fact come as close to science fiction as antimatter. Antimatter particles carry the same mass as normal matter, but the opposite charge. When matter and antimatter collide, they annihilate in a flash of energy.
Paul Dirac first theorized antimatter's existence in 1928, and since then researchers have studied antimatter particles created by nuclear decays and high-energy collisions of normal matter. Today, positrons -- antielectrons -- are even used in some kinds of medical imaging.
But antiatoms made up of antiprotons and antineutrons are still a rarity. Because we live in a world dominated by regular matter, antiprotons and antineutrons typically annihilate before they can form into antinuclei. To date, only a handful of groups have successfully coaxed antiparticles into atomic configurations.
Antimatter recipe
To make the antiatoms requires two conditions, says Declan Keane, a physicist at Kent State University in Ohio and a member of the STAR Collaboration, which discovered the new nuclei. First, you need enormously high energies to generate the antimatter. Second, you need enough of the stuff around that it has a chance to meet other antimatter particles and form atoms before it annihilates.
RHIC is perfect for this kind of work, Keane says. The collider smashes gold nuclei together at 200 gigaelectronvolts, an energy that approaches the earliest moments following the Big Bang. When they hit, nuclei quickly dissolve into a soup of quarks and antiquarks, the fundamental particles that make up protons and neutrons.
To form the new antihydrogen isotope, first an antistrange quark binds with an antiup and antidown quark to make an antilambda -- an antineutron-like particle. The antilambda, which is fractionally heavier than a neutron, must then combine with a conventional antineutron and an antiproton. The chances of this happening are very slim: out of 100 million collisions, RHIC generated just 70 of the new antihydrogen isotopes.
The little bang
The data "literally looked like haystacks", Keane says. To find the new antihydrogen, Jinhui Chen of the Shanghai Institute of Applied Physics, and Zhangbu Xu of Brookhaven National Laboratory, where RHIC is housed, developed sophisticated software that could pick out the new antinucleons.
Each one tipped the scales at just over 5.3x10-27 kilograms (2.991 gigaelectronvolts/c2) -- a vanishingly small amount, but with antihelium weighing in at 4.8x10-27 kg (2.72 GeV/c^2), still heavy enough to make this antihydrogen isotope the heaviest antinucleus discovered up to this point. The isotope didn't stick around for long however -- the half life of the antinuclei was just a few hundred trillionths of a second.
"The production of these 'strange' anti-nuclei is not really a surprise," says Jürgen Schukraft, a physicist on the ALICE heavy-ion experiment at CERN, Europe's particle-physics laboratory near Geneva, Switzerland. Nevertheless, he says, they will be interesting to study. Neutron stars, the remnants of once-massive stars, are thought to contain large numbers of strange quarks, and antinuclei containing strange quarks could play a part in their evolution. Moreover, studying the properties of antinuclei such as these might help physicists to better understand why the Universe is full of matter rather than antimatter.
The Big Bang may have given us plenty of regular matter to work with, Schukraft says, but "the little bang at RHIC has now delivered some of their most exotic antimatter partners for scientific investigation".
