Most people know two things about helium. One is that it makes your voice comically high-pitched when you inhale it; the other is that it is extremely light, which is why balloons filled with the stuff float upward through the heavier air. But in particle physics terms—and especially when it comes to the nuclear physics of antimatter—helium is no lightweight. With two protons and two neutrons, ordinary helium is four times as massive as hydrogen, the lightest element. (Both hydrogen and helium have other stable isotopes—atomic varieties with differing masses—but they are rare in nature.)

The realm of antimatter is a sort of shadow world in which the particles of our matter-dominated world have mutually annihilating counterparts—the electron has an antimatter partner in the positron, the proton has the antiproton, the neutron has the antineutron, and so forth. The big bang should have produced copious amounts of both matter and antimatter, but the latter is mysteriously rare in our experience, and physicists and cosmologists would like to know why. To investigate this seeming asymmetry of nature, scientists have manufactured subatomic antiparticles in high-energy collisions for decades and have even managed to produce short-lived nuclei and atoms of antimatter.

But those antinuclei and anti-atoms are difficult to corral—they annihilate in a burst of energy on contact with ubiquitous ordinary matter—and have only been created in the most rudimentary form, as tiny groupings of antiprotons, antineutrons and sometimes positrons. Now a research group using a particle collider at Brookhaven National Laboratory in Upton, N.Y., has produced the most massive antimatter assemblies yet: two antiprotons and two antineutrons, which together constitute the antimatter twin to the helium 4 nucleus, also known as the alpha particle. (Helium 4 is normal helium; an antimatter counterpart had already been observed for the rarer, lighter helium 3 isotope, which has two protons and one neutron.)

By sifting through the particulate wreckage of a billion smashups between gold ions, each traveling at 99.995 percent the speed of light in Brookhaven's Relativistic Heavy-Ion Collider before crashing together inside the STAR detector, the researchers identified 18 separate nuclei of antihelium 4. (The collider is known as RHIC for short, and STAR is an acronym for Solenoidal Tracker at RHIC.) The antinuclei, once created in the collisions, quickly annihilated against ordinary matter in the detector and vanished. The researchers announced their finding March 16 in a paper posted to the physics preprint Web site

The study's authors have submitted the paper to Nature, which has a strict policy of media silence prior to publication. So, they did not want to discuss their work. (Scientific American is part of Nature Publishing Group.)

But independent physicists call the result an impressive, if not entirely surprising, experimental coup. "It is an enormous technical achievement that they can extract these rarely produced objects," says Tom Cohen, a nuclear physicist at the University of Maryland, College Park. "But everybody believed—I should almost say knew—that anti-alpha particles could exist." Cohen compares the feat with climbing the world's tallest mountain: "It's really impressive that you can do it, but the fact that there's a summit to Mount Everest is not a big surprise."

Another physicist, who wished to remain anonymous because he had been asked to refrain from commenting publicly on the results, echoed Cohen's reaction. "It's really, really very impressive that they're able to do that, to see these rare events and convincingly isolate them," he says. "What they've found is that there is no shock; it's where it's predicted to be."

What would be a shock would be some deviation in the way antimatter behaves as compared with matter, which might help explain why our cosmic surroundings are dominated by matter and almost bereft of antimatter. Particle colliders such as the Large Hadron Collider outside Geneva are now pushing closer every year to big bang–like energy levels to look for such hints of new physics. And complementary experiments at lower energies have managed to produce and then trap—however briefly—atoms of antihydrogen with an eye toward making precision measurements of the anti-atoms' properties. But so far the cause of the matter–antimatter asymmetry remains an open question.

As for the STAR collaboration's newfound antihelium 4 nuclei, they are likely to hold the crown of most massive antimatter tidbits for some time. The next stable nucleus on the periodic table is lithium 6, with three protons and three neutrons; it, too, will have an antimatter counterpart. But the STAR researchers note that as rare as antihelium 4 nuclei are, revealing themselves only 18 times in one billion collisions, antilithium 6 is much rarer still. Its expected rate of production is roughly one millionth that of antihelium 4, leaving it beyond the reach of today's accelerators.