Heaviest antimatter found

... a three-dimensional rendering of the STAR time projection chamber surrounded by the time-of-flight barrel (the outermost cylinder). Particle tracks spray out from the collision, including a meter-long track from an antihelium-4 nucleus (highlighted in bold red)

The reports began circulating a few weeks ago, and today's publication in the journal Nature makes it official: Physicists have detected the heaviest bits of antimatter ever found on Earth. And that record is likely to stand for a long, long time.

Members of the STAR collaboration at the Relativistic Heavy Ion Collider, based at Brookhaven National Laboratory in New York, say they've seen the traces of 18 nuclei of antihelium-4 among about half a trillion particles produced by almost a billion gold-ion collisions at RHIC. These nuclei are like regular helium nuclei, except that instead of having two protons and two neutrons, they have two negatively charged antiprotons and two antineutrons.

The particles existed for only about 10 billionths of a second before they came in contact with ordinary matter particles and were annihilated, but that was long enough to register on STAR's detectors. Physicists can routinely produce antihydrogen nuclei (basically, antiprotons), and last year a research team reported the first detection of antihydrogen atoms (a positron going around an antiproton). Scientists have even detected antihelium-3 nuclei (two antiprotons and an antineutron). But until now, antihelium-4 has eluded them.

RHIC is best-known for smashing together gold ions so forcefully that particles like protons shatter into their constituent quarks and gluons, producing the kind of primordial soup that existed just an instant after the big bang. When that soup congeals, all sorts of combinations of quarks come together — and statistically, there's an ever-so-slight chance that the quarks will arrange themselves into two antiprotons paired with two antineutrons. The odds of that happening are so vanishingly small that RHIC's researchers had to sift through mountains of data to find the 18 events they were looking for.

The bad news is that the chances of finding anything even heavier are even more vanishingly small. So small, in fact, that physicists don't expect to detect them anytime in the foreseeable future, at RHIC or even at Europe's Large Hadron Collider.

The good news is that these 18 detections confirm the statistical model that theorists expected to see for the creation of antimatter in the lab.

Searching for natural-born antimatter in outer space is one of the top jobs for the $2 billion Alpha Magnetic Spectrometer, which is due to be delivered to the International Space Station a week from now. The AMS should be able to detect antihelium nuclei and other subatomic oddities during its years-long run in orbit.

via Cosmic Log - It's official: Heaviest antimatter found.