STAR Collaboration / RHIC / BNL
This image shows 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.
If the AMS comes up with different statistical balances of antimatter vs. matter, that would suggest that a cosmic source of antimatter somehow survived the big matter-vs.-antimatter blowup that scientists believe followed the big bang.
"The new measurement from the STAR experiment would provide the quantitative background rate for comparison," said Hank Crawford, a STAR collaborator from the University of California at Berkeley, said today in a news release. "An observation of antihelium-4 by the AMS experiment could indicate the existence of large quantities of antimatter somehow segregated from the matter in our universe."
Such findings could shed further light on one of the big mysteries about the universe's origins: How is it that matter won out over antimatter, resulting in the cosmos as we see it today? Is it possible that we merely live in a localized zone of the universe where matter dominates? Could there be huge reservoirs of antimatter, on the other side of a DMZ (dematterized zone)? During the next decade, the Alpha Magnetic Spectrometer just might help scientists put together more pieces of the matter-antimatter puzzle.
More about the antimatter mystery:
- Exotic antimatter particle exhibits new conduct
- Antimatter atoms caught at last
- Weird antimatter particles discovered
- Could X particle solve two puzzles?
Correction for 3:10 p.m. ET April 25: I initially had the constituents of the nucleus wrong in the second paragraph. It's two antiprotons and two antineutrons, not two antiprotons and two antiprotons. Sorry about that!
Hundreds of scientists in the STAR Collaboration equally share the credit for the research reported in the Nature paper, "Observation of the Antimatter Helium-4 Nucleus." A version of the paper was made publicly available on the arXiv.org website.
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