US team discovers heaviest antimatter yet

Posted by Emma Woollacott

Scientists at the US Department of Energy's Brookhaven National Laboratory have made the heaviest antimatter ever found.   

The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) recreates conditions immediately after the Big Bang, and has now successfully created eighteen examples of the nucleus of antihelium-4.

"STAR already holds the record for massive antiparticles, last year having identified the anti-hypertriton, which contains three constituent antiparticles," says spokesperson Nu Xu of Berkeley Lab.

"With four antinucleons, antihelium-4 is produced at a rate a thousand times lower yet. To identify the 18 examples required sifting through the debris of a billion gold-gold collisions."

For some reason, the equal amounts of matter and antimatter created in the Big Bang didn't completely annihilate one another, leaving us with the visible universe made of normal matter.

At RHIC, roughly equal amounts of matter and antimatter are produced, and do indeed wipe each other out. But the fireballs created expand and cool quickly, so the antimatter lasts long enough to be detected.   

The most common antiparticles are generally the least massive, because it takes less energy to create them.

"It's likely that antihelium will be the heaviest antiparticle seen in an accelerator for some time to come," says Xiangming Sun of Berkeley Lab.

"After antihelium, the next stable antimatter nucleus would be antilithium, and the production rate for antilithium in an accelerator is expected to be well over two million times less than for antihelium."

Meanwhile, the Alpha Magnetic Spectrometer (AMS) experiment, scheduled to be launched soon to the International Space Station, is designed to hunt for distant galaxies made entirely of antimatter.

"Collisions among cosmic rays near Earth can produce antimatter particles, but the odds of these collisions producing an intact antihelium nucleus are so vanishingly small that finding even one would strongly suggest that it had drifted to Earth from a distant region of the universe dominated by antimatter," says Hans Georg Ritter of Berkeley Lab.

"Antimatter doesn't look any different from ordinary matter, but AMS finding just one antihelium nucleus would suggest that some of the galaxies we see are antimatter galaxies."