The antimatter equivalent of helium nuclei has been produced by an international team of physicists working with the Relativistic Heavy Ion Collider at the U.S. Department of Energy’s Brookhaven National Laboratory in New York. Two University of California, Davis, professors are members of the team. A paper describing their results is published online this week by the journal Nature.

“This is the heaviest antimatter anyone has ever created,” said Manuel Calderon de la Barca Sanchez, professor of physics at UC Davis and an author of the paper. Authors also include Daniel Cebra, a professor of physics at UC Davis, and scientists from 54 other institutions in 12 countries.

The discovery will help physicists test theories about matter and antimatter, Calderon said. So far, the antihelium nuclei appear to have generally the same properties as regular helium, confirming existing theories, he said.

The physicists used Brookhaven’s Relativistic Heavy Ion Collider to smash gold particles into each other at almost the speed of light. The collisions briefly created a hot soup of subatomic particles called quarks and antiquarks, which then formed into new particles.

Sorting through data from almost a billion collisions, the research team found 18 examples of a stable antihelium-4 nucleus, made up of two antiprotons and two antineutrons.

If it were possible to “bottle” the antihelium particles, they would be as stable as regular helium, Calderon said; but, in practice, the particles fly through the accelerator until they hit a nucleus of regular matter and are annihilated.

One of the fundamental puzzles of modern physics is to understand why, if matter and antimatter were created in the Big Bang in equal amounts and annihilate each other when they meet, there was enough matter leftover to make up our universe.

“There is no process that we know that explains the amount of matter that we see in the universe,” Calderon said.

Understanding the rate at which antihelium is produced could help physicists interpret other experiments, including an instrument soon to be delivered to the International Space Station to search for antimatter in deep space.

The collaborators plan a second 10-week run on the collider to produce more antihelium particles this summer.

The research is supported primarily by the U.S. Department of Energy, as well as national and international collaborating institutions and many other funding agencies.