Scientists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) have discovered a new kind of antimatter nucleus, the heaviest ever detected. The nucleus, known as antihyperhydrogen-4, is composed of four antimatter particles: an antiproton, two antineutrons, and one antihyperon.
Members of RHIC’s STAR Collaboration utilized their particle detector to analyze collision debris. Their findings are reported in the journal Nature. "Our physics knowledge about matter and antimatter is that, except for having opposite electric charges, antimatter has the same properties as matter — same mass, same lifetime before decaying, and same interactions," said Junlin Wu, a graduate student at Lanzhou University and Institute of Modern Physics. Despite this understanding, our universe predominantly consists of matter rather than antimatter.
"Why our universe is dominated by matter is still a question, and we don’t know the full answer," Wu added.
RHIC's collisions recreate conditions similar to those just after the Big Bang. The collider accelerates heavy ions close to light speed, generating thousands of new particles from the resulting quark-gluon soup. This environment produces matter and antimatter in nearly equal amounts. Comparing these particles might offer clues to why matter dominates the universe.
“To study the matter-antimatter asymmetry, the first step is to discover new antimatter particles,” said Hao Qiu, Wu’s advisor at IMP.
STAR physicists previously observed other antimatter nuclei created in RHIC collisions. In 2010 they detected antihypertriton and in 2011 antihelium-4. Recent analyses suggested that detecting antihyperhydrogen-4 might be possible despite its instability.
“It is only by chance that you have these four constituent particles emerge from the RHIC collisions close enough together that they can combine to form this antihypernucleus,” said Lijuan Ruan from Brookhaven Lab.
To find antihyperhydrogen-4, scientists analyzed decay products: antihelium-4 nucleus and a pion (pi+). “Since antihelium-4 was already discovered in STAR, we used the same method used previously to pick up those events and then reconstructed them with pi+ tracks to find these particles,” Wu explained.
The STAR team sifted through billions of collision events to identify 22 candidate events with an estimated background count of 6.4. "That means around six of the ones that look like decays from antihyperhydrogen-4 may just be random noise," said Emilie Duckworth from Kent State University.
Subtracting this background gives confidence that approximately 16 actual antihyperhydrogen-4 nuclei were detected.
The results allowed for direct comparisons between matter and antimatter lifetimes without significant differences found. These experiments tested strong symmetry forms which physicists believe would rarely show violations concerning matter-antimatter imbalance.
“If we were to see a violation of [this particular] symmetry, basically we’d have to throw a lot of what we know about physics out the window,” Duckworth noted.
The next step involves measuring mass differences between particles and antiparticles—a task Duckworth is pursuing with DOE funding support.
This research was supported by multiple institutions including DOE Office of Science, U.S. National Science Foundation among others listed in their scientific paper. They utilized resources at Brookhaven Lab’s Scientific Data Center and NERSC at Lawrence Berkeley National Laboratory.