Scientists from the Relativistic Heavy Ion Collider’s (RHIC) Star Collaboration have unveiled the heaviest antimatter nucleus ever detected, which can enhance our understanding of the universe.
“I can see us checking matter-antimatter symmetry more precisely in the future with antihyperhydrogen-4, and we can discover other heavier antimatter (hyper)nuclei,” Hao Qiu, a STAR physicist, told Interesting Engineering .
This antinucleus, antihyperhydrogen-4, is composed of four antimatter particles: an antiproton, two antineutrons, and one antihyperon.
Challenge of detection
The detection of antihyperhydrogen-4 was a major feat. The STAR physicists focused on the tracks left behind by the particles resulting from the decay of this unstable antihypernucleus.
One of these decay products is the previously discovered antihelium-4 nucleus; the other is a positively charged particle called a pion.
However, distinguishing these decay signals from the cacophony of other particle interactions presented a significant challenge.
The researchers had to sift through billions of collision events, each with thousands of particle tracks, to pinpoint the rare instances where these two particles originated from the same point, signaling the decay of an antihypernucleus.
“First, we need to identify the decay daughters – antihelium-4 and pi+. We do this by using their energy loss vs. momentum relationship in the gas of the time projection chamber, and by using the mass^2 calculated from their momentum, track length, and time of flight,” Qiu explained to IE.
“Then, we need to reconstruct the decay vertices and reject most of the random combination background by requiring the decay vertices to be away from the collision point, because most background particles come directly from the collision point,” he added.
Overcoming the noise and comparative analysis
The researchers faced the challenge of distinguishing these decay signals from the overwhelming background noise of other particle interactions.
“The background is huge and the signal is very small. We find ~16 signal counts out of 6 billion events, each having thousands of tracks. We have tried many things – different datasets, different daughter particle identification criteria, different decay vertex reconstruction methods… In the 1st year we didn’t find a good signal, but we kept trying and did it,” highlighted Qiu.
With the discovery of antihyperhydrogen-4, the STAR team embarked on a direct comparison between matter and antimatter.
They meticulously measured the lifetime of antihyperhydrogen-4 and compared it to that of its matter counterpart, hyperhydrogen-4. Additionally, they compared the lifetimes of another matter-antimatter pair: the antihypertriton and the hypertriton.
Notably, the results of these comparisons were quite intriguing. No significant differences in lifetimes were observed, a finding that aligns with current theoretical models.
Precise approach
The identification of 22 candidate events for antihyperhydrogen-4, with an estimated background of 6.4, highlights the precision of their approach.
“We form the background invariant-mass distribution by rotating the anti helium-4 daughter by a random angle. So the signal invariant mass peak will be destroyed and a continuous background invariant-mass distribution shape can be obtained,” he asserted.
“We then do some normalization so that the rotational background match the candidate invariant-mass distribution outside the signal peak region. With this we can estimate the background. It has some statistical and systematic uncertainties, which are not large and properly considered,” he concluded.
While this may seem unremarkable, it serves as a crucial confirmation of the validity of these models. Had a discrepancy been found, it would have necessitated a reevaluation of the fundamental understanding of physics.
Although the discovery of antihyperhydrogen-4 does not directly solve the matter-antimatter asymmetry puzzle, it marks a significant stride in our understanding of antimatter.
Future research at RHIC and other cutting-edge facilities will continue to probe the mysteries of antimatter, inching us closer to unraveling the enigma of why our universe is dominated by matter. As scientists continue to push the boundaries of knowledge, the elusive answer to this fundamental question may one day be within reach.
Critical role of RHIC
The existence of antimatter has long captivated physicists, posing a fundamental question about the nature of our universe.
While the prevailing theory suggests that both matter and antimatter were created in equal amounts during the Big Bang, our universe is overwhelmingly composed of matter.
RHIC, often referred to as an “atom smasher,” serves as a powerful tool for recreating the conditions of the early universe.
By colliding heavy ions at near-light speeds, RHIC generates a maelstrom of thousands of new particles, including matter and antimatter, in near-equal proportions. This unique environment provides an ideal laboratory for studying antimatter.
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