New findings from the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) have provided compelling evidence that scientists can recreate an extraordinary form of matter from the earliest moments of the universe using surprisingly small atomic collisions. By smashing oxygen nuclei together at nearly the speed of light, researchers have uncovered strong, unmistakable signatures of quark-gluon plasma (QGP), an extremely hot and dense state of matter that filled the cosmos shortly after the Big Bang.
The findings, which have been published in the peer-reviewed scientific journal Physical Review Letters, reveal that high-energy streams of particles systematically lose energy as they travel through the exotic material created by these high-speed collisions. This critical phenomenon, known to physicists as jet quenching, provides a vital clue confirming that quark-gluon plasma has successfully formed on a microscopic scale.
While scientists have previously observed jet quenching in collisions involving much larger atomic nuclei at RHIC—a U.S. Department of Energy (DOE) Office of Science User Facility that operated at Brookhaven National Laboratory from 2000 until February 2026—the same effect has also been detected in oxygen collisions at CERN’s Large Hadron Collider (LHC), the premier European particle physics laboratory located near Geneva, Switzerland.
What makes these new RHIC findings especially significant to the international physics community is that the oxygen collisions occurred at substantially lower energies than those at the LHC. The results establish a crucial new lower boundary for the physical conditions required to produce quark-gluon plasma within such a small collision system. This breakthrough helps researchers closely narrow down the exact thresholds and parameters necessary to create matter that closely resembles the conditions of the infant universe.
"The QGP is believed to be the form of matter that filled the early universe—before the protons and neutrons that make up today’s atoms formed. Understanding its properties is essential for uncovering how the early universe evolved in its first microseconds," said Li Yi, a professor of physics at Shandong University who oversaw the complex STAR detector analysis.
"One of the most powerful ways scientists study the QGP is by identifying where its signatures appear—and just as importantly, where these signatures turn off," she added, emphasizing the importance of finding the absolute limits of this primordial state.
Rongrong Ma, a STAR physicist based at Brookhaven National Laboratory, explained why investigating smaller collision systems is so uniquely valuable to modern nuclear physics.
"This analysis is pushing the boundaries to an extreme case to help us learn something about this jet quenching phenomenon and the conditions needed to generate a QGP," Ma noted.
How Scientists Recreate Matter From the Early Universe
RHIC was originally designed and constructed specifically to produce and investigate quark-gluon plasma under controlled laboratory conditions. Traditionally, researchers have accomplished this monumental task by accelerating massive gold nuclei to enormous speeds and crashing them directly into one another within the heart of the collider ring.
Each gold nucleus contains a dense assembly of 79 protons and 118 neutrons. When two such heavy nuclei collide with immense kinetic energy, the individual boundaries of their constituent protons and neutrons effectively break down, violently releasing the fundamental quarks and gluons locked inside them. Furthermore, the sheer, tremendous energy of the impact generates thousands of additional quarks and gluons through the direct conversion of energy into matter, operating precisely as described by Albert Einstein’s famous relativistic equation, $E=mc^2$.
The result of this catastrophic subatomic impact is a fleeting, microscopic droplet of primordial matter. It is considerably smaller than an atomic nucleus and survives for less than a billionth of a billionth of a second before cooling and expanding. Almost immediately, the free quarks and gluons recombine into more familiar, stable particles that stream outward into detectors.
Because this exotic substance disappears almost as quickly as it forms, scientists cannot examine it directly with conventional instruments. Instead, they rely on enormous, highly sensitive detectors such as the STAR apparatus to analyze the complex array of particles that emerge from the collision debris.
One particularly revealing clue comes from high-energy particles that escape the collision zone with significantly less energy than expected. Their reduced energy clearly suggests they violently interacted with a dense, impeding mixture of quarks and gluons before managing to escape the interaction region.
Researchers pay special attention to jets, which are narrow, highly collimated sprays of extremely energetic particles. These dynamic jets frequently originate when a high-energy quark or gluon from one colliding nucleus strikes a quark or gluon from the opposite nucleus, sending two distinct particle jets traveling in opposite directions across the interaction space.
If these paired jets, often referred to by physicists as "di-jets," happen to form near the outer edge of the collision region, one jet may have to travel a much greater distance through the dense plasma than its counterpart. Carefully comparing their divergent behavior allows scientists to investigate how the surrounding primordial material drains energy away from particles passing through it.
Yi offered a familiar, everyday comparison to help illustrate the physical process taking place inside the collider.
"You can think of it like shooting a bullet through, say, water," Yi explained. "Because of the resistance, the bullet will slow down. It’s the same thing with these high-energy particles. When they go through the QGP, they will lose energy."
The Telltale Signs of Quark-Gluon Plasma
For decades, experiments conducted at RHIC have provided clear, reproducible evidence of jet quenching in collisions between large gold nuclei. The suppression effect is especially noticeable in jets that are forced to travel longer distances through the newly created matter.
Scientists believe these specific particles lose more energy because they encounter a greater number of free quarks and gluons along their extended path before finally escaping the system. Conversely, jets originating much closer to the outer boundary generally pass through a thinner layer of material and therefore experience significantly fewer interactions and energy loss.
These meticulous measurements ultimately helped establish that RHIC was successfully and consistently producing quark-gluon plasma. Additional, robust evidence soon followed from several other independent experiments, including an entirely unexpected discovery regarding how the resulting matter behaves on a macroscopic scale.
Physicists observed that relatively low-energy particles emerging from gold collisions did not move randomly, but instead moved collectively, following highly coordinated spatial patterns. This collective fluid-like behavior clearly indicated that the plasma flowed almost like a perfect liquid, exhibiting an extraordinarily low level of viscosity or internal fluid resistance.
The discovery was a major surprise when researchers first identified it in early data runs. Collective flow has since become another critically important "smoking gun" signature confirming the successful creation of quark-gluon plasma in the laboratory.
Can Tiny Atomic Collisions Create Primordial Matter?
After successfully establishing these definitive signatures in collisions involving large, heavy nuclei, experimental physicists began noticing similar patterns of collective motion in much smaller collision systems.
Subsequent experiments at the LHC, followed closely by detailed studies at RHIC, detected signs of collective flow when tiny projectiles—such as individual protons, deuterons, and helium-3 nuclei—struck much larger gold or lead target nuclei. Researchers at the LHC even observed comparable flow patterns in high-energy collisions between individual protons.
These surprising results immediately raised a major, highly debated question within the physics community. Did the coordinated particle motion mean that these much smaller, lighter collisions were also successfully producing quark-gluon plasma, or could an entirely different, conventional mechanism explain the observations?
Scientists realized they urgently needed a more definitive, unambiguous sign to settle the debate.
"The next natural question is: Can you see jet quenching, the other key signature that QGP is formed, in these very small collision systems?" asked Brookhaven’s Ma. "We haven’t seen any unambiguous signal, so it’s still kind of under debate in these small systems. We want to search for the smallest system where we can still see an unambiguous jet-quenching signal," he added.
To investigate this pressing question directly, researchers decided to turn their attention to controlled collisions between oxygen nuclei.
An oxygen nucleus contains a modest total of just eight protons and eight neutrons, making it vastly smaller and lighter than the heavy gold nuclei traditionally utilized in RHIC experiments. Furthermore, oxygen nuclei possess a relatively symmetrical, spherical geometric shape, which makes the complex geometry of their high-speed collisions much cleaner and easier for physicists to analyze theoretically.
"That means we can more easily look for signs of asymmetric quenching of di-jets produced close to the edge of the circular interaction region in head-on collisions," explained Maowu Nie, an associate professor at Shandong University who contributed significantly to the research effort.
The research team could also carefully examine how the final outcome changed depending entirely on whether the oxygen nuclei collided head-on or merely grazed one another during their passage.
"We can also compare how jets behave in head-on central collisions—those that produce the most particles and therefore have the highest likelihood of making a QGP—with collisions where the two oxygen nuclei barely strike each other, which aren’t expected to generate a QGP," Nie said.
Nie noted that these peripheral, grazing collisions are easily recognizable during data analysis because they generate far fewer total particles than direct, central impacts.
Oxygen Collisions Reveal a 20% Suppression
To successfully search for conclusive evidence of jet quenching, the research team developed a sophisticated analysis method designed to identify concentrated streams of high-energy particles and accurately measure how many emerge above a carefully selected energy threshold.
"Our measurements are essentially counting experiments," said Sijie Zhang, who conducted the underlying research for her master’s thesis under the supervision of Yi and Nie at Shandong University before moving on to pursue her Ph.D. at Yale University.
"We have an algorithm that finds the collimated groups of high-energy particles, the jets, and we count how many of these particles above a certain energy threshold come out," Zhang explained.
Yang He, a former Ph.D. student at Shandong University who developed the core analysis framework and now continues his studies on oxygen collisions at the LHC, described how the team leveraged different collision geometries to precisely identify particle energy loss.
"We compare what we see in back-to-back jets produced near the edge of the system, and we compare what we see in head-on collisions with the counts in non-central collisions where we don’t expect to see quenching," He stated.
The research team analyzed detailed STAR detector measurements gathered from oxygen collisions conducted at RHIC back in 2021, operating at a center-of-mass collision energy of 200 billion electron volts (GeV).
The finalized results revealed a clear, statistically significant 20% suppression of high-energy particles directly associated with the phenomenon of jet quenching.
Although this measured suppression percentage was noticeably smaller than what physicists typically record in massive gold-on-gold collisions, that difference was fully expected by the team because oxygen-oxygen collisions generate vastly smaller interaction systems.
Crucially, the observed quenching effect was still robust and distinct enough to provide unambiguous evidence that quark-gluon plasma had indeed formed even in this tiny nuclear system.
Comparing Big Bang Matter at Different Energies
Oxygen offers nuclear physicists another highly valuable experimental advantage. Aside from fundamental proton-proton collisions, oxygen is the only nuclear species that has now been investigated in high-energy collisions at both RHIC and the LHC.
This unique overlap gives researchers a rare opportunity to perform a direct, "apples-to-apples" comparison between two world-class accelerator facilities operating at vastly different collision energy scales.
The LHC successfully carried out its first dedicated oxygen collision runs in 2025. According to results reported by CERN, those high-energy experiments revealed clear signs of both collective particle flow and jet quenching, providing additional, complementary indications that quark-gluon plasma can form robustly in relatively small nuclear environments.
However, the LHC operated at collision energies exceeding 5,000 GeV, which is far above the 200 GeV utilized during the RHIC oxygen measurements.
"With a higher energy collision, it’s more likely to create more and more quarks and gluons and a hotter, denser QGP system," Yi noted. "That means the jet quenching signal, if it’s there, should be bigger at LHC compared to RHIC."
While higher collision energies certainly make it much easier to create the extremely hot, dense conditions required to form quark-gluon plasma, lower-energy experiments prove uniquely useful for determining precisely where those necessary conditions break down and are no longer sufficient.
By systematically reducing the collision energy and employing smaller atomic nuclei, physicists can closely approach the exact physical threshold at which the distinctive signatures of primordial cosmic matter finally disappear from view.
"We are pushing the boundaries in two ways to explore the minimal conditions for creating a QGP, both in terms of the nucleus size by looking at these small oxygen-oxygen collisions, but also by looking at data from lower collision energies—and we still see this crucial QGP signature. That’s a unique feature of our measurement," Ma emphasized.
New Experiments Could Reveal the Limits of Primordial Matter
Although RHIC has now officially completed its historic operational lifespan, scientists still have substantial amounts of valuable oxygen collision data awaiting thorough investigation.
During the collider’s final, intensive experimental run, researchers successfully collected vital measurements using both the STAR detector and sPHENIX, which serves as RHIC’s newest and most advanced detector system. The facility formally concluded its operational phase with final oxygen collisions recorded by both instruments on February 6, 2026.
These remaining, high-precision datasets are expected to provide further deep insights into how small a collision system can physically become, and how low its operating energy can fall, before quark-gluon plasma can no longer sustain its formation.
Gaining a comprehensive understanding of those fundamental limits will ultimately help physicists piece together the exact behavior and evolution of matter during the first microseconds of our universe, long before quarks and gluons had ever combined to form the stable protons and neutrons that constitute ordinary, everyday atoms today.
The research effort received vital financial support from the U.S. Department of Energy’s Office of Science, the U.S. National Science Foundation (NSF), and various other international scientific agencies and organizations cited in the published study.
The analysis team also utilized the Open Science Grid, which receives direct financial backing from the NSF, alongside advanced computing resources provided by Brookhaven Lab’s Scientific Data and Computing Facilities and the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility located at Lawrence Berkeley National Laboratory.
