New results from the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) provide strong evidence that collisions between relatively small oxygen nuclei can create quark-gluon plasma (QGP), an extremely hot and dense state of matter believed to have filled the universe during its first moments. The measurements, made at a collision energy of 200 GeV, detected jet quenching—a loss of energy by high-energy particle jets moving through the collision debris—and establish a lower limit for the conditions known to produce QGP in this small system.
The findings, reported in Physical Review Letters, extend observations previously made in collisions involving much larger nuclei. Similar effects have also been detected in oxygen collisions at CERN’s Large Hadron Collider (LHC), but the new RHIC measurements were made at substantially lower energies.
RHIC is a U.S. Department of Energy (DOE) Office of Science User Facility that operated at Brookhaven National Laboratory from 2000 to 2026. The new results help scientists narrow down the minimum nuclear size and collision energy needed to produce matter resembling the substance that existed shortly after the Big Bang.
“QGP is thought to be a form of matter that filled the early universe, before the protons and neutrons that make up today’s atoms were formed. Understanding its properties is essential to uncovering how the early universe evolved in the first microseconds,” said Li Yi, a physics professor at Shandong University who oversaw the STAR analysis.
“One of the most powerful ways scientists can study QGP is to identify where its signs appear. And just as important, to identify where these signs disappear,” she said.
Brookhaven Laboratory STAR physicist Rongrong Ma said that studying smaller collision systems can reveal the limits of the phenomenon.
“This analysis pushes the limits to extreme cases, which helps us learn something about this jet quenching phenomenon and the conditions necessary for QGP to occur,” Ma said.
Recreating matter from the early universe
RHIC was designed to create and study quark-gluon plasma. In traditional experiments, researchers accelerate gold nuclei to enormous speeds and collide them.
Each gold nucleus contains 79 protons and 118 neutrons. When two nuclei collide with enough energy, the boundary between their protons and neutrons is effectively destroyed, freeing the quarks and gluons inside. The collision energy also produces thousands of additional quarks and gluons as energy is converted into matter, as described by Einstein’s equation E=mc2.
The result is a fleeting droplet of primordial matter, smaller than an atomic nucleus and lasting less than a billionth of a second. Almost immediately, the quarks and gluons recombine into more familiar particles.
Because the substance disappears so quickly, scientists cannot study it directly. Instead, they use instruments such as the STAR detector to analyze the particles produced as the collision system evolves.
One important clue is that some particles emerge with less energy than expected. Their energy loss suggests that they interacted with a dense mixture of quarks and gluons before escaping.
Researchers focus particularly on jets, which are narrow sprays of highly energetic particles. Jets often form when a quark or gluon from one nucleus collides with a quark or gluon from the other, producing two sprays that move in opposite directions.
These paired sprays, known as dijets, can provide information about the material created in the collision. If the dijets form near the edge of the collision region, one may travel farther through the plasma than the other. Comparing the two allows scientists to investigate how the surrounding matter removes energy from passing particles.
Yi compared the process with firing a projectile through water.
“You can think of it like shooting a bullet through water,” Yi said. “Because of the resistance, the bullet is slowed down. This is also true for high-energy particles. When they pass through the QGP, they lose energy.”
Signs of quark-gluon plasma
For decades, RHIC experiments have observed jet suppression in collisions between gold nuclei. The effect is especially pronounced for jets that travel long distances through the newly formed matter.
Scientists believe those jets lose more energy because they encounter more quarks and gluons before escaping. Jets produced near the outer boundary pass through less material and undergo fewer interactions.
These measurements helped establish that RHIC can produce quark-gluon plasma. Other evidence came from the collective motion of the particles produced in gold collisions.
Relatively low-energy particles were found to move together in a coordinated pattern, indicating that the plasma has very low viscosity and internal resistance. It flows almost like a perfect liquid.
The observation was unexpected when it was first made. Since then, collective flow has become another important defining feature of quark-gluon plasma.
Can small collisions create primordial matter?
After identifying these signatures in collisions involving large nuclei, physicists began observing similar collective motion in much smaller systems.
Experiments at the LHC and later studies at RHIC detected collective flow when small projectiles—including protons, deuterons and helium-3 nuclei—collided with much larger gold and lead nuclei. LHC researchers also observed comparable flow patterns in collisions between individual protons.
Those observations raised an important question: Do such small collision systems produce quark-gluon plasma, or can another mechanism explain their coordinated motion?
Scientists needed a more definitive signal. Jet quenching is considered one of the strongest indicators of QGP formation, but a clear signal had not previously been established in the smallest collision systems.
“The next natural question is whether jet quenching, another important sign of QGP formation, is seen in these very small impact systems,” Ma said. “We haven’t seen a clear signal yet, so it’s still a discussion in these small systems. We want to look for the smallest systems where we can still see a clear jet quenching signal.”
To investigate the question, the researchers studied collisions between oxygen nuclei.
An oxygen nucleus contains eight protons and eight neutrons, making it much smaller than the gold nuclei typically used at RHIC. Oxygen nuclei are also relatively symmetrical and spherical, which makes the geometry of the collision easier to analyze.
“This means we can more easily look for signs of asymmetric extinction of the dijet produced near the edge of the circular interaction region during a head-on collision,” said Maowu Nie, an associate professor at Shandong University who contributed to the study.
The researchers could also compare collisions in which the oxygen nuclei struck one another directly with more peripheral collisions in which they barely grazed each other.
“We can also compare how the jet behaves in center-head-on collisions, where the most particles are produced and where QGP is most likely to occur, and how it behaves in collisions where two oxygen nuclei barely collide, where QGP is not expected to occur,” Nie said.
Grazing collisions can be identified because they produce far fewer particles than direct collisions.
Oxygen collisions show 20 percent suppression
To measure jet quenching, the researchers developed an analysis that identifies concentrated streams of high-energy particles and counts how many particles emerge above a selected energy threshold.
“Our measurements are essentially counting experiments,” said Shizhi Zhang, who conducted master’s thesis research with Yi and Nie at Shandong University and is currently pursuing a Ph.D. at Yale University.
“We have an algorithm that finds parallel groups of high-energy particles, jets, and counts how many of these particles emerge above a certain energy threshold,” Zhang said.
Yang He, a former Ph.D. student at Shandong University who developed the analytical framework and is now studying oxygen collisions at the LHC, explained how the team determined the energy loss.
“We compare what we observe with continuous jets produced near the edges of the system, and we compare what we observe with head-on collisions with counts in off-center collisions, where we would expect to see no quenching,” He said.
The team analyzed STAR measurements from oxygen collisions performed at RHIC in 2021 at a collision energy of 200 billion electron volts, or 200 GeV.
The results showed that high-energy particles associated with jet quenching were suppressed by 20 percent. That suppression was smaller than the level typically measured in gold collisions, as expected for the much smaller oxygen collision system.
Even so, the effect was strong enough to provide clear evidence that quark-gluon plasma had formed.
Comparing early-universe matter at different energies
Oxygen offers another advantage: apart from proton-proton collisions, it is the only nuclide whose collisions have been studied at both RHIC and the LHC. This gives scientists an opportunity to make an “apples-to-apples comparison” between two facilities operating at very different energies.
The LHC conducted its first oxygen collision in 2025. Results reported by CERN from those experiments revealed both collective flow and jet quenching, providing additional indications that QGP can form in relatively small nuclear collisions.
However, the LHC operated at collision energies above 5,000 GeV, far higher than the 200 GeV used for the RHIC measurements.
“The higher the collision energy, the more quarks and gluons are produced, which increases the likelihood of producing a hotter, denser QGP system,” Yi said. “So if a jet quenching signal exists, it should be larger at the LHC compared to RHIC.”
Higher energies make it easier to create the hot, dense conditions needed for QGP. Lower-energy experiments may therefore be more useful for determining when those conditions are no longer sufficient.
By reducing both the collision energy and the size of the colliding nuclei, physicists can approach the threshold at which the characteristic signatures of primordial matter disappear.
“We are pushing the limits in two ways to explore the minimum conditions for producing QGPs, both in terms of nuclear size by observing these small oxygen-oxygen collisions and by observing data from lower collision energies. And we still see this important QGP feature, which is a unique feature of our measurements,” Ma said.
Further data may define the limits
Although RHIC has completed its operations, scientists still have additional oxygen-collision data to analyze. During the collider’s final experimental run, researchers collected measurements with both STAR and RHIC’s newest detector, sPHENIX. The facility ceased operations on February 6, 2026.
The remaining data sets may offer further insight into how small a collision system can become, and how far its energy can be reduced, before quark-gluon plasma can no longer form.
Understanding those limits could help physicists reconstruct the behavior of matter during the universe’s first microseconds, when quarks and gluons had not yet bonded into the protons and neutrons that make up ordinary atoms today.
The research was supported by the DOE Office of Science, the U.S. National Science Foundation (NSF), and additional international agencies and organizations identified in the scientific publication. The researchers also used the Open Science Grid, which receives direct NSF support, as well as computing resources at Brookhaven Laboratory’s scientific data and computing facilities and the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility at Lawrence Berkeley National Laboratory.
Source: www.sciencedaily.com
