The findings, published in Physical Review Letters, reveal that high-energy streams of particles lose energy as they travel through the material created by these collisions. This effect, called jet quenching, provides an important clue that quark-gluon plasma has formed.

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 2026. The same effect has also been detected in oxygen collisions at CERN's Large Hadron Collider (LHC), the European Particle Physics Laboratory.

What makes the new RHIC findings especially significant is that the oxygen collisions occurred at substantially lower energies. The results establish a new lower boundary for the conditions known to produce quark-gluon plasma in this small collision system, helping scientists narrow down what it takes to create matter resembling that 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 STAR 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 said.

Rongrong Ma, a STAR physicist at Brookhaven Lab, explained why investigating smaller collisions is so valuable.

"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."

How Scientists Recreate Matter From the Early Universe

RHIC was designed specifically to produce and investigate quark-gluon plasma. Traditionally, researchers have accomplished this by accelerating gold nuclei to enormous speeds and crashing them into one another.

Each gold nucleus contains 79 protons and 118 neutrons. When two such nuclei collide with sufficient energy, the boundaries of their protons and neutrons effectively break down, releasing the quarks and gluons inside them. The tremendous energy of the impact also generates thousands of additional quarks and gluons through the conversion of energy into matter, as described by Einstein's famous equation, E=mc².

The result is a fleeting droplet of primordial matter. It is smaller than an atomic nucleus and survives for less than a billionth of a billionth of a second. Almost immediately, the quarks and gluons recombine into more familiar particles.

Because this exotic substance disappears so quickly, scientists cannot examine it directly. Instead, they use enormous instruments such as the STAR detector to analyze the particles that emerge from the collision.

One particularly revealing clue comes from particles that escape with less energy than expected. Their reduced energy suggests they interacted with the dense mixture of quarks and gluons before escaping.

Researchers pay special attention to jets, which are narrow sprays of extremely energetic particles. These often originate when a quark or gluon from one nucleus strikes a quark or gluon from another, sending two particle jets traveling in opposite directions.

If these paired jets, known as "di-jets," form near the edge of the collision region, one may have to travel farther through the plasma than the other. Comparing their behavior allows scientists to investigate how the surrounding material drains energy from particles passing through it.

Yi offered a familiar comparison to explain the process.

"You can think of it like shooting a bullet through, say, water," Yi said. "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, RHIC experiments have provided clear evidence of jet quenching in collisions between gold nuclei. The effect is especially noticeable in jets that travel longer distances through the newly created matter.

Scientists believe these particles lose more energy because they encounter more quarks and gluons before escaping. Jets originating closer to the outer boundary generally pass through less material and experience fewer interactions.

These measurements helped establish that RHIC was successfully producing quark-gluon plasma. Additional evidence came from several other experiments, including an unexpected discovery about how the resulting matter behaves.

Scientists observed that relatively low-energy particles emerging from gold collisions moved collectively, following coordinated patterns. This behavior indicated that the plasma flowed almost like a perfect liquid, with extraordinarily little viscosity or internal resistance.

The discovery was a major surprise when researchers first identified it. Collective flow has since become another important "smoking gun" signature of quark-gluon plasma.

Can Tiny Atomic Collisions Create Primordial Matter?

After establishing these signatures in collisions involving large nuclei, physicists began finding similar collective motion in much smaller systems.

Experiments at the LHC, followed by studies at RHIC, detected collective flow when tiny projectiles such as protons, deuterons, and helium-3 nuclei struck much larger gold or lead nuclei. Researchers at the LHC even observed comparable flow patterns in collisions between individual protons.

These results raised a major question. Did the coordinated particle motion mean that these much smaller collisions were also producing quark-gluon plasma, or could another mechanism explain the observations?

Scientists needed a more definitive sign.

"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 said.

To investigate, researchers turned to collisions between oxygen nuclei.

An oxygen nucleus contains just eight protons and eight neutrons, making it far smaller than the gold nuclei typically used in RHIC experiments. Oxygen nuclei also have a relatively symmetrical, spherical shape, which makes the geometry of their collisions easier to analyze.

"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," said Maowu Nie, an associate professor at Shandong University who contributed to the research.

The scientists could also examine how the outcome changed depending on whether the oxygen nuclei collided directly or merely grazed one another.

"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 explained that these grazing collisions are recognizable because they generate far fewer particles than direct impacts.

Oxygen Collisions Reveal a 20% Suppression

To search for jet quenching, the researchers developed an analysis that identifies concentrated streams of high-energy particles and measures how many emerge above a selected energy threshold.

"Our measurements are essentially counting experiments," said Sijie Zhang, who conducted the research for her master's thesis under Yi and Nie at Shandong University and is now pursuing a 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."

Yang He, a former Ph.D. student at Shandong University who developed the analysis framework and now studies oxygen collisions at the LHC, described how the team used different collision types to identify 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."

The team analyzed STAR measurements from oxygen collisions conducted at RHIC in 2021, at a collision energy of 200 billion electron volts (GeV).

The results revealed a 20% suppression of high-energy particles associated with jet quenching.

Although this suppression was smaller than what physicists typically measure in gold collisions, that difference was expected because oxygen collisions generate much smaller systems.

Crucially, the observed effect was still strong enough to provide clear evidence that quark-gluon plasma had formed.

Comparing Big Bang Matter at Different Energies

Oxygen offers scientists another important advantage. Apart from proton-proton collisions, it is the only nuclear species that has been investigated in collisions at both RHIC and the LHC.

That gives researchers an unusual opportunity to make an "apples-to-apples" comparison between two facilities operating at very different collision energies.

The LHC carried out its first oxygen collisions in 2025. According to results reported by CERN, those experiments revealed both collective flow and jet quenching, providing additional indications that quark-gluon plasma can form in relatively small nuclear collisions.

However, the LHC operated at collision energies exceeding 5,000 GeV, far above the 200 GeV used in the RHIC 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 said. "That means the jet quenching signal, if it's there, should be bigger at LHC compared to RHIC."

While higher energies make it easier to create the hot, dense conditions needed for quark-gluon plasma, lower-energy experiments may be more useful for determining precisely when those conditions are no longer sufficient.

By reducing the collision energy and using smaller nuclei, physicists can approach the threshold at which the distinctive signatures of primordial matter disappear.

"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 said.

New Experiments Could Reveal the Limits of Primordial Matter

Although RHIC has now completed its operations, scientists still have additional oxygen collision data to investigate.

During the collider's final experimental run, researchers collected measurements using both STAR and sPHENIX, RHIC's newest detector. The facility concluded its operations with oxygen collisions recorded by both instruments on February 6, 2026.

These remaining datasets could provide further insights into how small a collision system can become, and how low its energy can fall, before quark-gluon plasma can no longer form.

Understanding those limits may help physicists piece together the behavior of matter during the first microseconds of the universe, when quarks and gluons had not yet combined into the protons and neutrons that make up ordinary atoms today.

The research received financial support from 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 support from NSF, along with computing resources at 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 DOE's Lawrence Berkeley National Laboratory.