Oxygen collisions show evidence of early-universe matter at lower energies

Quark-gluon plasma is an extremely hot and dense state of matter believed to have existed shortly after the Big Bang, before protons and neutrons formed. RHIC, a US Department of Energy user facility at Brookhaven National Laboratory that operated from 2000 to 2026, was designed specifically to create and study this material, traditionally by colliding massive gold nuclei.

Reporting in Physical Review Letters, the STAR collaboration analysed oxygen-oxygen collisions conducted at RHIC in 2021 at an energy of 200 billion electron volts (GeV). They found a 20% suppression of high-energy particles associated with jet quenching. Jets often originate when a quark or gluon from one nucleus strikes one from another, sending two particle jets, known as "di-jets," in opposite directions; if one passes through more of the plasma, it loses energy. Shandong University professor Li Yi offered a familiar comparison: "You can think of it like shooting a bullet through, say, water. 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 result establishes a new lower boundary for the conditions known to produce quark-gluon plasma in this small collision system. "Understanding its properties is essential for uncovering how the early universe evolved in its first microseconds," Yi said. Rongrong Ma, a STAR physicist at Brookhaven, said the team is "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."

Oxygen also offers a rare cross-facility comparison: it is the only nuclear species apart from proton-proton collisions studied at both RHIC and the LHC. CERN's LHC carried out its first oxygen collisions in 2025 at energies above 5,000 GeV; results reported by CERN showed both collective flow and jet quenching. The RHIC detection at 200 GeV shows the signature persists at far lower energies. "That's a unique feature of our measurement," Ma said.

Researchers compared head-on central collisions, which generate the most particles and have the highest likelihood of producing a QGP, with grazing collisions that produce far fewer particles and are not expected to create the plasma. Sijie Zhang, who conducted the analysis for her master's thesis, described the work as "essentially counting experiments," using an algorithm that finds collimated groups of high-energy particles and counts how many emerge above a set threshold.

The question of whether small collision systems produce quark-gluon plasma remains under debate. "We want to search for the smallest system where we can still see an unambiguous jet-quenching signal," Ma said. RHIC has now concluded operations, ending with oxygen collisions recorded by the STAR and sPHENIX detectors on February 6, 2026. Those remaining datasets could reveal how small a collision system can become, and how low its energy can fall, before quark-gluon plasma can no longer form.