Collisions between oxygen nuclei at the Relativistic Heavy Ion Collider (RHIC) produced a clear signal of jet quenching, a signature of the hot, dense matter known as quark-gluon plasma (QGP), the STAR (Solenoidal Tracker at RHIC) Collaboration reported on October 20 in Physical Review Letters. The result shows the effect in a far smaller collision system than the gold nuclei used at RHIC previously.

Jet quenching is the energy loss of jets, the sprays of energetic particles produced in a collision, as they pass through hot, dense matter, according to Brookhaven National Laboratory, which hosts RHIC. Jets that travel farther through the plasma lose more energy. The effect is well established in RHIC’s gold-gold collisions, and the laboratory describes it as a “smoking gun” signature of QGP in collisions of larger nuclei.

The STAR team analyzed oxygen-oxygen collisions recorded in 2021 at a collision energy of 200 GeV per nucleon pair. According to the paper’s abstract, the researchers measured how often a high-momentum “trigger” particle, with transverse momentum between 7 and 30 GeV/c, was accompanied by recoiling particles of 3 to 7 GeV/c or by charged-particle jets above 8 GeV/c. They compared collisions with high event activity to those with low event activity.

The yields of recoiling particles and jets were suppressed by approximately 20% in the high-activity collisions, the authors wrote, “with an absence of suppression excluded with high significance.” For large-radius jets (R = 0.5), the suppression corresponds to a shift in transverse momentum of 0.70 ± 0.15 (stat.) ± 0.10 (syst.) GeV/c, which quantifies the energy redistributed by interactions after the collision. The abstract calls the measurements “strong evidence for jet quenching in O+O collisions.”

Oxygen nuclei each contain eight protons and eight neutrons, compared with 79 protons and 118 neutrons in gold. The 20% suppression is smaller than in gold-gold collisions, which the authors say is expected for a much smaller system. Brookhaven said the result sets a new lower boundary on the collision energy needed to produce a QGP in this small system.

“We want to search for the smallest system where we can still see an unambiguous jet-quenching signal,” said Rongrong Ma, a STAR physicist at Brookhaven. The analysis was overseen by Li Yi, a professor of physics at Shandong University in China, and involved physicists from Shandong University and Brookhaven. The paper was received on April 15, 2026, and accepted on August 19, 2026. The work was funded by the Department of Energy (DOE) Office of Science, the U.S. National Science Foundation and international agencies listed in the paper.

Why it matters

A central question in heavy-ion physics is how small a collision system can be and still create a quark-gluon plasma. Yi noted that the QGP “is believed to be the form of matter that filled the early universe,” and Brookhaven said understanding its properties is essential to learning how the universe evolved in its first microseconds. The STAR measurement adds a data point at a modest collision energy and a small nucleus, which helps define the conditions under which such matter forms.

Oxygen also links two facilities. The Large Hadron Collider (LHC) at CERN first collided oxygen in 2025 and has observed jet quenching in oxygen-oxygen collisions at more than 5,000 GeV, far above RHIC’s energy. Brookhaven said oxygen is the only species besides protons studied at both machines, allowing a direct comparison across a wide energy range. Yi noted the signal should be larger at the LHC, because higher energy creates a hotter, denser plasma.

What to watch

RHIC, a DOE Office of Science user facility, operated from 2000 until earlier in 2026. Its final run ended on February 6, 2026, with oxygen-oxygen collisions recorded by both STAR and sPHENIX, the collider’s newest detector, so further oxygen analyses from those data sets are possible. Maowu Nie, an associate professor at Shandong University, said the symmetric, round shape of oxygen nuclei means “we can more easily look for signs of asymmetric quenching of di-jets” produced near the edge of the collision zone in head-on collisions.

The caveats

The result is peer reviewed and published in Physical Review Letters. As defined in the abstract, the suppression is measured by comparing oxygen-oxygen collisions with high and low event activity, and the interpretation as evidence of QGP is the collaboration’s. The data come from a single experiment and a single collision energy, and the larger LHC signal Yi anticipates is an expectation rather than part of this measurement.

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