Researchers interpret the reduced yields as evidence of quark-gluon plasma, extending its energy-loss signature to smaller nuclei at lower collision energy.
The STAR experiment has measured approximately 20% fewer associated charged particles and jets per selected reference particle in high-activity oxygen–oxygen collisions than in low-activity collisions. Researchers interpret the suppression as strong evidence of jet quenching—energy lost by energetic particles passing through collision-created matter—and as evidence for quark-gluon plasma formation.
Earlier experiments found jet quenching in collisions of much larger gold nuclei. The oxygen analysis uses data collected in 2021 at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC), at an energy of 200 GeV per proton-or-neutron pair. GeV, or gigaelectron volt, is a unit of energy. Physical Review Letters published the result on 20 October 2026.
Quark-gluon plasma is an extremely hot, dense state in which quarks and gluons are no longer confined inside individual protons and neutrons. It resembles matter that filled the early universe. The plasma quickly disappears as its constituents recombine into familiar particles, so STAR studies the particles that emerge rather than inspecting the plasma directly.
Jets are narrow sprays of energetic particles. When escaping particles carry less energy than expected, researchers infer that they interacted with the material around them. That makes jet quenching an additional test of plasma formation: coordinated particle motion, called collective flow, had already appeared in smaller collision systems, but researchers debated whether plasma or another mechanism caused it.
STAR selected energetic charged hadrons—particles made of quarks—as reference particles, or triggers. It then counted associated charged hadrons and charged-particle jets recoiling in the opposite direction, dividing the counts by the number of triggers. Comparing these yields across collision-activity classes revealed the suppression. The researchers describe head-on collisions as producing more particles and being more likely to form plasma; grazing collisions produce fewer particles and provide a comparison where quenching is not expected.
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For charged-particle jets with radius parameter R = 0.5, which sets the jet’s angular size, STAR reports a transverse-momentum shift of 0.7 ± 0.15 (statistical) ± 0.10 (systematic) GeV/c. Transverse momentum is momentum perpendicular to the beams; statistical uncertainty reflects limited counts, while systematic uncertainty reflects measurement and analysis effects. STAR interprets the shift as energy redistribution through interactions after the collision and says the absence of suppression is excluded with high significance.
The researchers describe the suppression as weaker than typically measured in gold collisions, as expected for the smaller oxygen system. Their account gives the following comparison of the nuclei used at RHIC:
| Nucleus | Protons | Neutrons |
|---|---|---|
| Oxygen | 8 | 8 |
| Gold | 79 | 118 |
ATLAS has also reported jet quenching in oxygen–oxygen and neon–neon collisions at the Large Hadron Collider, at 5.36 TeV per proton-or-neutron pair. A TeV is a teraelectron volt. Brookhaven STAR physicist Rongrong Ma says the RHIC measurement pushes the search for minimum plasma-producing conditions in both nucleus size and collision energy while still finding the quenching signature.
RHIC ended operations with oxygen collisions recorded by STAR and sPHENIX on 6 February 2026. Additional oxygen datasets from that final run remain for analysis. Researchers say those measurements could help identify how small a collision system can become, and how low its energy can fall, before plasma formation ceases.
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