Physicists recreated Big Bang-era matter by smashing oxygen nuclei together at nearly light speed

Oxygen nuclei smashed together at nearly the speed of light have produced strong signs of quark-gluon plasma, the hot soup of quarks and gluons believed to have filled the early universe. The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory saw high-energy particles lose about 20% of their yield in the most active oxygen collisions, a signature physicists call jet quenching. The result appears in Physical Review Letters.

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Li Yi, a physics professor at Shandong University who oversaw the analysis, said the plasma is believed to be the form of matter that filled the early universe in the moments after the Big Bang. The finding matters because oxygen is a far smaller projectile than the gold nuclei RHIC used to first make the plasma, and it tests how little matter and how little energy are enough for quarks and gluons to behave collectively instead of staying locked inside protons and neutrons.

Jet quenching as a thermometer for early-universe matter

Quark-gluon plasma forms when nuclear collisions pack enough energy into a tiny volume that protons and neutrons dissolve into their constituents. Sprays of particles called jets, thrown out by the collision, lose energy when they plow through that dense medium. Measuring the shortfall reveals whether plasma formed. In the STAR paper posted to arXiv, associated particle yields per trigger fell by roughly 20% in high-activity collisions compared with low-activity ones, and the authors say a scenario with no suppression is excluded with high significance. For charged-particle jets the suppression corresponds to a momentum shift of 0.70 GeV/c, with a statistical uncertainty of 0.15.

Sijie Zhang, who carried out the work for her master’s thesis under Yi and Maowu Nie and is now a doctoral student at Yale, described the measurements as essentially counting experiments: tally the particles, compare the busy collisions with the quiet ones, and see what is missing. Yang He, a former Shandong University doctoral student who built the analysis framework and now studies oxygen collisions at the LHC, is also named in the release.

Comparing against grazing collisions matters. In a grazing collision the two nuclei barely overlap, so little hot matter should form and jets pass through nearly untouched, while head-on collisions put the most nucleons into the overlap region. Back-to-back jet pairs, or di-jets, produced near the edge of the collision zone give a second handle, because one jet of the pair can travel through more medium than the other. Seeing the yield drop as collision activity rises, rather than at random, is what separates a medium effect from detector quirks.

Oxygen at 200 GeV against gold collisions

An oxygen nucleus holds 8 protons and 8 neutrons, against 79 protons and 118 neutrons in gold. The STAR data came from oxygen collisions at 200 GeV per nucleon pair, recorded in 2021. Suppression of 20% is smaller than gold collisions produce, which the team attributes to the smaller system, yet it was strong enough to indicate plasma. According to the Brookhaven release distributed through Newswise, the result sets a new lower bound on the collision energy at which plasma can form in a small system. The Brookhaven National Laboratory newsroom carries the same announcement.

Rongrong Ma, a STAR physicist at Brookhaven, said the analysis pushes the boundaries to an extreme case. Ma noted that earlier small-system work had not delivered any unambiguous jet-quenching signal, so whether plasma forms in such small collisions was still under debate. Experiments had found collective flow in collisions of protons, deuterons and helium-3 nuclei with heavy nuclei, but jet quenching had not been unambiguously seen there, which left the picture lopsided until oxygen arrived.

The Large Hadron Collider ran its first oxygen collisions in 2025 at energies above 5,000 GeV and has reported collective flow and jet quenching of its own. Yi expects a stronger signal there, since a higher-energy collision tends to create more quarks and gluons, a hotter and denser plasma. With both facilities having studied oxygen, researchers can compare the results directly.

Oxygen’s shape helps the measurement. Its nuclei are close to symmetric spheres, which makes asymmetric quenching easier to study, according to the Brookhaven release.

What RHIC’s last oxygen run still holds

RHIC operated from 2000 until 2026, and its final run ended on February 6, 2026, with oxygen collisions recorded by STAR and the newer sPHENIX detector. Analysis of that data is still ahead. Per the ScienceDaily account, the remaining datasets could show how small and how low in energy a system can be while still forming plasma. RHIC is a U.S. Department of Energy Office of Science user facility, and the work drew support from that office, the National Science Foundation and international agencies, with computing from the Open Science Grid, Brookhaven’s data facilities and NERSC.

The paper, “Measurement of Jet Quenching in O O Collisions at 200 GeV by the STAR Experiment at RHIC,” is in Physical Review Letters volume 137, issue 17. The unresolved question is where the signal disappears. Oxygen at 200 GeV still shows it, the LHC data will show how it grows with energy, and the remaining RHIC runs may locate the edge where the suppression fades into the background of ordinary collisions.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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