STAR finds evidence of quark-gluon plasma in oxygen-nucleus collisions — ScienceDaily
The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at the US Brookhaven National Laboratory has found strong evidence of quark-gluon plasma formation during oxygen-nucleus collisions. As ScienceDaily reports, the study results were published in the journal Physical Review Letters.
Quark-gluon plasma is an ultra-hot and ultra-dense state of matter that, according to current understanding, existed in the first moments after the Big Bang, before protons and neutrons formed. It is usually created at RHIC by colliding much heavier gold nuclei. An oxygen nucleus contains eight protons and eight neutrons, so such collisions make it possible to study smaller systems.
Energy loss by jets
The key indication was so-called jet quenching — the loss of energy by narrow streams of high-energy particles passing through matter formed after a collision. Scientists analyzed data from oxygen collisions conducted at RHIC in 2021 at an energy of 200 GeV.
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The researchers compared jets in head-on nucleus collisions with the results of non-central collisions, where plasma formation was not expected. They recorded an approximately 20 percent suppression of high-energy particles associated with jet quenching. The effect is smaller than in gold-nucleus collisions, but sufficiently pronounced to indicate the formation of quark-gluon plasma.
A lower limit for plasma formation
Jet quenching in oxygen-nucleus collisions had previously also been observed at CERN's Large Hadron Collider, but at energies above 5,000 GeV. The RHIC measurement at 200 GeV sets a lower limit for the known conditions under which characteristic signs of this primordial state of matter can appear in a small system.
RHIC completed operations in 2026, but physicists still have to process data from the final run collected by the STAR and sPHENIX detectors. These measurements may help determine at what minimum system sizes and energies quark-gluon plasma ceases to form.
