CERN Oxygen Collision Discovery | Understanding Early Universe Matter

Oxygen Nuclei and the Early Universe

CERN researchers have detected evidence of quark-gluon plasma in oxygen-oxygen collisions. This result establishes oxygen as the smallest nuclear system to show direct jet quenching. The experiment, conducted at the Large Hadron Collider in 2025, involved collisions at a centre-of-mass energy of 5.36 teraelectronvolts per nucleon pair. This state of matter existed microseconds after the Big Bang, when temperatures were too high for protons and neutrons to hold together. Researchers describe this medium as a near-perfect liquid with low viscosity rather than a simple gas.

Previous experiments relied on lead nuclei to produce this plasma. Lead contains 208 protons and neutrons, which creates a large enough medium to reliably measure collective effects. Oxygen nuclei, containing only 16 nucleons, provide a much smaller test bed. While proton-based collisions previously showed patterns hinting at plasma, those results remained difficult to interpret due to the tiny scale. The oxygen-oxygen collisions provide a cleaner intermediate measurement between single protons and heavy lead nuclei.

Measuring Invisible Matter Through Jet Quenching

Physicists probe this medium by observing jets. A violent collision can launch a high-energy quark or gluon, which then fragments into a spray of particles known as a jet. When this parton crosses a droplet of quark-gluon plasma, the medium absorbs some of its energy. This effect, called jet quenching, results in fewer high-momentum particles reaching detectors than expected. Scientists use this signature to infer the presence of the hot medium, as they cannot directly photograph such a small, short-lived droplet.

The ALICE Collaboration, which published these findings in June 2026, measured neutral pions to track this energy loss. To ensure the observed suppression resulted from the plasma and not the internal structure of the nuclei, the team used proton-oxygen collisions as a control. The proton-oxygen data showed no suppression. By comparing these sets, researchers isolated the jet-quenching signal at a statistical significance of 4.9 standard deviations.

Implications for Future Nuclear Research

This finding does not prove every oxygen collision creates an equilibrated liquid. It does show that the system produces enough medium to measurably drain energy from passing partons. The data from 2026 align with other experiments, including ATLAS and CMS, which observed similar imbalances in jet pairs and charged-particle production. LHCb also contributed by studying neon collisions, confirming that heavier systems lead to stronger suppression of charm-containing particles.

Future research will focus on the lower boundaries of this phenomenon. By comparing proton, oxygen, neon, xenon, and lead systems, scientists can determine if jet quenching appears gradually or if it requires a specific threshold of size and particle density. These observations help clarify when fluid dynamics take over in these microscopic systems. While the LHC does not recreate the Big Bang in full, it allows physicists to analyze the physics of the early universe in controlled, sub-atomic increments.