CERN physicists Detect quark-gluon plasma in light nuclei

On Nov. 2, 2026, CERN physicists reported that the Large Hadron Collider had detected quark-gluon plasma, the hot, dense state of matter thought to exist in the first microseconds after the Big Bang, in collisions between light oxygen and neon nuclei. The announcement came exactly one year after the first-ever oxygen collisions at the collider. Until now, physicists had assumed that recreating this primordial soup required collisions between much heavier nuclei, such as lead, which are more than 200 times heavier than the protons typically used in LHC experiments. These findings suggest that the energy threshold for recreating the extreme conditions of the early universe is lower than believed, opening new avenues for studying the cosmos’s first moments.

Quark-gluon plasma is a state of matter that forms at temperatures exceeding 100,000 times the core of the Sun. Under such extreme heat and density, protons and neutrons melt into their fundamental constituents: quarks and gluons. By colliding atomic nuclei at near-light speeds, the LHC recreates these fleeting conditions, allowing physicists to study the universe as it was just moments after the Big Bang. This discovery builds on a timeline of earlier hints that included ALICE’s indication of plasma in proton-proton and proton-lead collisions earlier in 2026, and first hints from oxygen-oxygen collisions the previous year.

The detection relied on signatures observed across the LHC’s four main experiments, building on a timeline of earlier hints. ATLAS observed parton energy loss, where energetic quarks and gluons lose energy traversing the plasma, by studying pairs of particle jets and charged particles recoiling opposite from photons, with the effect most pronounced in head-on central collisions where the plasma volume is largest. CMS found suppressed production of charged particles in oxygen-oxygen and neon-neon collisions compared to proton-proton collisions, and detected varying suppression of upsilon mesons, tightly bound bottom quark-antiquark pairs. LHCb reported preliminary evidence of varying suppression using data from proton-oxygen and oxygen-oxygen collisions, and found greater suppression of particles containing charm quarks in heavier neon collisions than in oxygen collisions, though the collaboration cautioned that other physical processes could theoretically produce similar effects.

ALICE confirmed parton energy loss in oxygen-oxygen collisions by comparing neutral pion production to proton-oxygen collisions, ruling out alternative explanations for the effect. The collaboration also observed anisotropic flow, where particles containing three quarks, known as baryons, show a stronger directional preference than two-quark particles, known as mesons. This pattern arises from collective motion inherited by particles emerging from the plasma, reinforcing the liquid-like behavior of quark-gluon plasma even in these smaller collision systems.

The light nuclei used in these collisions also provided a cleaner window into nuclear geometry. A study published in Physical Review Letters by the CMS collaboration, led by A. Hayrapetyan et al. in 2026, analyzed particle flow from oxygen-oxygen and neon-neon collisions at 5.36 TeV per nucleon pair. Oxygen-16 nuclei appear to have a tetrahedron-like internal arrangement, while neon-20 nuclei are more elongated, resembling a bowling pin. Because these nuclei have similar mass numbers, their hydrodynamic evolution should be alike, so the observed differences in collective flow—particularly neon’s stronger elliptic flow in head-on collisions—trace back to differences in nuclear shape.

Symmetric light ion collisions are ideal for investigating this final-state collective response because the spatial distribution of nucleons sets the initial geometry, unlike proton-nucleus collisions where event-by-event fluctuations dominate. Previous high-energy collisions probed heavier nuclei such as uranium, and flow-like signals appeared in proton-proton and proton-nucleus collisions, but lighter oxygen and neon nuclei offer a cleaner test of collision geometry. None of the three model calculations reproduced the measured triangular flow ratios quantitatively, highlighting the need for improved treatment of initial-state fluctuations in small collision systems.

The discovery of quark-gluon plasma in such small systems reshapes our understanding of the conditions needed to recreate the early universe. For decades, scientists believed that only massive lead collisions could generate the extreme temperatures and densities of the primordial cosmos. The new results show that lighter nuclei can also reach temperatures exceeding 100,000 times the Sun’s core, breaking protons and neutrons into quarks and gluons.

To probe these fundamental questions further, CERN is upgrading the Large Hadron Collider to the High-Luminosity LHC, or HiLumi LHC, with operations expected to begin around 2030. This upgrade will increase the collider’s data output by roughly seven times, allowing scientists to study quark-gluon plasma and other fundamental particles in far greater detail. The increased collision rate will also enable precision measurements of the Higgs boson, the particle discovered at the LHC in 2012 that confirmed how elementary particles gain mass and completed the Standard Model’s inventory of particles. The upgrade prioritizes precision measurements of the Higgs, including its rare decays into pairs of muons or charm quarks, and searching for Higgs boson pairs to measure the Higgs self-coupling, an interaction that shapes the Higgs field filling all space.

These measurements test the Standard Model and search for new physics like dark matter and supersymmetry, as the Standard Model cannot explain dark matter or neutrino behavior. A long-term upgrade goal is determining whether particles now considered fundamental, such as quarks and the Higgs boson, are truly fundamental or made of something smaller still.

The upgrade requires more than just a more powerful collider; the ATLAS and CMS detectors are undergoing major overhauls to handle the intense particle bombardment. Entirely new silicon tracking detectors are replacing existing ones at the heart of both experiments, designed to survive radiation levels that would quickly destroy previous sensor generations. New timing detectors, including the High Granularity Timing Detector in ATLAS and a similar CMS system, will measure particle arrival times with precision of only a few tens of trillionths of a second, adding a fourth dimension to tracking. This will allow physicists to link each particle to the correct collision among up to 200 simultaneous proton-proton interactions occurring in every beam crossing, with the ATLAS detector sorting through roughly 10 times more collision data per second than it does now.

The massive undertaking involves thousands of scientists, engineers, and technicians worldwide, including roughly 245 institutions from more than 40 countries in the ATLAS experiment. Indiana University has participated since the detector began operating in 2009, with physics professor Harold “Hal” Evans serving as deputy project manager overseeing the United States’ roughly $300 million share of the detector upgrade. Evans is focused on detecting rare di-Higgs production events, which occur far less often than single Higgs production – roughly one out of every billion proton-proton collisions – and may reveal where things like supersymmetry could be hiding. Evans cited short-term CERN technology spin-offs, including the World Wide Web invented for data sharing, proton therapy for cancer, and medical imaging detector technology, as highly important to society, technology, and the economy.

He said the share of U.S. economic output tied to quantum mechanics technology runs somewhere between two-thirds and three-quarters, depending on the method of measurement.

On the CMS side, Daniela Bortoletto of the University of Oxford has been involved in the High-Luminosity program for nearly two decades, long before the Higgs boson was discovered in 2012. She served as the U.S. upgrade coordinator for the Compact Muon Solenoid and now works on ATLAS in Oxford, building silicon pixel detector modules for the upgraded inner tracker. Bortoletto compares the luminosity increase to a camera that takes seven photos per second instead of one, allowing subtle details to emerge from the larger dataset. A long-term upgrade goal is determining whether particles now considered fundamental, such as quarks and the Higgs boson, are truly fundamental or made of something smaller still, with Evans specifically searching for rare di-Higgs production events that may point to physics beyond the Standard Model.