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Oxygen and Neon Collisions Show Quark–Gluon Plasma Can Form in Small Systems
CERN’s 2025 oxygen–oxygen and neon–neon collisions at 5.36 TeV per nucleon pair produced converging signs of quark–gluon plasma like matter in nuclei containing just 16 and 20 nucleons. ALICE and CMS observed collective flow, while ATLAS reported jet energy imbalance consistent with parton energy loss; the experimen...
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Key takeaways
- CERN’s 2025 oxygen–oxygen and neon–neon collisions at 5.36 TeV per nucleon pair produced converging signs of quark–gluon plasma like matter in nuclei containing just 16 and 20 nucleons.
- ALICE and CMS observed collective flow, while ATLAS reported jet energy imbalance consistent with parton energy loss; the experiments also used flow patterns to distinguish relatively round oxygen 16 from elongated, “...
- A dynamical core–corona analysis found that the equilibrated core overtakes the corona above a midrapidity multiplicity of about 20, while a substantial non equilibrated component persists even in central oxygen–oxyge...
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openai.comCreate a landscape editorial hero image for this Studio Global article: How have CERN’s 2025 oxygen-16 and neon-20 collisions at 5.36 TeV per nucleon pair demonstrated quark-gluon plasma formation in systems much. Article summary: The 2025 O–O and Ne–Ne runs show that collisions of nuclei containing only 16 or 20 nucleons can produce strong collective behavior and parton-energy-loss signatures associated with quark–gluon plasma (QGP), extending QG. Topic tags: general, general web, academic. Style: premium digital editorial illustration, source-backed research mood, clean composition, high detail, modern web publication hero. Use reference image context only for broad subject, composition, and topical grounding; do not copy the exact image. Avoid: logos, brand marks, copyrighted characters, real person likenesses, fake screenshots, UI text, readable text, watermarks, charts with fake num
The Large Hadron Collider’s first oxygen–oxygen and neon–neon runs show that strong collective behavior and parton-energy-loss effects can emerge in systems far smaller than traditional lead–lead collisions. Together, the results provide converging evidence for quark–gluon-plasma-like matter—but not for a completely thermalized fluid in every event. 1
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What the four LHC experiments found
The light-ion campaign collided oxygen-16 and neon-20 nuclei at a centre-of-mass energy of 5.36 TeV per nucleon pair. The collision energy is comparable across the two systems, making their different flow patterns especially useful for separating effects caused by system size from effects caused by nuclear geometry. 8
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ALICE: collective flow and geometry
ALICE measured elliptic and triangular flow, commonly denoted v₂ and v₃, in oxygen–oxygen and neon–neon collisions. These coefficients describe how the directions of outgoing particles are distributed around the collision axis. A sizeable, correlated flow signal is consistent with matter responding collectively to the initial shape of the collision zone rather than particles escaping independently.
The measurements are important because flow is one of the main signatures used to study fluid-like QCD matter. In a small collision system, however, it is more precise to describe the result as evidence for collective, QGP-like behavior rather than as proof that the entire fireball reaches full local equilibrium.
ATLAS: energy loss in a hot, dense medium
ATLAS observed an imbalance between pairs of jets produced back to back in oxygen–oxygen and neon–neon collisions. The imbalance increases in more central, head-on events and is interpreted as parton energy loss while the jets pass through the matter created in the collision. CERN describes this observation as unambiguous evidence of the energy-loss effect. 1
This provides a complementary test to flow. Flow shows that the produced matter responds collectively; jet imbalance indicates that energetic partons interact with that matter as they travel through it.
CMS: long-range collective correlations
CMS measured long-range two- and four-particle correlations across nearly five units of pseudorapidity. Significant elliptic and triangular flow harmonics were observed in both oxygen–oxygen and neon–neon collisions at 5.36 TeV per nucleon pair. 17
Long-range correlations are consistent with a collective response extending across a substantial region of the collision. CMS also reported suppression of charged-particle production in the light-ion systems relative to proton–proton collisions, another QGP-sensitive effect summarized by CERN. 1
LHCb: another independent QGP-sensitive view
CERN reports that LHCb, along with ALICE, ATLAS and CMS, found signs associated with QGP in the oxygen and neon programme. 1
9 The supplied results do not identify a specific LHCb observable, so it would be misleading to assign one here. Its importance is that the light-ion evidence comes from all four major LHC collaborations rather than from a single detector or analysis.
How particle flow reveals nuclear shape
The same flow measurements that indicate collective behavior can also act as a probe of the nuclei before the collision. The basic idea is straightforward: if the initial overlap region is elongated, collective expansion converts that spatial asymmetry into a stronger elliptic pattern in the momenta of the final particles.
Oxygen-16 is comparatively round
In central oxygen–oxygen collisions, the overlap region is comparatively round. The elliptic flow that remains is therefore more strongly influenced by event-by-event fluctuations than by a large, persistent deformation of the oxygen nucleus. Hydrodynamic calculations that incorporate oxygen’s nuclear structure describe this behavior.
Neon-20 is elongated
Central neon–neon collisions show stronger elliptic flow than oxygen–oxygen collisions under comparable conditions. The preferred interpretation is that neon-20 has a prolate, intrinsically elongated shape—often described as resembling a bowling pin. Because the nuclei can be oriented differently when they collide, their orientation changes the geometry of the overlap and, consequently, the observed v₂ signal. 11
This makes light-ion collisions useful for two kinds of physics at once. They can test whether a tiny, short-lived system behaves collectively, while the collective response also provides information about how nucleons are arranged inside the colliding nucleus.
Why the core–corona model matters
Flow measurements show collective behavior, but they do not by themselves establish how much of the produced matter actually reaches local equilibrium. A study using Tetsufumi Hirano’s dynamical core–corona framework addresses that question by separating the collision into two evolving components:
- The core is the denser, interacting component treated as locally equilibrated and fluid-like.
- The corona consists of more dilute regions whose particles undergo too few interactions to thermalize and instead retain a non-equilibrated contribution.
For oxygen–oxygen collisions at 5.36 TeV, the model finds that the core contribution becomes larger than the corona contribution once the charged-particle multiplicity near midrapidity reaches approximately 20.
That crossover is not a sharp “QGP switch.” Even in the most central oxygen–oxygen collisions, a sizeable corona contribution remains. The study summary discussed here places that residual non-equilibrated component at roughly 30%, while emphasizing that it is not negligible.
The physical picture is therefore gradual thermalization. As multiplicity and interaction density rise, an increasingly large fraction of the system can be described by an equilibrated core, but the finite collision system still contains matter that does not fully join the fluid.
Why hydrodynamics alone is not enough
Hydrodynamics is useful for describing the collective core, but a small collision system cannot automatically be treated as a uniform fluid. A realistic description must also account for the corona, initial-state fluctuations, finite system size and the transition between dilute particle production and fluid-like QCD matter.
This distinction matters when interpreting the 2025 results. The observations support collective behavior associated with QGP formation, yet they do not imply that every produced particle passed through the same equilibrated medium. The strongest conclusion is more nuanced: QGP-like matter can emerge in small nuclei, while its equilibrated fraction changes continuously with event activity.
What small-nucleus collisions make possible
Oxygen and neon provide a useful size-and-shape scan between proton-based collisions and large lead–lead systems. They let researchers vary the nuclear geometry while keeping the collision energy and approximate system scale similar. 8
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That creates a new way to study three linked questions:
- When does collective QCD behavior begin? Light systems test how small a collision can be while still producing flow and energy-loss signatures.
- How does a QGP-like medium thermalize? The core–corona result points to a continuous change from dilute, non-equilibrated matter to a dominant equilibrated component.
- How does nuclear structure affect the final state? The contrast between rounder oxygen-16 and elongated neon-20 shows that the initial nuclear shape remains visible in the outgoing particle flow.
Future collisions with even lighter nuclei, such as helium-4, could push this test further. They would provide a stricter examination of whether collective signals survive as the system becomes smaller and whether microscopic transport increasingly has to replace a purely hydrodynamic description. That is a proposed direction for future study, not an established result from the 2025 run.
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CERN’s 2025 oxygen–oxygen and neon–neon collisions at 5.36 TeV per nucleon pair produced converging signs of quark–gluon plasma like matter in nuclei containing just 16 and 20 nucleons.
What are the key points to validate first?
CERN’s 2025 oxygen–oxygen and neon–neon collisions at 5.36 TeV per nucleon pair produced converging signs of quark–gluon plasma like matter in nuclei containing just 16 and 20 nucleons. ALICE and CMS observed collective flow, while ATLAS reported jet energy imbalance consistent with parton energy loss; the experiments also used flow patterns to distinguish relatively round oxygen 16 from elongated, “...
What should I do next in practice?
A dynamical core–corona analysis found that the equilibrated core overtakes the corona above a midrapidity multiplicity of about 20, while a substantial non equilibrated component persists even in central oxygen–oxyge...
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arxiv.orgObservation of long-range collective flow in OO and NeNe collisions and implications for nuclear structure studies