This summer, the Large Hadron Collider (LHC) took a breath of fresh air. Normally filled with beams of protons, the 27-km ring was reconfigured to enable its first oxygen–oxygen and neon–neon collisions. First results from the new data, recorded over a period of six days by the ALICE, ATLAS, CMS and LHCb experiments, were presented during the Initial Stages conference held in Taipei, Taiwan, on 7–12 September.

Smashing atomic nuclei into one another allows physicists to study the quark–gluon plasma (QGP), an extreme state of matter that mimics the conditions of the Universe during its first microseconds, before atoms formed. Until now, exploration of this hot and dense state of free particles at the LHC relied on collisions between heavy ions (like lead or xenon), which maximise the size of the plasma droplet created.

Collisions between lighter ions, such as oxygen, open a new window on the QGP to better understand its characteristics and evolution. Not only are they smaller than lead or xenon, allowing a better investigation of the minimum size of nuclei needed to create the QGP, but they are less regular in shape. A neon nucleus, for example, is predicted to be elongated like a bowling pin – a picture that has now been brought into sharper focus thanks to the new LHC results.

The experiments focused on measurements of subtle patterns in the angles and directions of the particles flying outward as the QGP droplet expands and cools, which are caused by small distortions in the original collision zone. Remarkably, these “flow” patterns can be described using the same fluid-dynamics calculations that are used to model everyday fluids, allowing researchers to probe both the properties of the QGP and the geometry of the colliding nuclei. Accurate model predictions enable a more precise exploration of flow in oxygen–oxygen and neon–neon collisions than in proton–proton and proton–lead collisions.

ALICE, which specialises in the study of the QGP, as well as the general-purpose experiments ATLAS and CMS, have measured sizeable elliptic and triangular flow in oxygen–oxygen and neon–neon collisions, and found that these depend strongly on whether the collisions are glancing or head-on. The level of agreement between theory and data is comparable to that obtained for collisions of heavier xenon and lead ions, despite the much smaller system size. This provides strong evidence that flow in oxygen–oxygen and neon–neon collisions is driven by nuclear geometry, supporting the bowling-pin structure of the neon nucleus and demonstrating that hydrodynamic flow emerges robustly across collision systems at the LHC.

Complementary results presented last week by the LHCb collaboration confirm the bowling-pin shape of the neon nucleus. The results are based on lead–argon and lead–neon collisions in a fixed-target configuration, using data recorded in 2024 with its SMOG apparatus. The LHCb collaboration has also started to analyse the oxygen–oxygen and neon–neon collision data.

“Taken together, these results bring fresh perspectives on nuclear structure and how matter emerged after the Big Bang,” says CERN Director for Research and Computing Joachim Mnich.

Further material

Animation showing side-by-side comparison of lead-lead and oxygen-oxygen collision

Animations showing the quark–gluon plasma formed in collisions between heavy ions

Animation showing the quark-gluon plasma produced in a lead-lead collision and formed by squeezing a group of protons together

Quark-gluon plasma explained - Youtube Video