The Large Hadron Collider (LHC) smashes together atomic nuclei to create a tiny, short-lived fireball of matter called quark-gluon plasma (QGP). The QGP is thought to have filled the Universe immediately after the Big Bang. Physicists at the ATLAS experiment are now studying the collisions of oxygen and neon nuclei, to understand how their shape impacts the dynamics of the QGP.

As the QGP expands and cools inside the ATLAS experiment, small distortions in the original collision zone transform into subtle patterns in the angles and directions of the particles flying outward. This transformation relies on momentary interactions of the particles, which is often modeled with hydrodynamics — the theory of how fluids behave. Hence, these patterns are known as flow harmonics, and allow researchers to probe both the properties of the QGP and the geometry of the colliding nuclei. The elliptic flow (v₂) is especially sensitive to whether the collision zone was elongated (oval-shaped) or more circular.

In summer 2025, the ATLAS experiment recorded collisions of oxygen–oxygen and neon–neon as part of a dedicated light-ion programme at the LHC. Oxygen and neon have similar masses, which makes them especially useful to compare as other size-related effects are minimized. Thus, any differences in their v₂ measurements would strongly hint that the two nuclei have different shapes.

The ATLAS Collaboration today presented their first study of oxygen and neon collisions at the Initial Stages conference, including first measurements of elliptic flow. This result comes just nine weeks after data-taking, reflecting the significant efforts of members across the ATLAS Collaboration, in particular its heavy-ion community.

When studying the most head-on collisions (i.e. most central), researchers found that neon’s elliptic flow was the largest, followed by oxygen, with lead’s v₂ substantially smaller (see Figure 1). These flow measurements map well with physicists’ understanding of the nuclei. Lead-208 is a “doubly-magic” nucleus. It has full “shells” of both protons and neutrons, leading to an especially tightly bound and nearly spherical ground state – a true bowling ball. When such spherical nuclei are collided head-on, they produce little intrinsic ellipticity, and thus a small v₂ measurement. By contrast, many nuclear-structure calculations predict that neon is noticeably elongated (a prolate shape) – akin to a bowling pin!

This built-in elongation produces a larger elliptic overlap, even in the most head-on collisions. This geometric bias translates into a larger v₂ measurement, as observed by the ATLAS Collaboration. Oxygen’s measured v₂ sits between neon and lead, consistent with models that describe it as having a roughly spherical or weakly clustered structure.

When comparing systems of near-equal mass, many detector effects and global features cancel out in ratios (see Figure 2). This makes the remaining difference a much cleaner fingerprint of initial geometry rather than, say, different amounts of energy deposited in the collision. In this sense, the pattern of particles emitted by the QGP fireball serves as an ultrafast snapshot of the nuclei’s shapes at the instant of overlap.

The impact of today’s results extends to both nuclear structure and QGP theory. These measurements provide new constraints on models that predict clustering and deformation in light nuclei, and they test how those initial-state geometries are translated into observable flow by the same hydrodynamic physics that underpins heavy-ion collisions. Future studies of more complex observables can reveal more about how a nuclei’s geometry impacts these collisions, including whether more energetic collisions (larger average momentum) yield more elliptic particle production.

Learn more

• Measurement of the azimuthal anisotropy of charged particles in 5.36 TeV ¹⁶O+¹⁶O and ²⁰Ne+²⁰Ne collisions with the ATLAS detector (arXiv:2509.05171)
• Initial Stages 2025 presentation by Brian Cole: Overview of the ATLAS experiment at LHC
• Initial Stages 2025 presentation by Aman Dimri: Measurements of particle correlations in O+O and Ne+Ne collisions with the ATLAS detector
• The unexpected uses of a bowling pin: exploiting 20Ne isotopes for precision characterizations of collectivity in small systems (Giacalone et al., arXiv:2402.05995)
• ATLAS takes a breath of oxygen (ATLAS News, July 2025)
• Looking inside trillion degree matter with ATLAS at the LHC (ATLAS Feature, May 2022)