⚛️ nuclear experiments

Anisotropic flow measurements in OO and Ne-Ne collisions with ALICE

This paper presents ALICE measurements of anisotropic flow in ${}^{16}$ O and ${}^{20}$ Ne collisions using multi-particle cumulants, including the first reconstruction of the elliptic flow probability distribution and the first measurement of the $v_3\{4\}/v_3\{2\}$ ratio in light-ion systems, to investigate the influence of well-constrained nuclear geometry on collective dynamics.

Original authors: Vojtěch Macháček, for the ALICE Collaboration

Published 2026-10-26

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Vojtěch Macháček, for the ALICE Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep within the heart of matter, protons and neutrons are not solid, unchanging spheres. Instead, they are bustling cities of quarks and gluons, constantly shifting and interacting. When scientists smash atomic nuclei together at nearly the speed of light, they create a fleeting, super-hot soup of these fundamental particles known as the quark-gluon plasma. This state of matter is believed to have filled the entire universe just moments after the Big Bang. For decades, researchers have studied this plasma by colliding heavy atoms like lead, which create large, chaotic fireballs. However, a surprising discovery in recent years showed that even when smashing much smaller particles, like protons against lead, this same fluid-like behavior emerges. This raised a profound question: is the fluid behavior driven solely by the sheer energy of the collision, or does the specific shape and internal structure of the colliding nuclei play a crucial role in how the plasma flows?

To answer this, a team of physicists using the ALICE detector at the Large Hadron Collider turned their attention to light ions, specifically oxygen and neon. Unlike the heavy lead nuclei, the nuclear structures of oxygen and neon are well-constrained from decades of low-energy physics experiments. By colliding these specific nuclei at an energy of 5.36 TeV, the researchers created a controlled environment to test how the initial geometry of the nuclei influences the resulting flow of particles. They analyzed billions of oxygen-oxygen collisions and hundreds of millions of neon-neon collisions to see if the specific structural differences between the nuclei left a distinct fingerprint on the plasma they created.

The researchers focused on a phenomenon called anisotropic flow, which describes how particles fly out from the collision point. If the collision zone were perfectly round, particles would spray out equally in all directions. However, because the nuclei are not always perfectly aligned, the collision zone often has an oval or triangular shape. This initial shape acts like a mold, pushing the particles to flow more strongly in certain directions than others. By measuring how particles move in relation to one another, the team could reconstruct the hidden shapes of the collision zones and the probability of different flow patterns occurring. They looked at correlations between groups of particles, ranging from pairs up to groups of eight, to get a detailed picture of the underlying distribution of these flows.

The results confirmed that the nuclear shape is indeed a dominant factor. In the most central collisions, where the two nuclei hit each other head-on and overlap completely, the neon-neon collisions produced a stronger elliptical flow than the oxygen-oxygen collisions. This difference suggests a more elliptical shape of the neon nucleus in comparison to the oxygen nucleus, as expected by current models. The data showed that the probability distribution of this flow was wider and more pronounced in neon, matching the expectation that a more elliptical nucleus creates a more elliptical overlap region than a less elliptical one. Furthermore, the study revealed that as the collisions became less central, the flow patterns evolved in a way that tracked the changing geometry of the overlapping nuclei, providing a clear link between the static structure of the atom and the dynamic behavior of the plasma.

Perhaps the most significant finding was the first-ever reconstruction of the full probability distribution for the elliptic flow in these light-ion systems. This allowed the team to see not just the average flow, but the full range of possible flow outcomes for every single collision. They also examined the triangular flow, which arises from random fluctuations in the positions of particles inside the nucleus. In heavy lead collisions, this triangular flow is usually driven almost entirely by random fluctuations because the lead nuclei are so large and round that their initial shape is nearly circular. However, in the neon collisions, the researchers found that the triangular flow was enhanced beyond what random fluctuations alone would produce. This suggests that the initial, non-circular shape of the neon nucleus is actively driving the flow, rather than just random noise. These measurements provide a new, precise way to disentangle the effects of nuclear structure from the properties of the plasma itself, offering a clearer window into the fundamental dynamics of the early universe.

Technical Summary: Anisotropic Flow Measurements in OO and Ne–Ne Collisions with ALICE

Problem and Motivation
While ultra-relativistic heavy-ion collisions at the LHC have established the existence of a quark–gluon plasma (QGP), recent observations of collective behavior in small collision systems (pp and p–Pb) have introduced a new frontier. However, interpreting collectivity in these small systems is complicated by uncertainties regarding proton subnucleonic structure. To disentangle the influence of nuclear geometry from other dynamic effects, the ALICE collaboration utilized light-ion collisions involving $^{16}\text{O}$ and $^{20}\text{Ne}$ . Unlike protons, the nuclear structures of these light ions are well-constrained by low-energy physics experiments, offering a unique opportunity to study how initial nuclear geometry drives anisotropic flow in systems where the QGP is formed.

Methodology
The analysis utilized data recorded by the ALICE detector during a dedicated light-ion campaign in July 2025 at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.36$ TeV. The dataset comprises approximately $3 \times 10^9$ oxygen-oxygen (OO) and $4.1 \times 10^8$ neon-neon (Ne–Ne) minimum-bias collisions.

Key methodological features include:

Event Selection and Kinematics: Collision centrality was estimated using multiplicity measurements in the forward region ( $-3.3 < \eta < -2.1$ ) via the FT0C detector. The analysis focused on charged particles with transverse momentum $0.2 < p_T < 3.0$ GeV/c and pseudorapidity $|\eta| < 0.8$ .
Correlation Analysis: The study employed the Generic framework based on multi-particle cumulants using weighted Q-vectors. To suppress non-flow effects (such as jet production and resonance decays), a two-subevent method with pseudorapidity gaps ( $|\Delta\eta| > 1.4$ for two-particle and $|\Delta\eta| > 0$ for four-particle correlations) was applied. Standard methods were used for higher-order correlations.
Systematic Uncertainties: These were evaluated by varying the primary vertex position and track selection criteria (distance of closest approach, TPC cluster counts, and crossed rows), with uncertainties combined in quadrature.
Observables: The analysis measured anisotropic flow coefficients ( $v_n$ ) up to the fourth harmonic using up to eight-particle correlations. It also reconstructed the probability distribution function (p.d.f.) of elliptic flow, $P(v_2)$ , and measured the ratio $v_3\{4\}/v_3\{2\}$ to probe triangular flow fluctuations.

Key Contributions and Results
The paper presents the first comprehensive measurements of multi-particle anisotropic flow in light-ion collisions, extending previous findings with higher-order correlations and new distributional analyses:

Flow Coefficient Ordering: In both OO and Ne–Ne collisions, an ordering of coefficients $v_2 > v_3 > v_4$ was observed. Furthermore, the multi-particle elliptic flow coefficients followed the hierarchy $v_2\{2\} > v_2\{4\} \approx v_2\{6\} \approx v_2\{8\}$ . This ordering supports the hypothesis that the underlying distribution of the elliptic flow coefficient $v_2$ follows an elliptic power distribution, a model previously validated in large collision systems.
Nuclear Geometry and $P(v_2)$ : The first reconstruction of the $P(v_2)$ distribution in light-ion systems revealed that the mean elliptic flow $\langle v_2 \rangle$ is significantly larger in central Ne–Ne collisions compared to OO collisions. This enhancement is attributed to the more elliptical shape of the $^{20}\text{Ne}$ nucleus relative to the spherical $^{16}\text{O}$ , consistent with current nuclear structure models. The evolution of $P(v_2)$ with centrality in OO collisions further reflects changes in the overlap geometry.
Geometry-Driven Ratios: The ratio of multi-particle elliptic flow measurements, $v_2\{m\}(\text{Ne–Ne})/v_2\{m\}(\text{OO})$ , deviates from unity, particularly in central collisions where nuclei fully overlap. This deviation directly reflects the differences in the initial nuclear shapes of the colliding systems.
Triangular Flow Fluctuations: While a full reconstruction of the $P(v_3)$ distribution was not possible due to statistical limitations, the ratio $v_3\{4\}/v_3\{2\}$ was measured. In Ne–Ne collisions, this ratio exceeds the universal limit expected for Bessel-Gaussian fluctuations (a limit typically seen in central Pb–Pb collisions where the overlap is circular). This enhancement suggests that the initial nuclear shape influences triangular flow, rather than flow being driven solely by fluctuations.

Significance
The paper claims that these measurements provide deeper insight into the role of nuclear geometry in driving collective flow in light-ion collisions. By utilizing systems with well-known nuclear structures, the results offer critical constraints on the initial state of the collision. Specifically, the findings provide essential input for Bayesian analyses aimed at disentangling the properties of the QGP in light-ion collisions, helping to distinguish between effects arising from initial nuclear geometry and those intrinsic to the QGP medium's transport properties.

Technical Summary: Anisotropic Flow Measurements in OO and Ne–Ne Collisions with ALICE

More like this