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Evidence of partonic collectivity in OO collisions through the measurement of $\phi$ , K $^0_{\rm S}$ , and $\Lambda$ elliptic flow

This paper presents the first measurement of elliptic flow for $\phi$ , K $^0_{\rm S}$ , and $\Lambda$ hadrons in O+O collisions at 5.36 TeV, revealing mass ordering at low transverse momentum and a significant baryon-meson splitting at intermediate transverse momentum that supports the existence of partonic collectivity and quark recombination in small collision systems.

Original authors: ALICE Collaboration

Published 2026-10-28

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CC BY 4.0

Original authors: 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

In the heart of matter, where atoms are stripped of their electrons and their nuclei are smashed together at nearly the speed of light, a strange and fleeting state of existence emerges. This is the quark-gluon plasma, a super-hot soup where the fundamental building blocks of the universe—quarks and gluons—roam freely instead of being locked inside protons and neutrons. Physicists have long studied this state using collisions of heavy atoms, like lead, which create a large, dense fireball that behaves like a perfect fluid, flowing together in a coordinated way. However, a lingering question has been whether this fluid-like behavior is a unique property of massive collisions or if it can also appear in much smaller systems, such as collisions between lighter nuclei. To answer this, scientists needed to watch how different types of particles move and organize themselves as the fireball expands and cools, looking for specific patterns that reveal whether the particles are flowing together or simply flying apart randomly.

A researcher using the ALICE detector at the Large Hadron Collider has now provided a definitive look at this phenomenon in a new, smaller setting: collisions between oxygen nuclei. By smashing these light nuclei together at an energy of 5.36 tera-electronvolts, the researcher created a tiny, short-lived fireball and measured how three specific types of particles moved within it. They focused on the phi meson, a particle made of a strange quark and its antiparticle; the K-short meson, a lighter particle also containing strange quarks; and the lambda baryon, a heavier particle made of three quarks. The researcher tracked the elliptic flow of these particles, a measure of how much they prefer to move in a specific direction relative to the shape of the collision, which acts as a fingerprint of the collective motion of the entire system.

The results reveal a clear and organized pattern that changes depending on the speed of the particles. When the particles move slowly, their behavior follows a strict rule based on their mass. The lighter particles move with a stronger directional flow, while the heavier ones lag behind, a pattern that matches what happens when a group of objects is pushed by a common wind. This observation confirms that even in these tiny oxygen collisions, the matter expands collectively, behaving like a fluid rather than a chaotic spray of independent particles. This finding is significant because it proves that the fluid-like state of matter is not exclusive to heavy collisions but can also form in the smallest nuclear systems accessible at the collider.

As the particles move faster, reaching speeds above 3 GeV/c, the pattern shifts dramatically. Instead of sorting by mass, the particles now sort by their family type. The lambda baryon, which is heavy, suddenly shows a much stronger directional flow than the phi meson, even though the phi meson is almost the same weight. The phi meson, despite its heavy mass, moves in step with the much lighter K-short meson. This separation is a powerful clue about how these particles are formed. If the flow were driven purely by mass, the heavy phi meson should have behaved like the heavy lambda. Instead, its behavior matches that of a meson, suggesting that these particles are not formed by simple cooling but are assembled from a flowing soup of quarks that recombine into new particles.

This specific behavior of the phi meson is particularly important because it rules out the idea that the observed patterns are just a result of the particles' weight. The fact that the phi meson follows the lighter meson rather than the similarly heavy baryon indicates that the particles inherit their motion from the quarks they are made of, which were flowing together before the particles even formed. The researcher measured this effect with high statistical certainty, finding that the difference between the baryon and the meson groups is so large that it is extremely unlikely to be a random fluctuation. These measurements provide the first direct evidence that the key signatures of a quark-gluon plasma persist in light nucleus collisions, completing the picture of how matter behaves in the smallest, most extreme environments created in the laboratory.

Technical Summary: Evidence of Partonic Collectivity in OO Collisions

Problem and Motivation
While the formation of a Quark-Gluon Plasma (QGP) and its collective hydrodynamic behavior are well-established in large nucleus-nucleus (e.g., Pb–Pb) collisions, the existence of similar collective phenomena in small collision systems remains a subject of intense investigation. Observations in high-multiplicity proton-proton (pp) and proton-lead (p–Pb) collisions—such as long-range azimuthal correlations and baryon-meson splitting in elliptic flow ( $v_2$ )—suggest collective behavior, but the interpretation is complicated by poorly constrained initial-state geometries and fluctuations in the proton structure.

Light-ion collisions, specifically Oxygen-Oxygen (OO) collisions, offer a unique intermediate regime. They bridge the gap between proton-induced and heavy-ion collisions while maintaining a well-constrained initial geometry due to the defined nuclear structure of oxygen. However, a complete understanding of how initial-state anisotropies translate into the hadronic final state requires measurements of identified hadrons. Specifically, distinguishing between mass-driven collective expansion and particle-type-dependent hadronization mechanisms (such as quark recombination) requires a probe that is sensitive to mass but distinct in quantum numbers. The $\phi$ meson is ideal for this purpose: it has a mass ($1.020 $GeV/$ c^2$) comparable to the $\Lambda$ baryon ($1.115 $GeV/$ c^2$) but possesses mesonic quantum numbers. Furthermore, its long lifetime ( $\sim 46$ fm/ $c$ ) ensures it decays outside the hadronic medium, minimizing contamination from late-stage rescattering.

Methodology
This paper presents the first measurement of the transverse momentum ( $p_T$ ) differential elliptic flow coefficient ( $v_2$ ) for $\phi$ , $K^0_S$ , and $\Lambda(\bar{\Lambda})$ hadrons in OO collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.36$ TeV, recorded by the ALICE detector during LHC Run 3.

Event Selection: Approximately 3 billion OO collisions were selected using the Fast Interaction Trigger (FIT) system. Events were classified into centrality classes (0–10%, 10–40%, and 65–100%) based on charged-particle multiplicity ( $N_{trk}$ ) at midrapidity ( $|\eta| < 0.8$ ) and forward rapidity (FT0C). The analysis focuses on the 0–10% and 10–40% classes.
Particle Reconstruction:
- $\phi$ meson: Reconstructed via the hadronic decay channel $\phi \to K^+K^-$ .
- $K^0_S$ and $\Lambda$ : Reconstructed via weak decay channels $K^0_S \to \pi^+\pi^-$ and $\Lambda \to p\pi^-$ .
- Strict topological and kinematic selections were applied to daughter tracks (e.g., Distance of Closest Approach to the primary vertex, decay radius, and Armenteros-Podolanski variables) to suppress combinatorial background.
Flow Measurement: The $v_2$ was measured using the two-particle azimuthal correlation technique. To suppress non-flow effects (e.g., resonance decays, jet fragmentation), a pseudorapidity gap of $|\Delta\eta| > 1.2$ was imposed between the particle of interest (POI) and reference particles.
Signal Extraction: For resonances decaying into pairs, a simultaneous fit was performed on the invariant mass distribution and the corresponding two-particle azimuthal correlation coefficient ( $V_{2\Delta}$ ). This allowed for the extraction of the signal $v_2$ ( $v^2_{sig}$ ) by separating the flow contribution from the combinatorial background.
Systematic Uncertainties: Extensive variations in event selection, track reconstruction, particle identification, and signal extraction were performed. Uncertainties were quantified via the root-mean-square of the spread in $v_2$ values relative to the default configuration.

Key Results
The analysis yields the $p_T$ -differential $v_2$ for $K^0_S$ , $\Lambda$ , and $\phi$ in two centrality classes:

Low $p_T$ ( $p_T \lesssim 2.2$ GeV/ $c$ ): A clear mass ordering of $v_2$ is observed: $v_2(K^0_S) > v_2(\Lambda) \approx v_2(\phi)$ .
- The separation between $K^0_S$ and $\Lambda$ exceeds $10\sigma$ , and between $K^0_S$ and $\phi$ exceeds $5\sigma$ .
- This hierarchy is consistent with hydrodynamic models where a common radial and anisotropic velocity field shifts heavier hadrons to higher $p_T$ , resulting in lower $v_2$ values at a fixed low $p_T$ .
Intermediate $p_T$ ( $p_T \gtrsim 3$ GeV/ $c$ ): The ordering pattern changes, revealing a distinct baryon-meson splitting.
- The $\Lambda$ baryon exhibits significantly larger $v_2$ values than the mesons ( $K^0_S$ and $\phi$ ).
- The separation between $\Lambda$ and $K^0_S$ exceeds $10\sigma$ , and between $\Lambda$ and $\phi$ exceeds $5\sigma$ .
- Crucially, despite having a mass similar to the $\Lambda$ , the $\phi$ meson follows the trend of the lighter $K^0_S$ meson rather than the $\Lambda$ baryon.

Significance and Claims
The paper claims these results provide the first evidence of partonic collectivity in OO collisions through identified hadron flow. The significance is articulated as follows:

Disentangling Mass and Hadronization: The behavior of the $\phi$ meson is the critical observation. If the intermediate- $p_T$ ordering were driven purely by hadron mass, the $\phi$ (mass $\approx \Lambda$ ) should exhibit $v_2$ values similar to the $\Lambda$ . Instead, the $\phi$ follows the $K^0_S$ (meson). This disfavors a purely mass-driven interpretation and supports the hypothesis that hadronization occurs via quark recombination from a collectively flowing partonic medium, where hadrons inherit the flow of their constituent quarks (baryons from three quarks, mesons from two).
Persistence of QGP Signatures: The observation of both mass ordering at low $p_T$ and baryon-meson splitting at intermediate $p_T$ in OO collisions demonstrates that the key signatures of partonic collectivity, previously established in large Pb–Pb systems, persist in the smallest nucleus-nucleus collisions accessible at the LHC.
Robustness: The results are consistent across different centrality estimators (midrapidity and forward rapidity), indicating that the observed collectivity is a robust feature of the system evolution rather than an artifact of the event selection.

In conclusion, the paper asserts that OO collisions provide a unique, well-constrained environment to confirm that the collective behavior observed in heavy-ion collisions extends to light nuclear systems, with the $\phi$ meson serving as a decisive probe for quark recombination mechanisms.

Technical Summary: Evidence of Partonic Collectivity in OO Collisions

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