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

ALICE Collaboration

AI summary · arXiv: 2610.36237v1 · nucl-ex

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

ALICE Collaboration

Original paper Submitted to arXiv on 26 Oct. 2026 23 pages, 3 captioned figures, 1 table, authors from page 17, submitted to PRL, figures at http://alice-publications.web.cern.ch/node/13688 License: CC BY 4.0 Also in: fr

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The essentials

ALICE measures identified-hadron elliptic flow in OO collisions and finds mass ordering at low momentum and significant baryon–meson splitting at intermediate momentum, supporting partonic collectivity.

Question

Do signatures of collective expansion and partonic hadronization observed in larger collision systems persist in light-ion OO collisions? The study uses the φ meson, whose mass is close to the Λ baryon’s but whose mesonic identity helps distinguish mass effects from particle-type effects.

Method

ALICE measured pT-differential v2 for K⁰S, Λ (and anti-Λ), and φ in OO collisions at √sNN = 5.36 TeV, using approximately 3 billion selected collisions and 0–10% and 10–40% centrality classes. The analysis covered 0.5 < pT < 8.0 GeV/c and used two-particle azimuthal correlations with a |Δη| > 1.2 gap and a low-multiplicity template fit to reduce non-flow effects; hadron signals were extracted with simultaneous invariant-mass and correlation fits.

Findings

Result At low pT, the measured flow follows a mass-ordered pattern, with K⁰S v2 greater than Λ and φ v2, and the stated pairwise separations exceed 10σ and 5σ, respectively.
See the passage in the paper
In both centrality classes, the separation between ${\rm K}^{0}_{\rm{S}}$ and $\Lambda$ exceeds $10\sigma$, while the difference between ${\rm K}^{0}_{\rm{S}}$ and $\phi$ exceeds $5\sigma$, demonstrating that the observed mass ordering is significant.
Result Above 3 GeV/c, Λ v2 exceeds meson v2, while φ and K⁰S v2 are compatible within uncertainties; the Λ–K⁰S separation exceeds 10σ and the Λ–φ separation exceeds 5σ in both centrality classes.
See the passage in the paper
In both centrality classes, the baryon–meson splitting of $v_{2}$ above 3 GeV/$c$ is quantified by separations exceeding $10\sigma$ between $\Lambda$ and ${\rm K}^{0}_{\rm{S}}$, and more than $5\sigma$ between $\Lambda$ and $\phi$.
Authors’ interpretation The φ follows the lighter K⁰S rather than the similarly massive Λ, which the authors interpret as evidence against mass alone determining the intermediate-pT pattern.
See the passage in the paper
In this context, the observation that the $\phi$ meson exhibits $v_{2}$ values comparable to those of the substantially lighter ${\rm K}^{0}_{\rm{S}}$ meson, rather than to those of the similarly massive $\Lambda$ baryon, provides a model-independent qualitative indication that the intermediate-$p_{\rm T}$ ordering of identified-hadron $v_{2}$ is not governed by hadron mass alone.
Authors’ interpretation The authors interpret the observed meson–baryon grouping and low-pT mass ordering together as evidence that signatures of partonic collectivity persist in OO collisions.
See the passage in the paper
Together, these measurements demonstrate that the key hallmarks of partonic collectivity persist in OO collisions, completing the identified-hadron flow picture in small nuclear systems and extending evidence for QGP-like behavior to the smallest nucleus–nucleus collisions accessible at the LHC.
Result The same qualitative ordering patterns were found when centrality was defined using the forward-rapidity FT0C estimator.
See the passage in the paper
The same qualitative features and ordering patterns are obtained using the forward-rapidity FT0C centrality estimator, indicating no significant dependence on the rapidity range of the centrality estimator.

Limitations

The measurement is restricted to identified-hadron v2 in two centrality classes and the stated transverse-momentum interval, so the conclusions do not directly establish the behavior of other observables or collision selections.
Extracting the single-particle flow coefficient relies on factorization of the residual two-particle correlation after non-flow suppression.
See the passage in the paper
The elliptic-flow coefficient of the POI is then obtained by assuming factorization of the residual two-particle correlation, after suppression of non-flow contributions through the $|\Delta\eta|>1.2$ requirement and the low-multiplicity template fit, into single-particle flow coefficients

Why it matters

The authors argue that identified-hadron flow patterns in OO collisions provide complementary evidence that key signatures of partonic collectivity, previously established in larger systems, persist in light-ion collisions. The φ–Λ comparison adds a probe for distinguishing mass-driven effects from particle-type-dependent hadronization patterns.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: oxygen–oxygen collisions elliptic flow partonic collectivity constituent-quark scaling hadronization

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Comprehensive Summary
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Original abstract

The paper presents the first measurements of elliptic flow (v₂) for three identified hadrons—K⁰S, Λ and ϕ—in oxygen–oxygen (OO) collisions at a centre-of-mass energy of 5.36 TeV per nucleon pair. The measurements use data from the ALICE detector at the LHC. The study asks whether flow-like patterns in these relatively small collision systems indicate collective motion of quarks before hadron formation, or whether they can be explained mainly by hadron mass and later-stage effects. The ϕ meson is particularly informative because its mass is close to that of the Λ baryon, but it is a meson. Comparing the two therefore helps distinguish mass-related effects from patterns associated with particle type and hadronization. The analysis uses approximately three billion selected collisions and reports results for the 0–10% and 10–40% centrality classes, defined primarily by charged-particle multiplicity at midrapidity. The particles are reconstructed through their decays: ϕ → K⁺K⁻, K⁰S → π⁺π⁻, and Λ → pπ⁻, with antiparticles included where applicable. The v₂ measurements cover transverse momenta from 0.5 to 8 GeV/c and are obtained from two-particle azimuthal correlations. A pseudorapidity gap greater than 1.2 units helps suppress short-range correlations unrelated to collective flow. The remaining correlations are analysed using a low-multiplicity template-fit method. Each identified particle’s flow is inferred relative to that of unidentified charged hadrons, under the assumption that residual correlations factorize into single-particle flow coefficients. Simultaneous fits to invariant-mass distributions and mass-dependent correlations separate signal from background. Systematic uncertainties are assessed by varying event, track, particle-identification, signal-extraction and non-flow selections. The method is also checked using alternative subtraction procedures and simulation-based tests. Below 2.2 GeV/c, the measurements show the expected mass ordering: K⁰S has a larger v₂ than Λ and ϕ, whose values are approximately equal. In both centrality classes, the K⁰S–Λ difference exceeds 10 standard deviations, while the K⁰S–ϕ difference exceeds 5. This ordering is consistent with collective expansion, in which radial and anisotropic flow combine to reduce the v₂ of heavier hadrons at a given momentum. Above roughly 3 GeV/c, the pattern changes. Λ v₂ exceeds that of the mesons, while ϕ and K⁰S have compatible values. The Λ–K⁰S separation exceeds 10 standard deviations in both centrality classes, and the Λ–ϕ separation exceeds 5. This is notable because the ϕ is nearly as massive as the Λ but follows the much lighter K⁰S, suggesting that mass alone does not determine flow at intermediate momentum. The pattern is qualitatively consistent with baryon–meson grouping from quark recombination, in which hadrons form from collectively flowing constituent quarks. The particles also follow an approximate common trend when momentum and v₂ are scaled by the number of constituent quarks, although the scaling is not exact. The authors conclude that key signatures associated with partonic collectivity in larger heavy-ion systems are also present in OO collisions. The findings support the presence of a collectively flowing medium and recombination-based hadronization in this smaller nuclear system. The ordering patterns remain similar when centrality is defined using the forward FT0C detector, supporting their robustness to the choice of multiplicity estimator. These results provide evidence for, rather than a direct observation of, a deconfined partonic medium.

What elliptic flow reveals about oxygen–oxygen collisions

In collisions between heavy atomic nuclei, particles can emerge in patterns that suggest the matter created in the impact behaves collectively. But do these patterns also appear when the colliding nuclei are smaller? A new study by the ALICE Collaboration investigates this question using oxygen–oxygen (OO) collisions at the Large Hadron Collider, at an energy of 5.36 TeV per nucleon pair.

The researchers report the first measurements of elliptic flow, known as v₂, for three identified particles: the K⁰S, the Λ baryon and the ϕ meson. Their results offer clues about whether quarks move collectively before forming hadrons, or whether the observed patterns can be explained largely by particle mass and effects that occur later.

Reading the shape of particle motion

Elliptic flow describes how particles are distributed in angle around the collision’s beam direction. If the system expands unevenly, more particles may travel in some directions than others. The strength of this azimuthal pattern is captured by v₂.

Measurements of v₂ can help scientists understand the matter created in a collision. In a collectively expanding system, particles of different masses often show different flow values at the same momentum. At higher momentum, a different pattern can emerge if hadrons form through the recombination of quarks that were already moving collectively.

The ϕ meson is especially useful for separating these possibilities. Its mass is close to that of the Λ, but the ϕ is a meson, while the Λ is a baryon. If mass alone controls their flow, their measurements might be expected to resemble one another. If their particle types matter too, their patterns could differ.

How the measurements were made

The analysis uses approximately three billion selected collisions. Results are reported for two centrality classes, 0–10% and 10–40%, defined mainly by the number of charged particles produced near the centre of the detector. The particles were identified through their decay products: ϕ → K⁺K⁻, K⁰S → π⁺π⁻ and Λ → pπ⁻, with antiparticles included where applicable.

The measured transverse-momentum range extends from 0.5 to 8 GeV/c. To estimate v₂, the team studied angular correlations between pairs of particles. A gap of more than 1.2 units in pseudorapidity helped reduce short-range correlations that do not arise from collective motion. The remaining correlations were analysed with a low-multiplicity template-fit method, relating each identified particle’s flow to that of unidentified charged hadrons.

The researchers also separated genuine particle signals from background by fitting invariant-mass distributions together with mass-dependent correlations. They tested how robust the results were by varying event, track, particle-identification and signal-selection criteria, and by comparing alternative subtraction methods and simulation-based checks.

Mass ordering at low momentum

Below about 2.2 GeV/c, the measurements show a familiar pattern: K⁰S has a larger v₂ than either Λ or ϕ, while the Λ and ϕ values are approximately equal. The difference between K⁰S and Λ exceeds 10 standard deviations in both centrality classes; the K⁰S–ϕ difference exceeds five.

This ordering is consistent with collective expansion. Radial expansion and anisotropic flow combine to shape the momentum distribution, and heavier particles can have a smaller v₂ than lighter ones at the same momentum. The low-momentum results therefore fit a picture in which the produced matter expands collectively.

A different pattern at higher momentum

Above roughly 3 GeV/c, the ordering changes. The Λ v₂ becomes larger than that of both mesons, while K⁰S and ϕ have compatible values. The Λ–K⁰S separation exceeds 10 standard deviations in each centrality class, and the Λ–ϕ separation exceeds five.

This contrast is striking because the ϕ and Λ have similar masses, yet their flow follows different groupings: the ϕ behaves more like the lighter K⁰S, while the Λ stands apart. That pattern suggests mass alone cannot explain the results at intermediate momentum.

Instead, the measurements are qualitatively consistent with quark recombination: hadrons may form from constituent quarks that already carry collective motion. Mesons contain two constituent quarks and baryons three, so their flow can group by particle type. When momentum and v₂ are scaled by the number of constituent quarks, the three particles follow an approximate common trend, although the scaling is not exact.

What the findings mean

The results show that key flow patterns associated with partonic collectivity in larger heavy-ion collisions are also present in the smaller OO system. They support the interpretation that the collision creates a collectively flowing medium and that recombination contributes to hadron formation.

The ordering remains similar when centrality is determined using the forward FT0C detector rather than the midrapidity multiplicity estimate, adding confidence that the pattern is not tied to one particular centrality measure. Still, the findings are evidence for a deconfined, collectively flowing partonic medium—not a direct observation of one. Together, the measurements provide a new way to study how collective behaviour and hadron formation develop in smaller nuclear collisions.

Created on 02 Nov. 2026

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Weekly letter Presented in "Particles, nuclei and gravitation this week" of 2 November 2026. Particles, nuclei and gravitation this week: Testing the hidden machinery of matter

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

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

What elliptic flow reveals about oxygen–oxygen collisions

Reading the shape of particle motion

How the measurements were made

Mass ordering at low momentum

A different pattern at higher momentum

What the findings mean

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