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Observation of partonic flow in proton-proton and proton-nucleus collisions

This paper reports the first observation of distinctive baryon-meson grouping in anisotropic flow within high-multiplicity proton-proton and proton-lead collisions at the LHC, providing strong evidence for the formation of partonic flowing systems consistent with quark-gluon plasma in these smaller collision systems.

Original authors: ALICE Collaboration

Published 2026-09-25

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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 first moments after the Big Bang, the universe was not made of the solid atoms that build our world today. Instead, it was a seething, super-hot soup of fundamental particles called quarks and gluons. These particles, which usually stick together to form protons and neutrons, were free-floating in a state of matter known as the quark-gluon plasma. As the universe cooled, this plasma froze into the hadrons we see today. To understand how this transition happened, scientists recreate these extreme conditions by smashing heavy atomic nuclei together at nearly the speed of light. When these massive collisions occur, they create a tiny, fleeting drop of this primordial plasma. By studying how the particles fly out of these collisions, researchers can learn how the plasma behaves, specifically looking for signs that the particles moved together in a coordinated, fluid-like way rather than scattering randomly.

For decades, this coordinated movement, or "flow," was only observed in collisions involving heavy ions, where the sheer number of particles suggested a large, liquid-like drop had formed. However, in recent years, scientists noticed strange similarities in much smaller collisions, such as those between a single proton and a lead nucleus, or even between two protons. These tiny systems were not expected to be large enough to form a fluid. The question became whether these small collisions also created a tiny drop of quark-gluon plasma, or if the observed patterns were caused by something else entirely. The ALICE collaboration at the Large Hadron Collider has now provided the clearest evidence yet that even in these smallest collisions, a fluid-like state of matter does indeed form.

The researchers focused on high-multiplicity events, which are collisions that produce a surprisingly large number of particles, and compared them to low-multiplicity events that produce fewer particles. They analyzed data from collisions of protons with lead nuclei at an energy of 5.02 tera-electronvolts, and collisions of two protons at 13 tera-electronvolts. To make sense of the chaos, they sorted the particles into two main families: mesons, which are made of a pair of quarks, and baryons, which are made of three quarks. They then measured how these particles were distributed as they moved outward from the collision point. In a fluid that expands, heavier particles tend to move differently than lighter ones at low speeds, a pattern the team confirmed they could see even in these tiny collisions.

The most significant discovery came when they looked at particles moving at intermediate speeds. In this range, the team found a distinct separation between the behavior of the baryons and the mesons. The baryons showed a stronger coordinated flow than the mesons. This specific grouping is a hallmark of a system where the particles are formed by the combination of flowing quarks. Imagine a crowd of people moving in a coordinated wave; if the wave is made of individuals, the groups formed by three people moving together will behave differently than groups of two. In the same way, the fact that the three-quark baryons and two-quark mesons split into different flow patterns suggests that the flow happened at the level of the individual quarks before they combined to form the particles we detect.

To confirm this interpretation, the team compared their measurements against sophisticated computer models. One model, which included the physics of a flowing quark-gluon plasma followed by the quarks combining to form particles, successfully reproduced the observed patterns. This model matched the data for both the proton-lead and proton-proton collisions. In contrast, other models that tried to explain the results without a fluid phase, or that relied on different mechanisms like string-like interactions, failed to capture this specific separation between baryons and mesons. These alternative models could not explain why the three-quark particles flowed more strongly than the two-quark ones at these speeds.

The study also addressed previous uncertainties in how to measure these effects. Earlier attempts to find this pattern in small systems were often confused by "noise" from unrelated particle interactions. The ALICE team used a refined technique to filter out this noise, ensuring that the signal they measured truly came from the collective motion of the system. By carefully separating the real flow from background effects, they were able to show that the baryon-meson grouping is a robust feature, appearing with high statistical certainty in both types of small collisions.

This work suggests that the formation of a quark-gluon plasma is not limited to the massive collisions of heavy ions. Even in the smallest collisions between protons, if enough energy is packed into a small space, a tiny droplet of this primordial fluid can form and expand. The findings indicate that the transition from a fluid of free quarks to solid matter is a fundamental property of high-energy collisions, occurring regardless of the size of the colliding objects, provided the conditions are right. The results provide a strong argument that the laws governing the behavior of the early universe apply even in the microscopic, high-speed collisions happening today in the laboratory.

Technical Summary: Observation of Partonic Flow in Proton–Proton and Proton–Nucleus Collisions

Problem and Context
Quantum Chromodynamics (QCD) predicts a phase transition from hadronic matter to a deconfined state of quarks and gluons, known as the quark–gluon plasma (QGP), at high energy densities. This state is traditionally created in ultrarelativistic heavy-ion collisions. A key signature of QGP formation is anisotropic flow, specifically the distinctive grouping of elliptic flow ( $v_2$ ) coefficients for baryons and mesons at intermediate transverse momentum ( $p_T$ ). In heavy-ion collisions, this phenomenon is interpreted as evidence of hadron formation via quark coalescence from a collectively flowing partonic medium.

However, the observation of QGP-like features, such as long-range azimuthal correlations and mass-dependent $v_2$ , in smaller collision systems (proton–proton and proton–nucleus) has sparked debate regarding the formation of a QGP in these environments. Previous measurements in small systems have yielded ambiguous results concerning baryon–meson $v_2$ splitting. Some studies failed to observe clear grouping, potentially due to difficulties in separating collective flow from "non-flow" effects (correlations from resonance decays and jets) or the use of subtraction methods that may over-estimate non-flow contributions. The central problem addressed by this work is determining whether the system created in high-multiplicity (HM) proton–lead (p–Pb) and proton–proton (pp) collisions at the Large Hadron Collider (LHC) undergoes a phase of hydrodynamically flowing partons, similar to that observed in heavy-ion collisions.

Methodology
The ALICE Collaboration analyzed data from pp collisions at $\sqrt{s} = 13$ TeV and p–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV collected during the LHC Run 2 (2016–2018).

Event Selection: Events were categorized into high-multiplicity (HM) and low-multiplicity (LM) classes. HM events were selected based on the highest 0.07% (pp) or 0–20% (p–Pb) of the V0 detector amplitude distribution, with an additional requirement on the number of reconstructed charged particles ( $N_{ch}$ ) at midrapidity ( $|\eta| < 0.8$ ) to ensure comparable average multiplicities ( $\langle N_{ch} \rangle \approx 35$ ) between the two systems.
Particle Identification: Identified hadrons included mesons ( $\pi^\pm, K^\pm, K^0_S$ ) and baryons ( $p+\bar{p}, \Lambda+\bar{\Lambda}$ ). Identification was achieved using specific energy loss ($dE/dx$) in the Time Projection Chamber (TPC) and Time-of-Flight (TOF) information, employing a Bayesian approach to ensure high purity.
Flow Measurement: The elliptic flow coefficient $v_2$ was measured using the two-particle correlation (2PC) method. To suppress non-flow effects, a large pseudorapidity separation ( $|\Delta\eta| > 1.1$ ) was enforced between the trigger particle (identified hadron at midrapidity) and reference particles (in forward/backward rapidity regions).
Non-Flow Suppression: A template fit method was employed to extract the flow signal. The correlation function from LM events, assumed to contain primarily non-flow effects, was used as a template to fit the HM correlation functions. This approach reduced residual non-flow contributions to approximately 6% at low $p_T$ and less than 1% at higher $p_T$ .
Theoretical Comparison: Results were compared with the Hydro-Coal-Frag hybrid model, which combines viscous hydrodynamics (for low $p_T$ ), quark coalescence (for intermediate $p_T$ ), and linear Boltzmann transport with fragmentation (for high $p_T$ ). Alternative models, including Hydro-Frag (without coalescence), AMPT, and PYTHIA 8 variants, were also considered.

Key Results

Baryon–Meson Grouping: In both high-multiplicity p–Pb and pp collisions, a clear separation in $v_2$ is observed at intermediate $p_T$ ( $3.0 < p_T < 8.0$ GeV/c). The $v_2$ values for baryons ( $p+\bar{p}, \Lambda+\bar{\Lambda}$ ) are significantly larger than those for mesons ( $\pi^\pm, K^\pm, K^0_S$ ) by approximately $5\sigma$ . Within their respective categories, baryons and mesons group together within $1\sigma$ .
Mass Ordering and Crossing: At low $p_T$ ( $p_T < 2.0$ GeV/c), a mass ordering of $v_2$ is observed (heavier particles have lower $v_2$ ), consistent with radial flow. Around $p_T \approx 2.0\text{--}3.0$ GeV/c, the $v_2$ curves for different particle species cross, transitioning from mass ordering to the baryon–meson splitting pattern.
System Similarity: The magnitude and $p_T$ dependence of the $v_2$ grouping in small systems are qualitatively similar to those observed in semicentral Pb–Pb collisions, although the absolute magnitude of $v_2$ is smaller in the smaller systems.
Model Comparison:
- The Hydro-Coal-Frag model, which includes partonic flow followed by hadronization via quark coalescence, successfully reproduces the baryon–meson grouping and splitting observed in the data for both p–Pb and pp collisions up to $p_T \approx 6\text{--}8$ GeV/c.
- The Hydro-Frag model (without coalescence) fails to reproduce the splitting, underestimating $v_2$ at intermediate $p_T$ and showing only mass ordering or reversed ordering.
- Other models, such as AMPT (string melting) and PYTHIA 8 with string shoving, either fail to generate the observed splitting or produce negative flow coefficients inconsistent with the data.

Significance and Claims
The paper claims that the observation of distinctive baryon–meson $v_2$ grouping in high-multiplicity pp and p–Pb collisions provides evidence for the formation of a system with hydrodynamically flowing partons in these small collision systems. The results suggest that the mechanism of hadron formation via quark coalescence, driven by a partonic phase, is active in high-multiplicity small systems, similar to the QGP formed in heavy-ion collisions.

The author emphasizes that previous discrepancies in small-system measurements likely arose from inadequate non-flow subtraction techniques. By utilizing a robust template fit method and large $|\Delta\eta|$ separations, this analysis isolates the collective flow signal, revealing a pattern consistent with partonic collectivity. The successful description of the data by the Hydro-Coal-Frag model, and the failure of models lacking quark coalescence or partonic flow, supports the interpretation that a deconfined, collectively flowing medium is formed even in high-multiplicity proton–proton and proton–nucleus collisions at the LHC.

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