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Hyperon ( $\Lambda$ ) polarization along the beam axis in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.36$ TeV

This paper presents the first observation of $\Lambda$ hyperon polarization along the beam axis relative to the third-order event plane in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.36$ TeV, providing new constraints on the bulk and shear viscosities of the quark-gluon plasma through comparisons with hydrodynamic calculations.

Original authors: ALICE Collaboration

Published 2026-06-17

📖 4 min read🧠 Deep dive

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

Imagine two heavy balls of lead smashing into each other at nearly the speed of light. When they collide, they don't just bounce; for a split second, they create a tiny, super-hot "soup" of matter called Quark-Gluon Plasma (QGP). This soup is so hot and dense that it behaves like a nearly perfect fluid, swirling and spinning with incredible speed.

This paper from the ALICE collaboration at CERN is like a detective story about that swirling soup. Here is what they found, explained simply:

1. The Spinning Soup and the "Soccer Ball" Effect

When these lead balls collide, they don't always hit perfectly head-on. Sometimes they graze each other, like two soccer balls rolling past one another. This creates a "twist" or a vortex in the resulting soup, much like water swirling down a drain.

Because of this swirling motion, the tiny particles created in the crash (specifically particles called Lambda hyperons) start to spin in a specific direction, like tops. This is called polarization. The scientists wanted to know: Does the direction these particles spin depend on how the soup is swirling?

2. The New Clue: The "Third-Order" Twist

In previous experiments, scientists looked at the main, big swirl of the soup (like the main current in a river). They found that the particles did spin in a pattern related to this main swirl.

In this new study, the scientists looked for something much more subtle: smaller, wiggly ripples in the soup's flow. They call these "higher-order event planes."

The Second-Order Plane: Think of this as the main oval shape of the collision (like a slightly squashed circle).
The Third-Order Plane: Think of this as a triangular wobble or a ripple that happens because the collision isn't perfectly smooth; it's a bit lumpy.

The Big Discovery: This is the first time anyone has measured the spin of these particles relative to that "triangular wobble" (the third-order plane) at the Large Hadron Collider. They found that the particles do spin in a pattern related to this wobble, just like they do with the main oval shape.

3. Why the "Sticky" Matters (Viscosity)

The paper compares their findings to computer simulations. They found that to get the simulation to match the real-world data, they had to account for the "stickiness" (viscosity) of the soup.

The Analogy: Imagine spinning a bowl of honey versus a bowl of water. Honey is thick and sticky (high viscosity); water is thin and runny (low viscosity).
The Finding: The scientists discovered that the "stickiness" of the QGP, specifically something called bulk viscosity, is crucial. If they ignored this stickiness in their models, the particles would spin in the opposite direction of what they actually observed. The "stickiness" changes how the soup evolves, which flips the direction of the spin.

4. What They Measured

More Data, Better Precision: They used a massive amount of data collected in 2023 (about 5 billion collisions). This is like taking a blurry photo and turning it into a high-definition 4K image. The results are much clearer than previous measurements.
The Pattern: The spin of the particles creates a wave-like pattern (a sine wave) as you look around the collision. They measured this wave for both the main oval shape and the triangular wobble.
The Result: The strength of the spin for the "triangular wobble" was surprisingly similar to the strength of the spin for the "main oval," even though the wobble itself is usually much weaker than the main flow.

Summary

In short, this paper tells us that the tiny particles created in these massive collisions are like little compass needles. They align themselves with the swirling currents of the super-hot soup. By measuring how they align with both the big swirls and the tiny ripples, scientists can figure out exactly how "thick" or "sticky" the soup is.

This is a major step forward because it gives us a new, independent way to understand the properties of the Quark-Gluon Plasma, proving that the "stickiness" of this primordial fluid plays a huge role in how the universe behaves in its very first moments.

Technical Summary: Hyperon ( $\Lambda$ ) Polarization along the Beam Axis in Pb–Pb Collisions at $\sqrt{s_{NN}} = 5.36$ TeV

Problem and Motivation
In relativistic heavy-ion collisions, the formation of a quark–gluon plasma (QGP) exhibiting collective fluid-like behavior is characterized by nontrivial velocity and vorticity fields. While global polarization of $\Lambda$ hyperons has been established as evidence of this vortical fluid, local vorticity fields arising from anisotropic flow are predicted to generate longitudinal polarization (along the beam axis) with specific azimuthal modulations. Previous measurements by the STAR and ALICE collaborations revealed a non-zero longitudinal polarization with a positive sine-modulating component, a result qualitatively opposite to early thermal-vorticity-based models but consistent with recent developments incorporating shear-induced effects. However, the sensitivity of longitudinal polarization to the transport properties of the QGP, particularly bulk viscosity, is expected to be more pronounced at Large Hadron Collider (LHC) energies than at Relativistic Heavy Ion Collider (RHIC) energies. Furthermore, the coupling of vorticity to higher-order flow harmonics remains an open area of investigation, offering potential constraints on the space–time evolution of the medium.

Methodology
This analysis utilizes a dataset of approximately 5 billion Pb–Pb collision events collected by the ALICE detector during the 2023 Run 3 data-taking period at $\sqrt{s_{NN}} = 5.36$ TeV, corresponding to an integrated luminosity of $L_{int} \simeq 3.2$ nb $^{-1}$ . The analysis focuses on the reconstruction of $\Lambda$ and $\bar{\Lambda}$ hyperons via their weak decay channels ( $\Lambda \to p\pi^-$ and $\bar{\Lambda} \to \bar{p}\pi^+$ ).

Key experimental components include:

Detectors: The Fast Interaction Trigger (FIT), specifically the FT0 detector for centrality and event plane determination; the Inner Tracking System (ITS) for vertex reconstruction; and the Time Projection Chamber (TPC) for track reconstruction and particle identification via specific energy loss ($dE/dx$).
Selection Criteria: Hyperon candidates are selected based on topological variables (transverse decay radius, distance of closest approach, pointing angle) and kinematic cuts ( $p_T > 0.5$ GeV/c for protons, $p_T > 0.2$ GeV/c for pions, $|y| < 0.5$ ).
Polarization Extraction: The polarization along the beam direction ( $P_z$ ) is accessed through the parity-violating angular correlation between the hyperon spin and the proton momentum in the hyperon rest frame. The analysis measures the longitudinal polarization relative to the second- and third-order event planes ( $\Psi_2$ and $\Psi_3$ ).
Analysis Technique: The signal is extracted using an invariant-mass analysis combined with a simultaneous fit to the polarization distribution. The polarization is corrected for detector acceptance and the decay parameter $\alpha_H$ . The event-plane resolution is evaluated using a three-sub-event method to mitigate longitudinal decorrelations.

Key Contributions

First Observation at LHC: This work presents the first observation of hyperon polarization along the beam direction measured relative to the third-order event plane at the LHC.
High-Precision Measurement: The study provides measurements of longitudinal polarization relative to the second-order event plane with improved statistical precision compared to previous results at $\sqrt{s_{NN}} = 5.02$ TeV, owing to the larger Run 3 data sample.
Viscosity Constraints: The analysis explicitly compares experimental data with hydrodynamic calculations to constrain the bulk ( $\zeta/s$ ) and shear viscosities of the QGP. It highlights the critical role of bulk viscosity in reproducing the sign and magnitude of the observed polarization at LHC energies.

Results

Azimuthal Modulation: The measured polarization exhibits clear azimuthal sine modulations relative to both the second- and third-order event planes.
Magnitude and Trends:
- The magnitude of the second- and third-order polarizations are found to be comparable, contrasting with the typical ordering observed for flow coefficients.
- The second-order longitudinal polarization increases monotonically with centrality (toward peripheral collisions) and shows a weak, mild dependence on transverse momentum ( $p_T$ ), with a tendency toward saturation at intermediate $p_T$ .
- The results at 5.36 TeV are consistent with previous measurements at 5.02 TeV within two standard deviations, though with reduced statistical uncertainties.
Model Comparison:
- Hydrodynamic calculations that include bulk viscosity (implemented via phenomenological temperature-dependent parametrizations peaking near the critical temperature) successfully reproduce the sign and trend of the experimental data.
- Calculations without bulk viscosity yield a longitudinal polarization with the opposite sign to the experimental data.
- Current viscous hydrodynamic calculations tend to overestimate the increase of the signal at high $p_T$ , indicating that the $p_T$ dependence is not fully reproduced by existing models.

Significance
The paper claims that the measurement of longitudinal polarization, particularly relative to the third-order event plane, provides an independent and complementary handle on the vortical structure of the QGP. The results demonstrate that longitudinal polarization is sensitive not only to the vortical structure but also to the transport properties of the medium, specifically the bulk viscosity. The inclusion of bulk viscosity is identified as essential for explaining the sign of the polarization at LHC energies, underscoring the role of dissipative effects in the development of local spin polarization. By probing both the average elliptic geometry (second order) and fluctuation-driven structures (third order), these measurements offer new constraints on the space–time evolution and transport coefficients of the quark–gluon plasma, distinguishing between different dynamical descriptions of the spin-polarization mechanism.

1. The Spinning Soup and the "Soccer Ball" Effect

2. The New Clue: The "Third-Order" Twist

3. Why the "Sticky" Matters (Viscosity)

4. What They Measured

Summary

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