Recent ALICE results from light-ion collision systems

This paper presents recent ALICE measurements from July 2025 LHC runs involving proton-oxygen, oxygen-oxygen, and neon-neon collisions, detailing charged-particle pseudorapidity densities, flow coefficients, and neutral pion suppression to advance the understanding of particle production and collective phenomena in small collision systems.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle accelerator, a giant racetrack where scientists smash tiny particles together at near-light speed to recreate the conditions of the universe just moments after the Big Bang.

For years, physicists have been trying to understand a mysterious substance called the Quark-Gluon Plasma (QGP). Think of the QGP as a super-hot, super-dense "soup" of fundamental particles. We know this soup exists when we smash heavy atoms (like Lead) together, creating a massive explosion that fills a large volume. But a big question has remained: Can we make this soup in smaller explosions?

In 2025, the ALICE experiment at the LHC decided to test this by smashing lighter atoms together: Oxygen (O) and Neon (Ne). This paper reports what they found. Here is the story of their discovery, broken down into simple concepts.

1. The Big Mystery: The "Small" vs. "Large" Collision

Previously, scientists saw strange behavior in small collisions (like Proton-Proton or Proton-Lead). These collisions acted like they had a "collective flow," as if the particles were dancing in a coordinated way, which usually only happens in the big, heavy-ion soup. However, they didn't see "jet quenching"—a phenomenon where high-speed particles get slowed down by the soup, like a runner trying to sprint through molasses.

This created a puzzle: Are these small collisions actually making the soup, or is it just a trick of the light?

To solve this, the team used Oxygen-Oxygen and Neon-Neon collisions. Imagine these as the "Goldilocks" zone: they are bigger than a single proton smash but smaller than a heavy Lead smash. They have a similar number of particles but a wider "footprint," which should make it easier to see if the "molasses" (the soup) is actually there.

2. Counting the Particles (The Crowd Density)

First, the scientists counted how many charged particles were created in the crash.

• The Analogy: Imagine a concert. In a small room (Proton-Proton), you have a few people. In a stadium (Lead-Lead), you have a massive crowd. In these new Oxygen collisions, they found a crowd size that fits perfectly between the small room and the stadium.
• The Result: The number of particles created matched what you would expect if the atoms were behaving like a fluid. The data lined up with complex computer models that assume a "soup" is forming.

3. The Dance of the Particles (Flow)

Next, they looked at how the particles moved. In a normal explosion, particles fly out randomly. But in a QGP soup, the pressure pushes them to flow in specific patterns, like water swirling down a drain.

• The Analogy: Think of a mosh pit. If everyone is just jumping randomly, it's chaotic. But if the crowd starts moving in a coordinated wave (an "elliptic" or "triangular" flow), it means they are all pushing against each other in a unified medium.
• The Result: The Oxygen and Neon collisions showed these coordinated waves! The particles didn't just fly apart; they flowed together. This suggests that even in these smaller systems, a tiny drop of the "soup" is forming and expanding.

4. The "Jet Quenching" Surprise (The Smoking Gun)

This is the most exciting part. In heavy-ion collisions, high-energy particles (called "jets") try to fly through the soup but get slowed down, losing energy. This is called "jet quenching."

• The Analogy: Imagine throwing a tennis ball through a thick fog. If the fog is just air (Proton-Proton), the ball flies straight. If the fog is thick soup (Lead-Lead), the ball slows down and loses energy.
• The Result: In the Oxygen-Oxygen collisions, the scientists looked at neutral pions (a type of particle) and found that fewer of them were coming out than expected.
  • They compared the Oxygen crash to a baseline Proton crash.
  • The "missing" pions suggest that the high-energy particles did hit something and lost energy.
  • This is the first strong evidence that jet quenching is happening in these smaller systems. It's like seeing the tennis ball slow down in the Oxygen fog, proving the fog is actually "soup."

The Conclusion

The ALICE team has successfully created a "miniature universe" in the lab using Oxygen and Neon.

• What they found: These small collisions create a fluid-like medium that behaves like the massive soup seen in heavy-ion crashes.
• Why it matters: It solves a long-standing debate. It proves that the "soup" isn't just about how big the collision is, but about how dense the particles get. Even in these smaller systems, the conditions are right to create a tiny, fleeting drop of the Quark-Gluon Plasma.

In short, by smashing lighter atoms together, scientists have confirmed that the "soup" of the early universe can form in much smaller, more manageable containers than previously thought, opening a new window to understand how the universe began.

Technical Summary

1. Problem Statement and Motivation

The paper addresses a significant tension in high-energy nuclear physics regarding the nature of the Quark-Gluon Plasma (QGP) in small collision systems.

• The Paradox: Recent LHC results in high-multiplicity proton-proton (pp) and proton-lead (p-Pb) collisions show signatures of collective flow and strangeness enhancement, typically associated with QGP formation. However, these small systems have not yet shown clear evidence of jet quenching (parton energy loss), a hallmark of QGP.
• The Gap: There is a discrepancy between "soft" observables (suggesting a medium) and "hard" observables (suggesting no medium).
• The Solution: The authors propose that collisions of light ions (Oxygen-16 and Neon-20) at the LHC offer a unique testing ground. These systems possess:
  • A similar number of participating nucleons and final-state multiplicity to high-multiplicity pp/p-Pb.
  • Larger geometrical transverse overlap, which increases the in-medium path length for partons, thereby enhancing potential jet-quenching effects.
  • A multiplicity range bridging the gap between small systems (pp/p-Pb) and large systems (Pb-Pb/Xe-Xe).

2. Methodology and Experimental Setup

The analysis utilizes data from the ALICE experiment collected during the LHC run in July 2025 at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.36$ TeV.

• Detector Upgrades: The analysis leverages the Long Shutdown 2 (LS2) upgrades, specifically:
  • ITS (Inner Tracking System) & TPC (Time Projection Chamber): Used for reconstructing charged particles at midrapidity to measure multiplicity and flow.
  • EMCal (Electromagnetic Calorimeter): Used for reconstructing photons from neutral pion ($\pi^0$) decays ($\pi^0 \to \gamma\gamma$).
  • FIT (Fast Interaction Trigger): Specifically FT0A and FT0C, used for event timing, centrality estimation, and vertex selection.
• Event Selection:
  • Minimum-bias events selected via hits in FT0A and FT0C.
  • Vertex selection within 10 cm of the interaction point; events with large discrepancies between FIT and tracked vertices are rejected.
  • Centrality classes defined using raw amplitudes from the FT0C detector.
• Analysis Techniques:
  • Multiplicity: Tracks categorized as "global" (ITS+TPC) or "ITS-only." TPC-only tracks are excluded due to poor pointing resolution. Corrections for acceptance and efficiency are applied using PYTHIA 8.3/Angantyr simulations.
  • Flow ($v_n$): Measured using two- and multi-particle cumulants with a generic framework. A pseudorapidity gap ($|\Delta\eta| > 1.4$) suppresses non-flow correlations (jets/resonances).
  • $\pi^0$ Suppression: The nuclear modification factor ($R_{OO}$) is calculated by comparing $\pi^0$ invariant yields in OO collisions to the pp baseline, normalized by the nuclear overlap function $\langle T_{OO} \rangle$. Background subtraction uses the rotational event mixing method.

3. Key Contributions and Results

A. Charged-Particle Pseudorapidity Density ($dN_{ch}/d\eta$)

• Observation: The average charged-particle density $\langle dN_{ch}/d\eta \rangle$ was measured as a function of centrality (0–5% to 60–90%).
  • Central (0–5%) OO collisions: $129.7 \pm 4.1$.
  • Peripheral (60–90%) OO collisions: $11.7 \pm 1.2$.
• Model Comparison:
  • Hydrodynamic models (IPGlasma + MUSIC + UrQMD, McDIPPER): IPGlasma describes the central data well. McDIPPER fails to reproduce the shape and magnitude from mid-central to peripheral collisions.
  • Event Generators: PYTHIA 8.3/Angantyr qualitatively captures the trend and matches peripheral data. AMPT shows an inconsistent trend.
• System Scaling: When normalized by the number of participating nucleon pairs ($\langle N_{part} \rangle/2$), the multiplicity in OO and Ne-Ne collisions rises more steeply with $\sqrt{s_{NN}}$ than in pp/pA collisions, aligning with the trend of heavy-ion collisions. The normalized multiplicity decreases by a factor of $\approx 1.7$ from central to peripheral collisions.

B. Anisotropic Flow ($v_2$ and $v_3$)

• Observation: Elliptic ($v_2$) and triangular ($v_3$) flow coefficients were measured for charged particles ($0.2 < p_T < 3$ GeV/c).
  • $v_2$: Shows weak centrality dependence (increasing slightly from central to mid-central, then decreasing). The non-zero $v_2\{4\}$ confirms the collective nature of the anisotropy.
  • $v_3$: Increases from peripheral to central collisions, opposite to initial-state eccentricity predictions but consistent with fluctuation-driven mechanisms seen in pp/p-Pb.
• Model Comparison: The data is compared with Trajectum hydrodynamic simulations using nuclear structure inputs from:
  • NLEFT (Nuclear Lattice Effective Field Theory): Reproduces both the trend and magnitude of $v_2$ and $v_3$ excellently.
  • PGCM (Projected Generator Coordinate Method): Shows slightly worse agreement in central collisions but remains within a few percent.
• Significance: The consistency with hydrodynamic models supports the emergence of collective behavior in light-ion collisions.

C. Parton Energy Loss (Neutral Pion Suppression)

• Observation: The nuclear modification factor $R_{OO}(p_T)$ for $\pi^0$ was measured in the range $1.2 < p_T < 20$ GeV/c.
• Result: A clear suppression of $\pi^0$ yields is observed relative to the pp baseline ($R_{OO} < 1$).
• Significance:
  • Cold Nuclear Matter (CNM) Effects: Comparisons with NLO pQCD calculations using nuclear PDFs (nPDFs like EPPS21, nNNPDF30) show that the observed suppression exceeds CNM expectations. The deviation reaches 2.4$\sigma$ significance when compared to the most suppressed EPPS21 prediction.
  • Hot Medium Effects: The suppression suggests the presence of parton energy loss (jet quenching) in the medium formed in OO collisions, a phenomenon previously elusive in smaller systems.

4. Significance and Conclusion

This paper represents a critical step in the LHC heavy-ion program by resolving the tension between soft and hard probes in small systems.

1. Collective Behavior Confirmed: The agreement between flow measurements and hydrodynamic models (specifically those using NLEFT nuclear structure inputs) confirms that light-ion collisions (OO, Ne-Ne) create a collectively expanding medium similar to larger heavy-ion systems.
2. Evidence of Jet Quenching: The observation of $\pi^0$ suppression beyond CNM effects provides the first strong experimental evidence of parton energy loss in light-ion collisions. This validates the hypothesis that increasing the geometrical overlap (via light ions) is sufficient to create a medium dense enough to quench jets.
3. Future Direction: These results highlight the necessity of collecting light-nuclei data to better constrain nuclear parton distribution functions (nPDFs) and to fully understand the transition from small to large collision systems in QGP formation.