Measurements of charged-particle pseudorapidity and transverse momentum distributions in O+O and Ne+Ne collisions at $\sqrt{s_{_\text{NN}}} = 5.36$ TeV with the ATLAS detector

The ATLAS experiment presents measurements of charged-particle pseudorapidity and transverse momentum distributions in O+O and Ne+Ne collisions at $\sqrt{s_{_\text{NN}}} = 5.36$ TeV, characterizing these observables across various centrality intervals and comparing the results with hydrodynamic calculations.

The Gist

The Big Picture: Smashing Tiny "Bowling Pins"

Imagine the Large Hadron Collider (LHC) as a massive, high-speed racetrack where scientists smash particles together to see what happens. Usually, they smash giant lead atoms (like bowling balls) together to create a tiny, super-hot drop of "primordial soup" called the Quark-Gluon Plasma (QGP). This soup is the state of matter that existed just after the Big Bang, where particles are so hot they melt into a fluid.

For a long time, scientists wondered: Does this fluid only form when you smash huge atoms together? Or can it form even when you smash tiny atoms?

To answer this, the ATLAS experiment at CERN decided to smash two types of small atoms: Oxygen (O) and Neon (Ne).

• Oxygen (16O) is like a perfect, round cluster of marbles.
• Neon (20Ne) is similar in size but shaped differently. Because of how its internal parts (alpha clusters) are arranged, it looks a bit like a bowling pin or a teardrop.

The scientists wanted to see if the shape of the "bowling pin" (Neon) versus the "round ball" (Oxygen) changed how the "primordial soup" flowed and expanded.

The Experiment: A High-Speed Photo Shoot

The team collected data from millions of these collisions. They used the ATLAS detector, which is like a giant, 3D camera surrounding the crash site.

1. The Crash: They smashed Oxygen and Neon atoms together at nearly the speed of light (5.36 TeV).
2. The "Centrality" (How Hard the Crash Was): Just like in a car crash, a head-on collision is different from a glancing blow.
   • Central collisions (0–5%): The atoms hit dead center. This creates the biggest, hottest, and most "fluid-like" soup.
   • Peripheral collisions (70–80%): The atoms just grazed each other. This creates a smaller, cooler splash.
3. The Measurement: They counted how many charged particles (like tiny shrapnel) flew out and measured how fast they were moving. They looked at this from two angles:
   • Pseudorapidity ($\eta$): A measure of the angle relative to the beam, which is easy to measure but slightly distorts the view of slow-moving particles.
   • Rapidity ($y_\pi$): A more "true" measure of the particle's motion, calculated by assuming the particles are pions (a common type of particle). This gives a clearer picture of the physics.

The Key Findings

1. The "Bowling Pin" Effect
The scientists found that Neon collisions (the bowling pin shape) produced slightly more particles than Oxygen collisions (the round shape), especially in the most violent, head-on crashes. This suggests that the initial shape of the nucleus matters. The "pin" shape creates a slightly different starting point for the fluid, leading to a different flow pattern.

2. The Flow of the Soup (Radial Flow)
When the atoms smash, the resulting soup expands outward like an exploding balloon. This is called radial flow.

• The data showed that in the most central (head-on) collisions, the soup expands faster and pushes particles harder.
• This "hardening" of the particle speeds happened in both Oxygen and Neon, proving that even these tiny collisions create a fluid-like state that behaves like a hydrodynamic system (like water flowing).

3. Comparing the Shapes
When they compared the ratio of Neon to Oxygen results, they found:

• Particle Count: Neon produced about 5% to 20% more particles than Oxygen, depending on how central the collision was.
• Particle Speed: The average speed of the particles was almost identical for both. This is a crucial clue: it suggests that while the shape changes the initial explosion, the flow of the fluid is remarkably similar once it gets going.

4. Checking the Math
The scientists compared their real-world data with computer simulations (theoretical models).

• Some models (like EPOS) did a great job predicting the results.
• Other models based on fluid dynamics (like IPGlasma and Trento) were close but had some trouble getting the details right, especially in the "grazing" (peripheral) collisions.
• The fact that the real data didn't perfectly match the models tells scientists they need to refine their understanding of how these tiny nuclei are structured inside.

The Conclusion

This paper is a landmark because it is the first detailed look at how Oxygen and Neon collisions behave at the LHC.

The main takeaway is that size isn't everything. Even with atoms as small as Oxygen and Neon, you can create a state of matter that flows like a fluid. Furthermore, the specific shape of the atom (round vs. bowling pin) leaves a fingerprint on the collision, changing how many particles are produced. This helps scientists understand the very earliest moments of the universe and the fundamental rules that govern how matter behaves under extreme heat and pressure.

In short: They smashed tiny bowling pins and round balls together, watched the splash, and confirmed that even the smallest crashes can create a fluid that flows just like the giant ones.

Technical Summary

Technical Summary: Measurements of Charged-Particle Pseudorapidity and Transverse Momentum Distributions in O+O and Ne+Ne Collisions at $\sqrt{s_{NN}} = 5.36$ TeV

Problem and Motivation
Heavy-ion collisions at the Large Hadron Collider (LHC) are understood to create a quark–gluon plasma (QGP) that evolves hydrodynamically. While viscous hydrodynamic calculations have successfully described soft single- and multi-hadron measurements in large systems (e.g., Pb+Pb), a critical open question remains: whether the paradigm of QGP creation and hydrodynamization holds for systems with transverse sizes comparable to a single proton. Recent observations of collective-like behavior in proton–proton and proton–nucleus collisions suggest the QGP may form in very small systems. To investigate the dependence of this evolution on system size and nuclear geometry, collisions of light nuclei with distinct shapes are required. Specifically, the $^{16}$O nucleus and the $^{20}$Ne nucleus have similar sizes but are predicted to possess different ground-state shapes due to $\alpha$-clustering effects (with $^{20}$Ne potentially exhibiting a "bowling pin" shape). Measuring observables sensitive to initial-state geometry and subsequent radial flow in these systems is essential to test hydrodynamic models and initial-state descriptions.

Methodology
This analysis utilizes data recorded by the ATLAS detector during special LHC runs in July 2025, comprising $27.7~\mu\text{b}^{-1}$ of O+O collisions and $53.1~\mu\text{b}^{-1}$ of Ne+Ne collisions at $\sqrt{s_{NN}} = 5.36$ TeV.

• Event Selection and Centrality: Events were selected using minimum-bias triggers. Collision centrality (0–80%) was characterized by the total transverse energy ($\Sigma E_T^{\text{FCAL}}$) measured in the ATLAS forward calorimeters. A Glauber-model-based analysis, incorporating nucleon density distributions from ab initio nuclear-structure models (PGCM and NLEFT), was used to map the transverse energy distribution to centrality percentiles and estimate the average number of participating nucleons ($\langle N_{\text{part}} \rangle$).
• Track Reconstruction: Charged-particle tracks were reconstructed in the inner detector (ID) within the fiducial range $|\eta| < 2.5$ and $0.27 < p_T < 5$ GeV. Primary particles were distinguished from secondary tracks (from weak decays or material interactions) and fake tracks (combinatorial) using Monte Carlo (HIJING) simulations and data-driven template fits based on the distance of closest approach ($d_0$).
• Rapidity Variables: To mitigate distortions arising from the difference between pseudorapidity ($\eta$) and true rapidity ($y$), particularly at low $p_T$, measurements were also performed using a pion-mass hypothesis rapidity ($y_\pi$). This variable provides results closer to true rapidity, which is the natural variable for hadronic particle production.
• Extrapolation: Invariant per-event yields were measured in the fiducial range and extrapolated to $p_T > 0$ MeV. This was achieved by fitting the measured $p_T$ spectra with a "Hagedorn + negative exponential" function (augmented by a negative exponential term to improve fit quality) and integrating the function analytically. Corrections for the migration between $\eta$ and $y_\pi$ were applied using measured ratios of invariant yields.
• Systematic Uncertainties: Comprehensive uncertainties were evaluated, including track selection, detector material modeling, centrality definition, Monte Carlo reweighting (to match ALICE flavor ratios), and the functional form and range used for $p_T$ extrapolation.

Key Contributions and Results
The paper presents the first detailed ATLAS measurements of charged-particle pseudorapidity density ($dn/d\eta$), mean transverse momentum ($\langle p_T \rangle$), and invariant transverse momentum spectra in O+O and Ne+Ne collisions.

• Spectral Shapes and Radial Flow: The $p_T$ spectra exhibit a hardening from peripheral to central collisions, with yields below $p_T \sim 0.5$ GeV suppressed and yields at higher $p_T$ enhanced in central events. This trend is consistent with the development of stronger radial flow in more central collisions. The hardening is observed to be approximately boost-invariant over $|\eta| < 2.5$.
• System Dependence (Ne+Ne vs. O+O):
  • Multiplicity: Ne+Ne collisions produce higher charged-particle multiplicities than O+O collisions for a given centrality. The ratio $dn/d\eta(\text{Ne+Ne})/dn/d\eta(\text{O+O})$ varies from approximately 1.20 in the most central (0–1%) collisions to 1.04 in peripheral (75–80%) collisions.
  • Mean $p_T$: The mean transverse momentum is nearly identical for both systems, with differences less than 0.5% (5 MeV) across the centrality range.
  • $p_T$ Spectra Ratios: The ratio of $p_T$ spectra between Ne+Ne and O+O is nearly independent of $p_T$ (varying by $<10\%$), indicating that the relative particle yield is largely constant across the measured momentum range.
• Comparison with Theory:
  • Hydrodynamic Models: Calculations using the IPGlasma and Trento frameworks (coupled with MUSIC and Trajectum hydrodynamic solvers, respectively) generally reproduce the observed trends. However, deviations increase in peripheral collisions. The IPGlasma-based predictions show slightly better agreement with the data than the Trento-based predictions.
  • EPOS Model: The EPOS event generator (core–corona configuration) provides the best overall agreement with the data across centrality intervals.
  • Nuclear Structure Sensitivity: The measurements of the Ne+Ne/O+O ratios for $dn/d\eta$ and $\langle p_T \rangle$ are found to be sensitive to the differences between nuclear-structure models (PGCM vs. NLEFT), particularly in the most central collisions where the hydrodynamic models predict different initial geometries.

Significance
This work constitutes the first detailed ATLAS results for charged-particle production in O+O and Ne+Ne collisions at the LHC. By comparing two systems of similar size but different nuclear shapes, the measurements provide important experimental constraints on models of particle production and collective behavior in small- and intermediate-sized nuclear systems. The results complement existing data from $pp$, $p$+A, and heavy-ion collisions, contributing to a more complete picture of the onset and evolution of QGP dynamics as a function of system size. The data serve as a baseline for understanding how initial-state nuclear geometry influences the development of radial flow and particle multiplicities in light-ion collisions.