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Three Predictions for Partonic Energy Loss from Light to Heavy Ion Collisions

This paper presents three predictions for partonic energy loss in ultrarelativistic light ion collisions by combining BDMPS-Z, weighted path, and JEWEL models with realistic $^{16}$ O and $^{20}$ Ne geometries to demonstrate how ion size characteristically influences energy loss in small quark-gluon plasma systems.

Original authors: Wilke van der Schee, Isobel Kolbé, Govert Nijs, Kumail Ruhani, Ishtiaq Ahmed, Shahin Iqbal

Published 2026-09-07

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CC BY 4.0

Original authors: Wilke van der Schee, Isobel Kolbé, Govert Nijs, Kumail Ruhani, Ishtiaq Ahmed, Shahin Iqbal

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 heart of particle physics, scientists use massive machines to smash atoms together at speeds close to that of light. When heavy atoms like lead collide, they create a fleeting, super-hot soup of fundamental particles known as the quark-gluon plasma. This state of matter existed just moments after the Big Bang, and by studying it, researchers hope to understand how the universe cooled down to form the protons and neutrons that make up everything we see today. A key feature of this plasma is that it acts like a thick fog; when a high-speed particle tries to move through it, the plasma drags on it, robbing it of energy. This phenomenon, called partonic energy loss, has been well observed in collisions of large, heavy ions. However, a lingering question has remained: does this same energy loss happen in much smaller systems? If the plasma is too tiny, perhaps the drag is too weak to measure, or perhaps the rules change entirely. Answering this helps physicists define the smallest possible droplet of this exotic matter and tests whether our current theories of how particles interact hold up at the very smallest scales.

In July 2025, the Large Hadron Collider at CERN began a new phase of experiments, smashing together lighter ions—specifically oxygen and neon—instead of just heavy lead. The goal was to create these smaller droplets of quark-gluon plasma and see if the energy loss effect still appeared. A team of theorists, led by Wilke van der Schee and colleagues, set out to predict exactly what would happen before the data was even collected. They did not rely on a single guess but instead built three distinct models to simulate the collisions. The first model used a standard theoretical framework that assumes particles lose energy through many small, gentle nudges as they pass through the plasma. The second model took a more practical approach, using a formula that weighed the temperature of the plasma along the path the particle traveled. The third and most complex model used a sophisticated computer program that simulated the entire journey of particles, including how they bounce off the plasma and how the plasma itself reacts. To make these predictions as realistic as possible, the team incorporated detailed maps of the oxygen and neon nuclei, which are not perfect spheres but have specific, slightly irregular shapes.

The researchers compared their predictions for oxygen-on-oxygen collisions against neon-on-neon collisions. Because neon atoms are slightly larger and heavier than oxygen atoms, they expected the resulting plasma droplets to be bigger. If the theory of energy loss holds true, the larger neon droplets should cause a greater slowdown for the particles passing through them. The simulations confirmed this intuition, but the size of the effect depended heavily on which model was used. The model based on the "many small nudges" theory predicted a difference between the two types of collisions that was within the uncertainty, though ratios showed clear distinctions. In contrast, the model that weighed the temperature along the particle's path predicted a much clearer difference: particles in neon collisions would lose significantly more energy than those in oxygen collisions. The most complex simulation, which tracked every bounce and interaction, showed a similar trend to the temperature-weighted model regarding the ion-size dependence, suggesting that the larger neon nuclei do indeed create a more effective barrier.

A crucial part of this study involved the internal structure of the nuclei themselves. The team used two different advanced methods to calculate the exact shape and density of the oxygen and neon atoms. One method suggested a larger difference in size between the two types of atoms, while the other suggested a smaller difference. This variation turned out to be important. In the temperature-weighted model, the difference in predicted energy loss between oxygen and neon was about twice as large when using the first structural method compared to the second. This highlighted that our understanding of the precise shape of these atomic nuclei is still a source of uncertainty. The researchers noted that while their models agreed on the general direction—that neon should show more energy loss than oxygen—the exact amount of that loss remains a question that depends on how we view the atomic building blocks.

When the actual experimental data from the 2025 run became available, it provided a decisive test. The measurements showed that the energy loss observed in the light-ion collisions gave an excellent match to the pathlength approach. The other two models, including the one based on the "many small nudges" theory and the complex JEWEL simulation, underpredicted the energy loss. This outcome suggests that for these small, fleeting droplets of plasma, the simple idea of particles losing energy through a series of tiny, independent interactions is not enough to describe reality. Instead, the energy loss appears to depend more strongly on the total distance traveled through the hot medium and the temperature along that path. The study concludes that while the smallest droplets of quark-gluon plasma are indeed capable of slowing down fast-moving particles, the physics governing this process in small systems is more complex than previously thought, requiring models that account for the full journey of the particle through the expanding fireball.

Technical Summary: Three Predictions for Partonic Energy Loss from Light to Heavy Ion Collisions

Problem Statement
The Large Hadron Collider (LHC) has initiated a program of ultrarelativistic light-ion collisions, specifically colliding $^{16}\text{O}$ and $^{20}\text{Ne}$ isotopes. A primary motivation for this run is to investigate partonic energy loss (jet quenching) in systems characterized by small quark-gluon plasma (QGP) droplets. While signatures of collectivity and strangeness enhancement have been observed in high-multiplicity proton-proton and proton-lead collisions, direct observation of in-medium parton energy loss in such small systems has remained elusive. Theoretical models predict non-zero energy loss in these systems, but experimental observation is hindered by the need to select high-activity events, which introduces large systematic uncertainties. The paper aims to provide theoretical predictions for the charged hadron nuclear modification factor ( $R_{AA}$ ) in $^{16}\text{O}^{16}\text{O}$ and $^{20}\text{Ne}^{20}\text{Ne}$ collisions to facilitate the identification of energy loss and distinguish between different theoretical approaches.

Methodology
The authors present three distinct theoretical frameworks to compute the charged hadron nuclear modification factor, incorporating realistic nuclear structure inputs for $^{16}\text{O}$ and $^{20}\text{Ne}$ derived from Nuclear Lattice Effective Field Theory (NLEFT) and the ab initio Projected Generator Coordinate Method (PGCM).

The Simple Model (BDMPS-Z based):
- Utilizes the "Simple formula" by Peter Arnold, which solves full BDMPS-Z emission equations in the harmonic oscillator approximation for a dynamic medium.
- Includes realistic quark/gluon spectra, fragmentation functions, and EPPS16 nuclear PDFs.
- Geometry is boost-invariant but matches Trento calculations for anisotropy and size.
- Contains a single free parameter ( $\hat{q}$ ), fitted to minimum-bias charged hadron $R_{AA}$ data from PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV.
Temperature-Weighted Path Approach:
- Assumes a power-law spectrum ( $\sigma_{pp} \propto p_T^{-6}$ ) and a $p_T$ -dependent energy loss $\delta e(p_T)$ proportional to a temperature-weighted path integral $I = \int T^3 u \cdot dL$ .
- The proportionality constant $\kappa(p_T)$ is fixed using experimental PbPb reference data ( $R_{AA}^{ref}$ ).
- Predictions are generated using 10 different parameter settings from the Trajectum hydrodynamic posterior to estimate systematic uncertainties.
- This approach validates well against PbPb centrality dependence above $p_T > 5$ GeV.
JEWEL-Trajectum Interface:
- Couples the Monte Carlo jet evolution code JEWEL (Jet Evolution With Energy Loss) with the Trajectum hydrodynamic framework.
- Unlike standard JEWEL configurations, this interface uses event-by-event fluctuating hydrodynamic profiles (energy density, temperature, flow) from Trajectum as the background medium.
- Uses default JEWEL parameters (version 2.4.0) and EPPS21 nuclear PDFs (using the $^{16}\text{O}$ set for both isotopes due to lack of dedicated $^{20}\text{Ne}$ sets).
- This setup allows for the study of parton-medium interactions including medium recoil and fluctuations in small systems.

Key Results

Model Discrepancies: The three models yield different absolute predictions for $R_{AA}$ . The Simple model and the Path-length approach, both fitted to PbPb data, predict significantly different suppression levels for light ions. The Path-length approach predicts lower $R_{AA}$ values (indicating higher suppression) for small systems compared to the Simple model.
Isotope and Structure Dependence:
- In the Path-length approach, $^{20}\text{Ne}$ collisions show significantly more suppression than $^{16}\text{O}$ collisions. Furthermore, the ratio $R_{AA}(^{20}\text{Ne})/R_{AA}(^{16}\text{O})$ differs by a factor of approximately two between PGCM and NLEFT nuclear structure inputs. This difference is attributed to the interplay between nuclear size, initial QGP temperature, and temperature-weighted path lengths.
- In the JEWEL-Trajectum model, the $p_T$ dependence differs from the other models, and the ratio between PGCM and NLEFT inputs is nearly identical, suggesting that jet fragmentation fluctuations dominate over the subtle differences in nuclear geometry in this framework.
- The Simple model predicts a clear difference between PGCM and NLEFT inputs, though the absolute uncertainty in $R_{AA}$ is large.
Comparison to Data (Post-Prediction): Upon the final submission of the paper, experimental data from the light-ion run became available. The data shows an excellent match to the Path-length approach (specifically the Trajectum-based prediction). Conversely, both the Simple model and JEWEL underpredict the observed energy loss. The authors note that the Simple model's underprediction is consistent with its performance in the 50-70% centrality bin of PbPb collisions.

Significance and Claims
The paper claims to provide the first set of true predictions for partonic energy loss in light-ion collisions prior to the availability of experimental data. The primary significance lies in demonstrating that:

System Size Sensitivity: Different theoretical models respond characteristically to the change in system size from $^{16}\text{O}$ to $^{20}\text{Ne}$ , offering a potential discriminator for the underlying physics of energy loss.
Nuclear Structure Impact: The choice of nuclear structure model (PGCM vs. NLEFT) introduces a systematic uncertainty that is significant enough to be distinguishable in ratios, particularly in path-length based approaches.
Model Validity: The comparison with newly available data suggests that the path-length approach, which does not rely on the coherence effects inherent in the BDMPS-Z harmonic oscillator approximation (which scales as $L^2$ for small lengths), may be more appropriate for describing energy loss in small QGP droplets. The results imply that BDMPS-Z based approaches might underestimate energy loss in small systems.

The authors conclude that distinguishing between these models is crucial for understanding the path-length dependence of energy loss and the nature of the smallest deconfined quark matter droplets. They emphasize that while the study is not complete regarding all uncertainties (e.g., nPDF uncertainties were not fully propagated in all sections), the agreement of the path-length approach with data provides a strong constraint on theoretical descriptions of small-system QGP.

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