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Quarkonium production in light-ion collisions with the ALICE experiment

This paper presents new measurements of charmonium production in oxygen-oxygen and proton-oxygen collisions collected by the ALICE experiment at the LHC in 2025, utilizing forward rapidity coverage to test theoretical models of quark-gluon plasma formation and regeneration mechanisms in intermediate collision systems.

Original authors: Rebecca Cerri (on behalf of the ALICE Collaboration)

Published 2026-09-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Rebecca Cerri (on behalf of the 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

Deep inside the heart of matter, protons and neutrons are not solid, indivisible spheres. They are bustling cities made of even smaller particles called quarks and gluons, held together by a force so strong that they can never escape their confinement under normal conditions. However, if you could smash two heavy atoms together at nearly the speed of light, the heat and pressure would become so intense that this confinement breaks down. For a fleeting instant, the quarks and gluons melt into a free-flowing, super-hot soup known as the quark-gluon plasma. This state of matter is believed to have existed just microseconds after the Big Bang, before the universe cooled enough for particles to stick together again. Physicists are eager to understand how this primordial soup behaves, how it forms, and how it cools down, because studying it reveals the fundamental rules that govern the building blocks of our universe.

To probe this invisible, fleeting state, scientists look for specific messengers: heavy particles called quarkonia. These are pairs of heavy quarks and their antimatter twins that bind together tightly. In the extreme heat of the quark-gluon plasma, the bond between these pairs is expected to weaken or break entirely, much like a magnet losing its grip in a scorching fire. By measuring how many of these pairs survive the journey through the collision, researchers can gauge the temperature and density of the plasma. For years, this experiment has been performed by smashing the heaviest atoms available, like lead, into each other. But a new question has emerged: does this hot soup only form in the largest collisions, or can it appear in smaller, lighter crashes as well?

In 2025, the ALICE experiment at the Large Hadron Collider took a significant step to answer this by analyzing data from collisions involving lighter atoms for the first time. Instead of the usual heavy lead nuclei, the team collided oxygen nuclei with other oxygen nuclei, and also smashed protons into oxygen nuclei. These events were recorded during a short two-day data-taking period, reaching recorded integrated luminosities of 5.01 nb⁻¹ for oxygen-oxygen collisions and 7.27 nb⁻¹ for proton-oxygen collisions. The researchers focused their attention on a specific type of heavy particle, the J/psi, which is a bound pair of a charm quark and an anti-charm quark. They tracked how often these particles were produced in these new, smaller collision systems and compared the results to what happens when protons collide with other protons.

The results revealed a clear difference between the two types of light-ion collisions. When protons hit oxygen, the production of J/psi particles dropped by about 30 percent at lower speeds. This reduction aligns perfectly with models that account for "cold nuclear matter" effects—changes that happen simply because the quarks are passing through the dense environment of the oxygen nucleus before any hot plasma could possibly form. However, when oxygen nuclei collided with other oxygen nuclei, the suppression was stronger, dropping by roughly 40 percent. This extra loss suggests that something more than just the cold nucleus is at play. The data hints that even in these smaller oxygen-oxygen crashes, a tiny droplet of the hot quark-gluon plasma may be forming, enough to break apart some of the heavy quark pairs.

This finding is crucial because it helps map the boundary of where this exotic state of matter can exist. The study shows that while the hot plasma likely plays a role in the oxygen-oxygen collisions, it is not as dominant as it is in the massive lead-lead crashes. In the larger lead collisions, the suppression of these particles is even more severe, and the physics is complicated by a process where broken quark pairs recombine to form new particles. In the smaller oxygen systems, this recombination effect is much weaker, likely because fewer heavy quarks are created in the first place. The researchers also noted that the way the particles behave at different speeds in these light collisions looks very similar to what is seen in proton-lead collisions, suggesting a consistent pattern across different sizes of atomic crashes.

While these measurements are preliminary, they provide a vital piece of the puzzle. They confirm that the transition from simple nuclear effects to the complex behavior of a hot plasma is not a sudden switch that only flips in the largest collisions. Instead, it appears to be a gradual shift where the influence of the hot medium begins to emerge even in collisions involving lighter atoms. By comparing these new results with existing data from lead and proton collisions, the team has created a more complete picture of how the universe's most extreme state of matter scales with the size of the collision. This work does not claim to have solved the mystery of the quark-gluon plasma, but it offers a clearer view of the conditions required to create it, guiding future experiments toward a deeper understanding of the early moments of our universe.

Technical Summary: Quarkonium Production in Light-Ion Collisions with the ALICE Experiment

Problem and Motivation
Quarkonium production serves as a primary probe for investigating the Quark-Gluon Plasma (QGP), a state of deconfined matter formed in ultra-relativistic heavy-ion collisions. Heavy quarks ( $c\bar{c}$ and $b\bar{b}$ ) are created in initial hard scatterings and traverse the full evolution of the system. In the QGP, quarkonium states are expected to dissociate due to color screening and in-medium interactions. Conversely, at LHC energies, the recombination of uncorrelated charm quarks (regeneration) can counteract suppression, particularly at low transverse momentum ( $p_T \lesssim 5$ GeV/c). While the nuclear modification factor ( $R_{AA}$ ) has been extensively studied in proton-proton (pp), proton-lead (p–Pb), and lead-lead (Pb–Pb) collisions, intermediate collision systems offer a critical test ground for theoretical models. Specifically, light-ion collisions (such as oxygen-oxygen and proton-oxygen) allow researchers to investigate whether observed modifications in smaller systems arise predominantly from cold nuclear matter (CNM) effects or if hot nuclear matter effects (QGP formation) also play a role.

Methodology
This study presents preliminary measurements of the $J/\psi$ nuclear modification factor in oxygen-oxygen (OO) and proton-oxygen (pO) collisions using data collected by the ALICE experiment in July 2025.

Data Samples: The analysis utilizes data from a short two-day run, yielding integrated luminosities of 5.01 nb $^{-1}$ for OO collisions at $\sqrt{s_{NN}} = 5.36$ TeV and 7.27 nb $^{-1}$ for pO collisions at $\sqrt{s_{NN}} = 9.62$ TeV.
Detector Setup: The analysis relies on the forward muon spectrometer (MCH) covering $-4.0 < \eta < -2.5$ , the Fast Interaction Trigger (FIT) for event selection and multiplicity measurement, and the Inner Tracking System (ITS) for primary vertex determination.
Event Selection: Events are selected via coincidence in FIT detectors. $J/\psi$ candidates are reconstructed via their decay into opposite-sign muon pairs within the rapidity range $2.5 < y < 4$ . In pO collisions, the laboratory acceptance ( $2.5 < y_{lab} < 4.0$ ) corresponds to a center-of-mass frame of $2.15 < y_{cms} < 3.65$ due to the asymmetric beam energies.
Analysis Procedure: The number of $J/\psi$ candidates is extracted by fitting the opposite-sign dimuon invariant-mass spectra using a double-sided Crystal Ball function for the signal and empirical functions for the background.
Calculation of $R_{AA}$ : The nuclear modification factors ( $R_{OO}$ and $R_{pO}$ ) are calculated as the ratio of the $J/\psi$ cross-section in the respective system to the reference pp cross-section, normalized by the mass number ( $A$ or $A^2$ ). The pp reference cross-section is derived from an interpolation of previously published ALICE measurements at $\sqrt{s} = 2.76$ to 13 TeV.

Key Results

OO Collisions: The measured $J/\psi$ $R_{OO}$ exhibits a suppression of approximately 40% at low $p_T$ , with a weak dependence on $p_T$ . These results are consistent with transport model predictions that incorporate both CNM effects (shadowing, Cronin effect) and hot nuclear matter effects (dissociation and regeneration).
pO Collisions: The $J/\psi$ $R_{pO}$ shows a suppression of roughly 30% at low $p_T$ , followed by a gradual increase toward unity at higher $p_T$ . These data are consistent with the Fully Coherent Energy Loss (FCEL) model, which attributes nuclear effects to the interference between radiation amplitudes of incoming and outgoing partons, assuming $c\bar{c}$ production in a color octet state without nuclear PDF effects.
System Comparison:
- $R_{OO}$ is systematically lower than $R_{pO}$ across the measured $p_T$ range, suggesting that while pO is dominated by CNM effects, OO collisions may involve additional hot nuclear matter contributions.
- Comparisons with p–Pb and Pb–Pb collisions reveal that while $R_{pO}$ and $R_{pPb}$ show similar $p_T$ dependence, their magnitudes differ.
- Pb–Pb collisions exhibit a stronger $p_T$ dependence and a deeper suppression at high $p_T$ ( $R_{PbPb} \approx 0.4$ ) compared to OO. This indicates that regeneration plays a more significant role in the larger Pb–Pb system, likely due to the higher abundance of heavy-quark pairs and higher initial energy density.

Significance and Claims
The paper claims that these first measurements of $J/\psi$ production in OO and pO collisions represent a new step in bridging the understanding of pp, p–Pb, and Pb–Pb systems. The results provide initial indications that hot nuclear matter effects may already be at play in light-ion collisions (specifically OO), as evidenced by the stronger suppression in OO compared to pO and the agreement with models including regeneration. However, the author notes that the recombination contribution in OO is likely smaller in magnitude than in Pb–Pb due to the smaller system size. The study concludes that while the results are preliminary, they offer important insights into the system-size dependence of quarkonium suppression and the transition from cold to hot nuclear matter effects. The author suggests that future comparisons of $R_{AA}$ as a function of $N_{part}$ and measurements of $J/\psi$ elliptic flow ( $v_2$ ) would further clarify the role of hot nuclear matter in these intermediate systems.

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