Evidence for parton energy loss in oxygen$-$oxygen collisions at $\mathbf{\sqrt{s_{\rm NN}}=5.36}$ TeV

This paper presents the first unambiguous evidence of parton energy loss (jet quenching) in oxygen-oxygen collisions at $\sqrt{s_{\rm NN}}=5.36$ TeV, demonstrated by a significant suppression of the double ratio $R_{\rm OO}/R_{\rm pO}$ at a $4.9\sigma$ level that cannot be explained by cold nuclear matter effects alone.

The Gist

The Big Picture: Smashing Atoms to Find "Perfect" Liquid

Imagine you have a giant, high-speed slingshot (the Large Hadron Collider) that smashes tiny particles together at nearly the speed of light. When you smash heavy atoms like Lead (Pb) together, they melt into a super-hot, super-dense soup called Quark-Gluon Plasma (QGP). Scientists think this soup behaves like a "perfect liquid"—it flows without any friction, much like water flowing down a drain, but it's made of the fundamental building blocks of matter.

For a long time, we knew this perfect liquid existed in big collisions (like Lead-Lead). But a big question remained: Does this liquid form in smaller collisions?

Think of it like this: If you drop a large rock into a pond, you get huge waves. If you drop a pebble, you get small ripples. But what if you drop a grain of sand? Do you still get ripples, or does the water just stay calm? Scientists wanted to know if the "perfect liquid" forms even when the collision is as small as two Oxygen atoms hitting each other.

The Mystery: The "Jet Quenching" Test

To test if this liquid exists, scientists look for a specific sign called "jet quenching."

• The Analogy: Imagine two cars speeding toward each other. In a normal crash (like two protons hitting), they might shoot out a spray of sparks (high-energy particles called "partons") that fly straight out.
• The Liquid Effect: If those cars crash inside a thick, sticky mud pit (the QGP), the sparks won't fly far. The mud will slow them down, absorb their energy, and stop them. This slowing down is called "energy loss" or "jet quenching."

In big collisions (Lead-Lead), we see the sparks get stopped. In tiny collisions (Proton-Proton), they fly straight through. The mystery was: What happens in the middle-sized collision (Oxygen-Oxygen)?

The Experiment: A New Kind of Collision

In July 2025, the ALICE experiment at CERN performed a special test. They smashed:

1. Oxygen against Oxygen (OO): A medium-sized collision.
2. Proton against Oxygen (pO): A smaller collision to act as a control.
3. Proton against Proton (pp): The baseline (no liquid expected).

They looked specifically at neutral pions (a type of particle that decays into two photons, or light particles). These pions are like the "sparks" in our car analogy. By measuring how many pions came out and how fast they were going, the team could see if they lost energy.

The Results: The "Double Check"

The scientists had to be very careful. Sometimes, the nucleus of an atom itself can slow down particles before the collision even happens (like driving through a light fog). This is called "Cold Nuclear Matter" effect. They needed to make sure the slowing down they saw was actually due to the "hot liquid" (QGP) and not just the fog.

Here is how they solved it:

1. The First Look (OO vs. pp): They found that in Oxygen-Oxygen collisions, the pions were significantly slower and fewer than expected. It looked like they hit a wall.
2. The Control Check (pO vs. pp): They looked at Proton-Oxygen collisions. Here, the pions flew straight through with no slowing down. This proved that the Oxygen nucleus itself (the "fog") wasn't the problem. If the fog were the cause, the Proton-Oxygen crash would have shown slowing down too.
3. The "Double Ratio" (The Magic Trick): To be absolutely sure, they created a special math formula: (Oxygen-Oxygen result) divided by (Proton-Oxygen result) squared.
   • Think of this as a "noise-canceling" headphone for physics. It cancels out the "fog" (cold effects) and leaves only the "mud" (hot effects).
   • The Result: Even after canceling out the fog, the Oxygen-Oxygen collisions still showed a massive slowdown. The data was 4.9 times more extreme than what you would expect if there were no liquid at all.

The Conclusion: The Smallest Liquid Yet

The paper concludes that yes, the perfect liquid forms even in the smallest nuclear system studied to date: Oxygen-Oxygen collisions.

• The Evidence: The particles lost energy just like they do in giant Lead collisions.
• The Significance: This proves that the "perfect liquid" doesn't need a huge amount of matter to form. It can appear in a system as small as two Oxygen atoms smashing together.
• The Theory Check: The data matched perfectly with computer models that assume a hot, dense liquid exists. Models that assumed no liquid (only cold effects) failed to explain the results.

In short: Scientists smashed Oxygen atoms together and found that even in this tiny, intermediate-sized crash, the matter melts into a super-fluid that acts like a sticky trap, slowing down high-speed particles. This is the smallest "perfect liquid" ever observed.

Technical Summary

Technical Summary: Evidence for Parton Energy Loss in Oxygen–Oxygen Collisions at $\sqrt{s_{NN}} = 5.36$ TeV

Problem and Motivation
The formation of the quark–gluon plasma (QGP), a deconfined state of quarks and gluons, is well-established in large nuclear systems such as Pb–Pb and Au–Au collisions, where "jet quenching" (parton energy loss) serves as a primary probe of the medium's properties. However, the onset of QGP formation in smaller collision systems remains a subject of investigation. While high-multiplicity proton–proton (pp) and proton–lead (p–Pb) collisions have exhibited signatures of collectivity, unambiguous direct evidence of parton energy loss in these small systems has been elusive. A critical gap exists in understanding the transition from small to large systems. Oxygen–oxygen (OO) collisions, with an intermediate nuclear mass number ($A=16$), offer a unique opportunity to bridge the gap between small systems (pp, p–Pb) and large systems (Xe–Xe, Pb–Pb). A significant challenge in interpreting suppression in these intermediate systems is distinguishing "hot" medium effects (QGP-induced energy loss) from "cold" nuclear matter (CNM) effects, such as nuclear parton distribution function (nPDF) modifications (e.g., gluon shadowing) or fully coherent energy loss. Previous measurements in OO collisions by CMS indicated suppression but lacked a robust data-driven constraint on CNM contributions specific to the oxygen projectile.

Methodology
This study utilizes data recorded by the ALICE detector in July 2025 from OO collisions at $\sqrt{s_{NN}} = 5.36$ TeV and proton–oxygen (pO) collisions at $\sqrt{s_{NN}} = 9.62$ TeV. The analysis focuses on the production of neutral pions ($\pi^0$) in the transverse momentum range $1.2 < p_T < 20$ GeV/c.

• Detector and Reconstruction: The Fast Interaction Trigger (FIT) system provided minimum-bias triggers and collision timing, while the Electromagnetic Calorimeter (EMCal) was used for photon reconstruction. Photons were identified by clustering energy deposits and applying selection criteria based on cluster shape (roundness) and timing to reject hadronic backgrounds and pileup. $\pi^0$ mesons were reconstructed via the invariant mass of photon pairs ($M_{\gamma\gamma}$), with combinatorial background subtracted using mixed-event and rotation methods.
• Reference Baselines: To calculate nuclear modification factors, reference cross-sections from pp collisions were required.
  • For OO collisions ($\sqrt{s_{NN}} = 5.36$ TeV), pp data at the same energy ($\sqrt{s} = 5.36$ TeV) served as the baseline.
  • For pO collisions ($\sqrt{s_{NN}} = 9.62$ TeV), the baseline was constructed by interpolating between pp measurements at $\sqrt{s} = 5.36$ TeV and $\sqrt{s} = 13.6$ TeV using a power-law parameterization of the energy dependence.
• Observables:
  • $R_{OO}$: The nuclear modification factor for OO collisions, defined as the ratio of the $\pi^0$ yield in OO to that in pp, scaled by the number of binary collisions ($A^2$).
  • $R_{pO}$: The nuclear modification factor for pO collisions, defined similarly but scaled by $A$. This provides a first-time data-driven constraint on CNM effects for an oxygen projectile.
  • Double Ratio ($R_{OO}/R_{pO}^2$): Constructed to largely cancel CNM contributions present in both $R_{OO}$ and $R_{pO}$, isolating final-state hot-medium effects.

Key Contributions and Results

1. Measurement of $R_{OO}$: The $R_{OO}$ factor exhibits a clear suppression relative to unity, with a significance of $9.2\sigma$. The suppression pattern as a function of $p_T$ is similar to that observed in Pb–Pb collisions. When compared to Next-to-Leading Order (NLO) pQCD calculations incorporating various nPDF sets (TUJU21, nNNPDF3.0, nCTEQ15HQ, EPPS21), the data shows deviations. While some nPDF sets predict deviations of up to 20% (EPPS21), the suppression observed in the data exceeds theoretical CNM expectations with a significance of $1.8\sigma$ against the most suppressed prediction.
2. Measurement of $R_{pO}$: For the first time, $R_{pO}$ is measured for oxygen projectiles. The results are compatible with unity across the full $p_T$ range within uncertainties. This indicates that CNM effects alone do not lead to significant suppression in pO collisions. The data is consistent with both standard nPDF calculations and models based solely on fully coherent energy loss (FCEL).
3. Double Ratio Analysis: The double ratio $R_{OO}/R_{pO}^2$ is presented to isolate hot-medium effects. Theoretical predictions including only CNM effects (nPDFs) cluster around unity. The measured double ratio deviates significantly from these expectations, showing a suppression consistent with parton energy loss. The significance of this deviation is $4.9\sigma$ (for the nNNPDF3.0 baseline).
4. Theoretical Comparison: The observed suppression in the double ratio is compared to two theoretical models incorporating parton energy loss: the "hybrid coupling approach" and the "Cape Town model." Both models, which include energy loss mechanisms in a hot medium, successfully describe the magnitude and shape of the suppression observed in the data.

Significance
The paper establishes evidence for parton energy loss in oxygen–oxygen collisions. By combining the measurement of $R_{OO}$ with the novel $R_{pO}$ measurement and the double ratio observable, the study successfully constrains cold nuclear matter effects and isolates final-state interactions. The results demonstrate that the observed suppression cannot be explained by CNM effects alone. This finding extends the experimental evidence for jet quenching to the smallest nuclear system studied to date, providing a crucial data point for understanding the onset of QGP formation at low mass numbers. The findings are consistent with theoretical models that assume the presence of a hot, deconfined medium in OO collisions.