⚛️ nuclear experiments

Observation of enhanced baryon production using strange hadrons in oxygen-oxygen collisions

Using the CMS detector at the LHC, this study reports the first observation of enhanced baryon-to-meson production in oxygen-oxygen collisions at 5.36 TeV, providing evidence for the formation of a quark-gluon plasma and the influence of collective radial flow and quark coalescence in small collision systems.

Original authors: CMS Collaboration

Published 2026-08-20

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: CMS 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

In the heart of matter, where protons and neutrons are built, there exists a state of existence so extreme that the usual rules of atomic structure dissolve. Under normal conditions, quarks and gluons—the fundamental building blocks of protons and neutrons—are permanently bound together, never found alone. However, when atomic nuclei are smashed together at speeds approaching the speed of light, the energy is so intense that these bonds break. For a fleeting instant, a hot, dense soup of free-floating quarks and gluons forms, a state of matter known as the quark-gluon plasma. This is the same kind of matter that filled the entire universe just microseconds after the Big Bang. Scientists study this plasma to understand how the universe cooled down to form the stable atoms that make up everything we see today. A key question in this field is whether this exotic soup can form in collisions that are much smaller than the massive lead-on-lead crashes usually studied. If it can, it would suggest that the conditions required to create this primordial state are far more common and easier to achieve than previously thought.

To investigate this, researchers at the Large Hadron Collider used a specialized detector called CMS to observe collisions between oxygen nuclei. While oxygen is a small ion compared to the heavy lead ions typically used, these collisions still pack enough energy to potentially create a tiny droplet of quark-gluon plasma. The team analyzed data from oxygen-oxygen collisions and compared them directly to collisions between single protons. They focused their attention on two specific types of particles that emerge from the wreckage of these collisions: a type of particle called a kaon, which is a meson, and a particle called a lambda, which is a baryon. Both of these particles contain a strange quark, a heavier cousin of the up and down quarks that make up ordinary matter. By counting how many of these particles were produced at different speeds, the scientists could look for signs that the particles were interacting with a hot, dense medium rather than just flying apart from a simple collision.

The results revealed a distinct difference in how these particles behaved depending on the size of the collision. In the oxygen-oxygen crashes, the production of the lambda baryons was significantly enhanced compared to the production of the kaon mesons, particularly in a specific range of speeds. This ratio of baryons to mesons was much higher in the oxygen collisions than in the proton-proton collisions, where no such enhancement was seen. This finding is crucial because it mirrors a phenomenon observed in much larger, heavy-ion collisions, where the formation of a quark-gluon plasma is well established. The data suggests that in these smaller oxygen collisions, the quarks and gluons are not just scattering randomly but are flowing together in a coordinated way, a behavior known as collective radial flow. This flow, combined with a process where quarks join together to form particles, appears to boost the number of baryons produced relative to mesons.

The study provides the first clear observation of this baryon-to-meson enhancement in oxygen-oxygen collisions. While the effects are less pronounced than in the massive lead collisions, the pattern is unmistakable. The researchers found that the strange baryons were being produced in greater numbers than expected if the collisions were just simple interactions between individual particles. This points toward the formation of a deconfined medium, a tiny, short-lived version of the quark-gluon plasma, even in these intermediate-sized systems. The findings help refine our understanding of how this exotic state of matter forms and how it influences the creation of new particles. It suggests that the transition from a hot, free-flowing soup of quarks to the solid matter we know involves complex mechanisms like the coalescence of quarks and the push of collective flow, and that these mechanisms are active even in collisions that are much smaller than previously believed necessary to create such a state.

Technical Summary: Observation of enhanced baryon production using strange hadrons in oxygen-oxygen collisions

Problem and Motivation
The formation of a hot, deconfined medium known as the quark-gluon plasma (QGP) in ultrarelativistic heavy-ion collisions is a well-established phenomenon. In these systems, hadron production is significantly modified compared to proton-proton (pp) interactions, particularly in the transverse momentum ( $p_T$ ) region of 2–5 GeV. Two primary mechanisms are responsible for these modifications: collective radial flow and quark coalescence. While these effects are well-documented in large ion systems (e.g., lead-lead), it remains an open question whether they manifest in smaller collision systems. Previous observations in high-multiplicity pp, pPb, and even photon-ion collisions have suggested QGP-like behavior, but these findings are often complicated by event activity selections that may introduce biases. The oxygen-oxygen (OO) collision system, with a mass number $A=16$ , offers a unique intermediate-sized system to investigate these phenomena. While signatures of QGP production have recently been identified in OO collisions, potential modifications to strange hadron and baryon yields had not yet been quantified. This study aims to measure the production of strange hadrons ( $K^0_S$ and $\Lambda$ ) in OO collisions at $\sqrt{s_{NN}} = 5.36$ TeV to determine if baryon-to-meson enhancement and strangeness enhancement—hallmarks of QGP formation—are present.

Methodology
The analysis utilizes data collected by the CMS detector at the LHC. The dataset comprises OO collisions with an integrated luminosity of $6.8 \pm 0.3$ nb $^{-1}$ and a baseline pp dataset collected at the same energy ( $\sqrt{s_{NN}} = 5.36$ TeV) with an integrated luminosity of $1.06 \pm 0.03$ pb $^{-1}$ .

Event Selection: OO events were selected requiring energy deposits above 4 GeV in two or more detector elements on each side of the Hadron Forward (HF) calorimeter to suppress beam background and ultraperipheral collisions. pp events were triggered by coincident colliding bunches. Pileup corrections were applied to both datasets, and events were required to have a primary vertex within 15 cm of the detector center.
Particle Reconstruction: $K^0_S$ mesons and $\Lambda$ baryons were reconstructed via their decay channels $K^0_S \to \pi^+\pi^-$ and $\Lambda \to p\pi^-$ . Candidates were identified using oppositely charged tracks with $p_T > 0.1$ GeV and $|\eta| < 2.4$ , fitted to a common secondary vertex. Strict selection criteria regarding track quality, impact parameter significance, and decay length significance were applied to ensure high purity.
Yield Extraction: Signal yields were extracted by fitting the invariant mass spectra in $p_T$ intervals. The signal was modeled with a double-Gaussian function, while the combinatorial background was described by a third-order polynomial.
Corrections and Systematics: Yields were corrected for detector acceptance, reconstruction efficiency, and selection efficiency using Monte Carlo simulations (PYTHIA 8 for pp, PYTHIA overlaid with HIJING for OO, and GEANT4 for detector response). A specific feed-down correction was applied to $\Lambda$ yields to account for contributions from weak decays of heavier strange baryons ( $\Xi$ and $\Omega$ ), scaled according to charged-particle multiplicity density. Systematic uncertainties were evaluated for pileup, feed-down modeling, track reconstruction, candidate selection, and luminosity.
Observables: The study presents the $p_T$ -differential invariant cross sections, the nuclear modification factor ( $R_{AA}$ , denoted as $R_{OO}$ for this system), and the $\Lambda/K^0_S$ cross-section ratio.

Key Contributions and Results

Cross Sections and Spectral Modification: The $p_T$ -differential cross sections for $K^0_S$ and $\Lambda$ were measured for the first time in OO collisions. The spectral shape of $K^0_S$ mesons in OO collisions relative to pp collisions shows a modification similar to that of inclusive charged hadrons.
Nuclear Modification Factor ( $R_{OO}$ ):
- For $K^0_S$ mesons, $R_{OO}$ is consistent with that of charged hadrons, sitting slightly below unity in the intermediate $p_T$ region (2–8 GeV).
- For $\Lambda$ baryons, $R_{OO}$ exhibits a pronounced enhancement relative to both $K^0_S$ mesons and charged hadrons in the range $2 < p_T < 8$ GeV.
- At $p_T > 10$ GeV, all particle species show significant suppression ( $R_{OO} < 1$ ), consistent with parton energy loss in a deconfined medium.
Baryon-to-Meson Enhancement: The ratio of $\Lambda$ to $K^0_S$ cross sections was measured. In OO collisions, this ratio shows a significant enhancement compared to pp collisions in the intermediate $p_T$ region ( $2.3 < p_T < 5.2$ GeV). The peak of the ratio shifts from $p_T \approx 2.3$ GeV in pp to $p_T \approx 2.8$ GeV in OO.
Statistical Significance: The difference between the $\Lambda/K^0_S$ ratios in OO and pp collisions was evaluated accounting for correlated systematic uncertainties. In the $p_T$ range of 2.3–5.2 GeV, the significance of the enhancement ranges from 5.0 to 6.3 standard deviations.

Significance and Claims
This paper claims the first observation of a baryon-to-meson enhancement in oxygen-oxygen collisions relative to proton-proton collisions. The observed enhancement of $\Lambda$ baryons over $K^0_S$ mesons, combined with the shift in the peak position of the ratio to higher $p_T$ , provides strong evidence for the formation of a deconfined medium in OO collisions.

The results are interpreted as consistent with hadronization mechanisms involving quark coalescence and collective radial flow. In the coalescence picture, baryons (three quarks) gain more transverse momentum than mesons (two quarks) when combining with partons from the medium. Additionally, radial flow imparts a larger boost to heavier hadrons like the $\Lambda$ . The observation suggests that even in intermediate-sized systems like OO, where the system size is smaller than in lead-lead collisions, the conditions for QGP formation and the associated collective effects are present. This finding offers new insights into the roles of strangeness, radial flow, and quark coalescence in the hadronization process within a strongly interacting deconfined medium.

More like this