⚛️ nuclear experiments

First measurement of $\mathbf{\rm \Xi_{\rm c}^{0}}$ production in $\mathbf{Pb}-\mathbf{Pb}$ collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}}}$ = 5.02 TeV

The ALICE Collaboration reports the first measurement of prompt $\Xi_{\mathrm c}^{0}$ baryon production in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV, revealing a nuclear modification factor reaching up to 3 and yield ratios that challenge current theoretical models of charm hadronization in the quark-gluon plasma.

Original authors: ALICE Collaboration

Published 2026-07-21

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: 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

Imagine the universe as a giant, cosmic kitchen where the most extreme cooking imaginable takes place. In this kitchen, scientists smash heavy atoms together at nearly the speed of light to recreate the conditions that existed just a tiny fraction of a second after the Big Bang. The result is a super-hot, super-dense soup called the quark-gluon plasma (QGP). Think of this soup not as a liquid you can drink, but as a chaotic, glowing fog where the tiny building blocks of matter—quarks and gluons—are free to roam instead of being stuck together inside particles like protons and neutrons.

In our everyday world, heavy particles called "charm quarks" are like rare, heavy ingredients that get thrown into this soup. They are created in the very first split-second of the collision, long before the soup fully forms. As the soup cools down, these heavy ingredients have to decide how to get dressed up again. Do they grab a single partner to form a simple particle (a meson), or do they team up with two others to form a more complex trio (a baryon)? This "dressing up" process is called hadronization. Scientists are obsessed with watching this process because it reveals the hidden rules of how matter forms in the universe. If the rules are different in this cosmic soup compared to normal collisions, it tells us something profound about how the universe works at its most fundamental level.

The First Look at a Rare Cosmic Trio

In a recent study, the ALICE Collaboration at CERN's Large Hadron Collider decided to hunt for a very specific, rare particle in this cosmic soup: the $\Xi^0_c$ baryon. You can think of this particle as a "charm-strange" trio—a heavy charm quark holding hands with a strange quark and a down quark. While scientists have seen other charm particles before, this is the first time anyone has successfully spotted the $\Xi^0_c$ in the heavy-ion collisions (specifically, smashing lead nuclei together) that create the quark-gluon plasma.

The team analyzed data from lead-lead collisions at an energy of 5.02 TeV. They focused on two types of collisions: the most violent, head-on crashes (called 0–10% centrality) and slightly less violent, glancing blows (30–50% centrality). By looking at the particles produced in the middle of the collision zone, they managed to count how many $\Xi^0_c$ particles were born in different speed ranges (transverse momentum, or $p_T$ ).

The Big Surprise: A Particle Explosion

The most exciting discovery is how many of these particles showed up. When the scientists looked at the slower-moving $\Xi^0_c$ particles (in the range of 3 < $p_T$ < 4 GeV/c) in the most violent collisions, they found something astonishing. The number of these particles was three times higher than what you would expect if the particles were just flying through empty space without interacting with the soup.

To put it simply: the quark-gluon plasma seems to be a "factory" that is incredibly good at building these specific charm-strange trios. The paper reports a "nuclear modification factor" ( $R_{AA}$ ) reaching a value of 3.0 ± 1.0 (stat.) +0.9 −0.8 (syst.) in that specific speed range. This is the highest boost ever measured for any charm particle in these conditions. It suggests that the heavy charm quarks aren't just floating around; they are actively grabbing onto strange and down quarks from the soup to form these new particles, a process scientists call coalescence.

The Mystery of the Missing Models

However, the story isn't a complete "we solved it" moment. When the scientists compared their real-world data to the best computer simulations (models) they have, the models mostly failed to predict the results.

The Yield Problem: The models predicted far fewer $\Xi^0_c$ particles than the scientists actually saw. The models underestimated the production yield.
The Ratio Problem: The paper also looked at how many $\Xi^0_c$ particles were made compared to other common particles like $D^0$ mesons, $D^+_s$ mesons, and $\Lambda^+_c$ baryons. In the heavy-ion collisions, the ratio of $\Xi^0_c$ to these other particles was significantly higher (by factors of 2 to 10 depending on the speed) than in normal proton-proton collisions. The models tried to predict these ratios but consistently underestimated them, meaning the real universe is making even more of these rare trios than the theories suggest.

One model, called TAMU, did a decent job of predicting the "boost" (the $R_{AA}$ value) for the slower particles, getting close to the value of 3. But even this model, along with others like QCM, EPOS4HQ, and POWLANG, couldn't fully explain why there were so many of these particles or why the ratios were so high.

What This Means

The paper suggests that the formation of the $\Xi^0_c$ is likely driven by the unique environment of the quark-gluon plasma. The "soup" is so rich in strange quarks that the heavy charm quarks have an easier time finding partners to form these specific trios. The data hints that the process of "coalescence" (particles sticking together) is much stronger here than in normal collisions, and that the strange quarks in the plasma are playing a bigger role than we thought.

While the paper doesn't claim to have the final answer, it provides a crucial new piece of the puzzle. It shows that our current theories need to be tweaked to account for this massive production of charm-strange baryons. The authors note that to fully understand this phenomenon, they need to look at even slower particles (lower momentum) and reduce the uncertainties in their measurements. With the ALICE detector getting upgrades and more data coming in from future runs, scientists hope to turn this "hint" into a clear picture of how matter is born in the hottest, densest environments in the universe.

Technical Summary: First Measurement of $\Xi^0_c$ Production in Pb–Pb Collisions at $\sqrt{s_{NN}} = 5.02$ TeV

Problem and Motivation
The production of charm hadrons in high-energy heavy-ion collisions serves as a critical probe for understanding the hadronization phase of the quark–gluon plasma (QGP). While charm quarks are produced primarily in hard-scattering processes early in the collision, their subsequent hadronization into various species (mesons vs. baryons) is governed by soft processes. Previous measurements in proton–proton (pp) and proton–lead (p–Pb) collisions revealed that the yield of charm-strange baryons, specifically $\Xi^0_c$ , is significantly larger than model expectations based on the universality of hadronization observed in $e^+e^-$ collisions. This suggests a modification in hadrochemistry where charm quarks have a higher probability of forming baryons in hadronic environments, potentially driven by coalescence mechanisms or statistical hadronization.

While the $\Lambda^+_c$ baryon has been extensively studied in nucleus–nucleus (A–A) collisions, the $\Xi^0_c$ (containing one charm, one strange, and one down quark) had not been measured in this regime. Studying $\Xi^0_c$ is essential to test theoretical models—such as coalescence and statistical hadronization—in a system where the abundance of strange quarks in the QGP is expected to enhance the production of strange-charmed baryons relative to non-strange counterparts.

Methodology
The ALICE Collaboration analyzed data from Pb–Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.02$ TeV, collected during the 2018 LHC run. The analysis focused on prompt $\Xi^0_c$ baryons (produced directly in hadronization or via hadronic decay chains, excluding weak decays from beauty hadrons) reconstructed at midrapidity ( $|y| < 0.5$ ).

Decay Channel: The $\Xi^0_c$ was reconstructed via the decay chain $\Xi^0_c \to \pi^+ \Xi^-$ , followed by $\Xi^- \to \pi^- \Lambda$ and $\Lambda \to \pi^- p$ .
Detector Systems: Reconstruction utilized the Inner Tracking System (ITS) for vertexing, the Time Projection Chamber (TPC) for tracking and particle identification (PID) via specific energy loss ( $\langle dE/dx \rangle$ ), and the Time-Of-Flight (TOF) detector for PID via flight time.
Event Selection: Events were categorized into 0–10% (central) and 30–50% (semi-central) centrality intervals based on the V0 detector signal amplitude. Approximately 100 million and 85 million events were used for the respective intervals.
Signal Extraction:
- Candidates were selected using kinematic and topological criteria (e.g., Distance of Closest Approach to the primary vertex).
- A Boosted Decision Tree (BDT) algorithm, trained on Monte Carlo simulations (QCM model) and real-data sidebands, was employed to suppress the large combinatorial background.
- The invariant mass distribution of the $\Xi^0_c$ candidates was fitted with a Gaussian signal function and a second-order polynomial background.
Corrections and Subtractions:
- Raw yields were corrected for acceptance, reconstruction efficiency, and branching ratios.
- The non-prompt contribution (from beauty-hadron decays) was estimated and subtracted, found to be less than 3% of the total yield.
- Systematic uncertainties were evaluated by varying fit ranges, bin widths, Gaussian widths, selection criteria, and efficiency models.

Key Contributions and Results
This work presents the first measurement of prompt $\Xi^0_c$ production in Pb–Pb collisions.

Production Yields: The transverse-momentum ( $p_T$ ) differential spectra were measured in the intervals $3 < p_T < 12$ GeV/c (0–10% centrality) and $4 < p_T < 12$ GeV/c (30–50% centrality).
Nuclear Modification Factor ( $R_{AA}$ ):
- The $R_{AA}$ , representing the modification of production in A–A collisions relative to pp collisions, reaches a maximum value of $3.0 \pm 1.0 \text{ (stat.)} ^{+0.9}_{-0.8} \text{ (syst.)}$ in the $3 < p_T < 4$ GeV/c interval for central collisions.
- This is the largest $R_{AA}$ value measured to date for charm hadrons.
- The $R_{AA}$ decreases with increasing $p_T$ , approaching unity at higher momenta.
Yield Ratios: The paper reports the ratios $\Xi^0_c/D^0$ $Ξ_{c}^{0} / D^{0}$ , $\Xi^0_c/D^+_s$ $Ξ_{c}^{0} / D_{s}^{+}$ , and $\Xi^0_c/\Lambda^+_c$ $Ξ_{c}^{0} / Λ_{c}^{+}$ .
- All ratios are systematically higher in Pb–Pb collisions compared to pp collisions.
- In central collisions, the enhancement factors range from a factor of 2 to 10 depending on the ratio and $p_T$ interval.
- Specifically, in the 0–10% centrality interval for $3 < p_T < 8$ GeV/c, the $\Xi^0_c$ yield is comparable to that of $D^0$ and $\Lambda^+_c$ , and larger than that of $D^+_s$ .

Comparison with Models
The measured spectra and $R_{AA}$ were compared to predictions from four models: Quark (Re)Combination Model (QCM), TAMU, EPOS4HQ, and POWLANG.

Spectra: All models underestimate the measured $\Xi^0_c$ production yield in both centrality intervals across the full $p_T$ range.
$R_{AA}$ : The TAMU model predicts an $R_{AA} \approx 3$ at $p_T \approx 3$ GeV/c, reproducing the data well. The QCM, EPOS4HQ, and POWLANG models predict lower values but remain compatible with the data within $3\sigma$ .
Ratios: All theoretical models tend to underestimate the measured baryon-to-meson and baryon-to-baryon yield ratios, although large uncertainties (particularly in branching ratios) prevent definitive conclusions.

Significance and Claims
The paper claims that the observation of a large $R_{AA}$ peak and enhanced yield ratios supports the hypothesis that $\Xi^0_c$ baryons are formed via the coalescence of charm quarks with strange and down quarks (or diquarks) within a radially expanding partonic medium. The results suggest that the charm hadrochemistry is modified in Pb–Pb collisions compared to pp collisions.

The authors note that while the data is compatible with models incorporating coalescence, the current theoretical descriptions generally fail to fully capture the magnitude of the enhancement. The paper concludes that further measurements extending to lower $p_T$ and a reduction in branching ratio uncertainties are necessary to conclusively determine the contribution of $\Xi^0_c$ to total charm production and to fully validate the mechanisms of strangeness enhancement and coalescence in the QGP. The upgraded ALICE detector and future data from Run 3 are expected to facilitate these improvements.

The First Look at a Rare Cosmic Trio

The Big Surprise: A Particle Explosion

The Mystery of the Missing Models

What This Means

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