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Investigating charm quark energy loss in medium with the nuclear modification factor of D $^0$ -tagged jets

The ALICE collaboration reports the first measurement of the nuclear modification factor for D $^0$ -tagged charm jets in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, observing a hint of reduced suppression compared to inclusive jets that supports the influence of both color-charge and dead-cone mass effects on in-medium energy loss, with the LIDO model providing the best description of the data.

Original authors: ALICE Collaboration

Published 2026-09-25

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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

In the heart of matter, where atoms dissolve into a primordial soup of their smallest constituents, lies a state of existence that filled the universe mere microseconds after the Big Bang. Scientists call this the quark-gluon plasma, a seething, super-hot fluid where the building blocks of protons and neutrons—quarks and gluons—roam free, unbound by the forces that usually hold them together. To study this fleeting state, researchers smash heavy atomic nuclei together at nearly the speed of light, recreating conditions so extreme that a tiny droplet of this ancient plasma forms for a fleeting instant. Within this chaotic environment, scientists watch how high-speed particles lose energy as they plow through the dense medium, a process that reveals the fundamental rules governing how matter interacts at its most basic level.

A new study by the ALICE Collaboration at the Large Hadron Collider has taken a closer look at this energy loss, focusing specifically on particles containing charm quarks. These heavy particles are created in the very first moments of the collision and must travel through the entire life of the plasma, acting as probes that carry information about the medium they traverse. By tracking jets of particles that contain a specific type of charm particle called a D0 meson, the team measured how much these jets were slowed down compared to similar jets in empty space. The results show that charm jets are indeed suppressed, or slowed, by the plasma, but they lose less energy than the average jet, which is a mix of lighter particles. This difference suggests that the mass of the particle and its specific electrical charge both play a role in how it interacts with the hot soup, offering a clearer picture of the forces at play in the early universe.

The experiment took place in 2018, using lead ions accelerated to a center-of-mass energy of 5.02 tera-electronvolts per nucleon pair. When these ions collided, they created a fireball of quark-gluon plasma. Inside this fireball, the researchers looked for "jets," which are narrow sprays of particles that shoot out from the collision point. These jets are born from high-energy quarks or gluons that fragment as they move. The team specifically hunted for jets that contained a D0 meson, a particle made of a charm quark and an antiquark. To find these, they reconstructed the D0 meson from its decay products, a kaon and a pion, and then grouped it with other charged particles nearby to form the jet. They focused on the most violent collisions, the top ten percent of events where the nuclei smashed together most directly, ensuring the plasma was dense enough to cause significant effects.

The core of the investigation was to measure the "nuclear modification factor," a ratio that compares how often these jets appear in heavy-ion collisions versus how often they appear in proton-proton collisions, where no plasma is formed. If the plasma did nothing, this ratio would be one. If the plasma absorbed or slowed the jets, the ratio would drop below one. The data revealed that charm jets were indeed suppressed, with the ratio dropping to about 0.32 at lower speeds and rising to 0.5 at higher speeds. This confirms that the plasma is a dense, obstructive medium. However, when the researchers compared these charm jets to "inclusive" jets—which contain a random mix of light quarks and gluons—they found something intriguing. In the higher energy range, between 20 and 50 GeV/c, the charm jets were less suppressed than the inclusive jets. The difference was statistically significant, standing out with a confidence level of about two standard deviations, suggesting that the heavy charm quarks are not losing energy as quickly as their lighter counterparts.

This observation points to two competing physical effects. First, the "dead-cone effect," a phenomenon where heavy particles suppress the emission of radiation at small angles, effectively shielding them from losing energy. Second, the "Casimir effect," which relates to the color charge of the particle; gluons carry a stronger charge than quarks and are expected to lose more energy. Since inclusive jets contain a large fraction of gluons, they suffer more energy loss. The charm jets, being heavy and composed of quarks, experience less suppression. The researchers tested these ideas against several sophisticated computer models. One model, called LIDO, which accounts for both the mass of the quark and its interactions with the medium, provided the best match to the data. It successfully predicted the ordering where charm jets lose less energy than the average jet, and it showed that removing the dead-cone effect from the calculation would have led to a much larger suppression, proving that this mass-related effect is crucial.

The study does not claim to have solved the entire puzzle of how quarks interact with the plasma, but it provides a vital piece of evidence. The data suggests that the mass of the initial particle and its color charge are both essential factors in determining how much energy it loses. While the results are consistent with current theories, the author notes that more precise measurements are needed to fully separate the influence of the dead-cone effect from other mechanisms. By extending the measurement of charm jets down to lower energies, this work opens a new window into the microscopic world of the quark-gluon plasma, helping scientists understand how the fundamental properties of matter dictate its behavior in the most extreme environments imaginable.

Technical Summary: Investigating Charm Quark Energy Loss in Medium with the Nuclear Modification Factor of D0-Tagged Jets

Problem Statement
In ultrarelativistic heavy-ion collisions, the formation of a deconfined state of quarks and gluons, the Quark-Gluon Plasma (QGP), allows for the study of parton energy loss mechanisms. According to Quantum Chromodynamics (QCD), the energy loss of a parton traversing the medium depends on its color charge (Casimir factor) and its mass (dead-cone effect). While inclusive jets (a mixture of quark- and gluon-initiated jets) and heavy-flavor hadrons have been extensively studied, a direct measurement of the nuclear modification factor ( $R_{AA}$ ) for charm jets—specifically those tagged by a reconstructed $D^0$ meson—has been lacking. This gap limits the ability to disentangle the specific contributions of quark mass and color charge to in-medium energy loss, particularly in the low transverse momentum ( $p_T$ ) region where mass effects are expected to be most pronounced.

Methodology
The ALICE Collaboration analyzed data from central (0–10%) 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 charm jets identified by the presence of a $D^0$ meson among their constituents ( $D^0$ -tagged jets).

Reconstruction: $D^0$ mesons were reconstructed via the hadronic decay channel $D^0 \to K^-\pi^+$ (and charge conjugate) within the pseudorapidity range $|\eta| < 0.9$ . Jets were reconstructed using the anti- $k_T$ algorithm with a resolution parameter $R=0.3$ , combining $D^0$ candidates and charged particles ( $p_T > 150$ MeV/c). The analysis covered a jet transverse momentum range of $5 < p_T^{ch\ jet} < 50$ GeV/c.
Background Subtraction and Unfolding: A significant challenge in central heavy-ion collisions is the fluctuation of the underlying event. The analysis employed an area-based method to subtract the average background momentum density. To correct for detector resolution and background fluctuations, an iterative Bayesian unfolding procedure was applied using a response matrix generated from PYTHIA 8 simulations embedded in real Pb–Pb data.
Feed-down Correction: The contribution of non-prompt $D^0$ mesons (originating from beauty hadron decays) was estimated using POWHEG+PYTHIA 6 templates and subtracted to isolate prompt charm jets.
Reference: The nuclear modification factor was calculated by comparing the $p_T$ -differential yield in Pb–Pb collisions to the production cross-section in pp collisions (at the same energy), scaled by the average nuclear overlap function $\langle T_{AA} \rangle$ .

Key Contributions

First Measurement of $R_{AA}$ for $D^0$ -Tagged Jets: This work presents the first measurement of the nuclear modification factor for charm jets identified by a reconstructed $D^0$ meson in Pb–Pb collisions.
Extension to Low $p_T$ : The use of $D^0$ tagging allows the investigation of the charm-jet production yield down to $p_T^{ch\ jet} = 5$ GeV/c, a region where mass effects are theoretically expected to dominate.
Direct Comparison with Inclusive Jets: The study provides a direct comparison between the suppression of charm jets and inclusive jets within the same kinematic and centrality class, isolating the effects of the initiating parton's properties.

Results

Suppression: The measured $R_{AA}$ for $D^0$ -tagged jets shows a strong suppression in central Pb–Pb collisions, varying from approximately 0.32 at low $p_T$ to 0.5 at high $p_T$ .
Reduced Suppression Relative to Inclusive Jets: In the overlapping transverse momentum region ( $20 < p_T^{ch\ jet} < 50$ GeV/c), the $R_{AA}$ of $D^0$ -tagged jets is observed to be higher than that of inclusive jets. The difference has a statistical significance of approximately $2\sigma$ (specifically $2.1\sigma$ in the $20-30$ GeV/c interval).
Model Comparison: The data were compared with several theoretical models (JETSCAPE, Dai et al., and LIDO) that incorporate mass and color-charge effects.
- The LIDO model provided the best description of the data, successfully reproducing the ordering between inclusive and $D^0$ -tagged jets.
- Comparisons with LIDO calculations performed without the dead-cone effect showed a larger suppression for charm jets, indicating that the dead-cone effect plays a significant role in reducing energy loss for charm quarks in this kinematic region.
- The residual difference between models with and without dead-cone effects, as well as the difference between charm and inclusive jets, is attributed to the interplay of the quark mass (suppressing energy loss) and the color charge (enhancing energy loss for gluon-initiated jets, which dominate the inclusive sample).

Significance
The paper claims that these results provide crucial insights into the mechanisms of in-medium energy loss. The observation of reduced suppression for charm jets compared to inclusive jets supports the hypothesis that energy loss depends on both the quark mass (via the dead-cone effect) and the color charge. The successful description of the data by the LIDO model highlights the importance of including mass effects in theoretical frameworks. Furthermore, the measurement demonstrates that $D^0$ -tagged jets can serve as a sensitive probe for studying the QGP at low transverse momenta, where the interplay between mass and color charge is most distinct. The results suggest that while the dead-cone effect is a fundamental factor differentiating charm jets from inclusive jets in the measured kinematic range, more precise measurements are required to fully constrain the relative contributions of dead-cone effects versus other mechanisms like mass-dependent fragmentation.

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