Probing the charm-quark dead cone effect in heavy ion collisions

CMS Collaboration

AI summary · arXiv: 2611.01285v1 · nucl-ex

Probing the charm-quark dead cone effect in heavy ion collisions

CMS Collaboration

Original paper Submitted to arXiv on 31 Oct. 2026 Submitted to Physical Review Letters. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-25-013 (CMS Public Pages) License: not stated (all rights reserved)

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The essentials

CMS observes fewer collinear intrajet emissions in prompt D⁰ jets than in inclusive jets in PbPb collisions, consistent with the charm-quark dead-cone effect.

Based on the abstract only: check the details in the original paper.

Question

The study asks whether the charm-quark dead cone is observable in heavy-ion collisions and whether medium-induced radiation fills it.

Method

The analysis uses 2018 PbPb collision data at a nucleon-pair center-of-mass energy of 5.02 TeV, corresponding to an integrated luminosity of 1.7 nb⁻¹. CMS compares angular distributions of hard, collinear intrajet emissions selected with late-kT grooming in inclusive jets and jets containing prompt D⁰ mesons.

Findings

Result Late-kT emissions in D⁰ jets are shifted toward larger angles compared with those in inclusive jets.
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The angular distribution of late-$k_\mathrm{T}$ emissions in PbPb collisions is found to be shifted towards larger angles in D$^0$ jets relative to inclusive jets
Authors’ interpretation The observed angular shift indicates suppression of collinear splittings in D⁰ jets.
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indicating a suppression of collinear splittings for D$^0$ jets.
Result The PbPb observation is consistent with a similar observation in proton-proton collisions.
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This shift, which is expected from the dead-cone effect, is consistent with a similar observation made in proton-proton collisions.
Authors’ interpretation The measurement disfavors hypotheses in which medium-induced radiation fills the charm-quark dead cone.
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The new measurement disfavors hypotheses that predict a filling of the charm-quark dead cone by medium-induced radiation in heavy ion collisions.

Limitations

The reported study is limited to PbPb collisions at 5.02 TeV and the late-kT emission angular distribution in inclusive and prompt D⁰ jets.

Why it matters

The authors present this as the first experimental study of the charm-quark dead-cone effect in heavy-ion collisions. The measurement provides evidence against scenarios predicting that medium-induced radiation fills the dead cone.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: charm-quark dead-cone effect heavy-ion collisions CMS D-zero jets late-kT grooming

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Comprehensive Summary
Layman's Summary
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Original abstract

The paper reports the first experimental study of the charm-quark dead-cone effect in heavy-ion collisions. The dead cone is the suppression of gluon radiation near the direction of a massive quark compared with radiation from a light quark. The study examines whether this suppression can be observed in charm jets produced in the dense medium created by lead–lead collisions, and whether the medium fills the suppressed region with additional radiation. The analysis uses lead–lead collision data recorded by the CMS experiment at the CERN Large Hadron Collider in 2018. The collisions had a center-of-mass energy per nucleon pair of 5.02 TeV and an integrated luminosity of 1.7 inverse nanobarns. CMS compares inclusive jets with jets containing prompt D-zero mesons, which identify jets associated with charm. The measured quantity is the angular distribution of hard, collinear emissions within jets. These emissions are identified using the late-kT grooming algorithm, which selects a particular splitting in the jet’s branching history. The main result is that late-kT emissions in D-zero jets are shifted toward larger angles than emissions in inclusive jets. In physical terms, charm-associated jets have fewer emissions close to the jet direction, consistent with the suppression of collinear splittings expected from the dead-cone effect. The abstract reports the data set’s energy and luminosity but does not provide numerical values for the angular shift, the measured distributions, uncertainties, or statistical significance. These quantities therefore cannot be specified from the available information. The observed shift is consistent with a similar observation in proton–proton collisions, supporting the interpretation that the angular pattern in charm jets is compatible with the dead-cone effect in both dense and less dense collision environments. For the lead–lead data, the result disfavors hypotheses predicting that medium-induced radiation fills the charm dead cone. It does not establish that no medium-related modification occurs; rather, the measurement is inconsistent with predictions in which such radiation substantially fills the suppressed region. This summary is limited to the paper’s title and abstract. The available information does not include the full analysis details, such as jet and D-zero selection criteria, grooming and background-subtraction procedures, detector corrections, model comparisons, systematic uncertainties, or quantitative results. Consequently, the measurement’s precision, the robustness of the comparison, and the extent to which alternative explanations are excluded cannot be assessed. The supported conclusion is that CMS reports a larger-angle emission pattern for D-zero jets than for inclusive jets in PbPb collisions, consistent with charm dead-cone suppression and disfavoring the described dead-cone-filling hypotheses.

Looking for the Charm Quark’s “Dead Cone” in Heavy-Ion Collisions

When a high-energy quark travels through space, it can radiate gluons, producing a branching cascade that becomes a jet of particles. But a quark’s mass changes how that radiation is distributed. Massive quarks are expected to emit fewer gluons close to their direction of travel than light quarks do. This suppressed region is known as the “dead cone.”

In Probing the charm-quark dead cone effect in heavy ion collisions, the CMS Collaboration reports the first experimental study of this effect for charm quarks in heavy-ion collisions. The result offers a way to investigate how charm jets develop inside the exceptionally dense matter formed when lead nuclei collide.

A Radiation Pattern Shaped by Mass

The dead-cone effect is a prediction of quantum chromodynamics, the theory describing the strong force. Because a charm quark is heavier than the light quarks, its mass is expected to suppress gluon radiation emitted at small angles relative to the quark’s direction. In a jet, that would mean fewer close-to-the-axis splittings and a radiation pattern shifted toward larger angles.

Heavy-ion collisions add a further question. They create a dense medium through which energetic quarks and gluons travel. That medium can influence how jets radiate and evolve. Could extra radiation produced in the medium fill in the region where the charm quark’s mass would otherwise suppress emissions? The CMS study tests whether the observed charm-jet pattern is compatible with that possibility.

Comparing Charm-Associated and Inclusive Jets

The analysis uses lead–lead collision data collected by CMS at the CERN Large Hadron Collider in 2018. The collisions had a center-of-mass energy per nucleon pair of 5.02 TeV, and the data set had an integrated luminosity of 1.7 inverse nanobarns.

CMS compares two categories of jets: inclusive jets and jets containing prompt D-zero mesons. A prompt D-zero meson helps identify a jet associated with a charm quark. Rather than simply counting particles in each jet, the analysis examines the angular distribution of hard, collinear emissions—relatively energetic emissions occurring close to a jet’s direction.

To identify these emissions, the researchers use the late-kT grooming algorithm. Grooming examines a jet’s branching history and selects a particular splitting for study. This makes it possible to compare the angular pattern of emissions in charm-associated jets with the pattern in the inclusive-jet sample.

What CMS Observed

The central result is that the selected emissions in D-zero jets are shifted toward larger angles than those in inclusive jets. Put simply, charm-associated jets show fewer emissions close to the jet’s direction. That pattern is consistent with the expected suppression of collinear radiation from a massive quark—the charm dead-cone effect.

The result also bears on how the dense collision environment might modify the dead cone. The lead–lead data disfavor hypotheses in which medium-induced radiation substantially fills the suppressed region. This does not show that the medium has no effect on charm jets. It means that the observed pattern is inconsistent with the particular picture in which extra radiation fills the dead cone to a substantial degree.

The angular pattern is also consistent with a similar observation in proton–proton collisions. Seeing compatible behavior in both less-dense and heavy-ion collision environments supports the interpretation that charm jets retain a radiation pattern associated with the dead-cone effect.

What the Result Can—and Cannot—Tell Us

The reported summary does not provide numerical values for the angular shift, the measured distributions, uncertainties, or statistical significance. It also does not include the detailed selection criteria, background-subtraction procedures, detector corrections, systematic uncertainties, or model comparisons. Without those details, readers cannot assess the measurement’s precision or how strongly alternative explanations are excluded.

The careful takeaway is therefore specific: CMS reports that emissions selected in D-zero jets in lead–lead collisions tend to occur at larger angles than those in inclusive jets. This is consistent with charm-quark dead-cone suppression and disfavors predictions in which medium-induced radiation substantially fills the suppressed region. It is evidence about the radiation pattern, not proof that heavy-ion media leave charm jets unchanged.

Why the Measurement Matters

Jets provide a record of how energetic particles radiate and interact. Studying their internal structure can help separate effects associated with a quark’s own properties, such as its mass, from changes caused by the surrounding medium. By examining charm-tagged jets in lead–lead collisions, the CMS Collaboration brings the dead-cone idea into a setting where both effects may be relevant.

The study’s main contribution is an experimental look at that distinction. Its observed larger-angle emission pattern is compatible with the expected charm dead cone, while challenging the described scenarios in which the heavy-ion medium would substantially erase it. More detailed quantitative results are needed to judge the measurement’s precision and the reach of its conclusions.

Created on 09 Nov. 2026

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Probing the charm-quark dead cone effect in heavy ion collisions

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Looking for the Charm Quark’s “Dead Cone” in Heavy-Ion Collisions

A Radiation Pattern Shaped by Mass

Comparing Charm-Associated and Inclusive Jets

What CMS Observed

What the Result Can—and Cannot—Tell Us

Why the Measurement Matters

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