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

This paper presents the first experimental observation of the charm-quark dead-cone effect in heavy ion collisions, demonstrating that the angular distribution of emissions in D $^0$ jets is shifted toward larger angles compared to inclusive jets, thereby confirming the suppression of collinear splittings and ruling out hypotheses that the dead cone is filled by medium-induced radiation.

Original author: CMS Collaboration

Published 2026-11-03

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CC BY 4.0

Original author: 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 author. For technical accuracy, refer to the original paper. Read full disclaimer

In the earliest moments after the Big Bang, the universe was not a collection of solid stars and planets, but a seething, ultra-hot soup of fundamental particles. Physicists call this state the quark-gluon plasma. It is a phase of matter where the building blocks of protons and neutrons, known as quarks, are free to roam rather than being locked together. To study how this primordial soup behaves, scientists smash heavy atomic nuclei together at nearly the speed of light. These collisions create tiny, fleeting droplets of the quark-gluon plasma, allowing researchers to observe how energy and matter interact under extreme conditions. One of the key questions in this field is how the heavy particles within this plasma, specifically those containing charm quarks, lose energy as they move through the hot medium. Theoretical models suggest that the mass of a particle should influence how it radiates energy, creating a specific "shadow" or gap in the pattern of emitted particles, a phenomenon known as the dead-cone effect.

For years, scientists have observed this dead-cone effect in collisions between protons, where no hot plasma is present. However, it remained a mystery whether this effect would survive the chaotic environment of a heavy-ion collision, where the dense medium might fill in that gap with its own radiation. A researcher using the CMS detector at the Large Hadron Collider in Switzerland has now answered this question for the first time. They analyzed data from lead-lead collisions recorded in 2018, a dataset corresponding to an integrated luminosity of 1.7 inverse nanobarns. By focusing on jets of particles that contained a specific type of meson called a D0, which is formed from a charm quark, they were able to trace the path of energy loss with high precision.

The researcher looked at the angles at which particles were emitted inside these jets. In a vacuum, a heavy charm quark is expected to emit fewer particles at very small angles compared to a lighter quark, creating a cone-shaped region where emissions are suppressed. This is the dead cone. They measured the distribution of these emissions in the lead-lead collisions and compared them to similar measurements taken in proton-proton collisions, where no plasma exists. They found that the pattern of emissions in the heavy-ion collisions was shifted toward larger angles for the charm-quark jets. This shift indicated that the suppression of small-angle emissions was still present, meaning the dead cone had not been filled in by the surrounding medium.

This finding challenges a long-standing hypothesis that the intense radiation from the quark-gluon plasma would overwhelm the dead-cone effect, effectively filling the gap with new particles. Instead, the data suggests that for charm quarks, the dead cone remains largely intact, behaving much like it does in a vacuum. The results are consistent with theoretical predictions that the angle of the dead cone for a charm quark is smaller than the characteristic angle at which the plasma can resolve individual emissions. Consequently, the plasma cannot easily penetrate this small region to add its own radiation. The study provides the first experimental evidence that the dead-cone effect for charm quarks persists even in the extreme environment of heavy-ion collisions, offering a clearer picture of how heavy particles navigate the quark-gluon plasma.

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

Problem Statement
The primary objective of this study is to investigate the "dead-cone effect" for charm quarks within the environment of the quark-gluon plasma (QGP) generated in heavy ion collisions. In Quantum Chromodynamics (QCD), the dead-cone effect describes the suppression of gluon emissions off a massive quark within a cone of angle $\theta_d = m_q/E_q$ around the emitting quark. While this effect has been experimentally confirmed in proton-proton ($pp$) collisions for both charm ( $c$ ) and bottom ( $b$ ) quarks, its behavior in heavy ion collisions remains a subject of theoretical debate.

Theoretical predictions suggest that in a dense medium like the QGP, medium-induced gluon radiation could "fill" the dead cone, effectively washing out the mass-dependent suppression observed in vacuum. Conversely, other models propose that if the medium's coherence angle ( $\theta_c$ ) is larger than the dead-cone angle ( $\theta_d$ ), the emission pattern within the dead cone should remain vacuum-like. Previous studies have not detailed the flavor dependence of jet quenching in this regime, and specifically, the interplay between the charm-quark dead cone and medium-induced radiation in heavy ion collisions had not been experimentally characterized.

Methodology
The analysis utilizes data from lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.02$ TeV, recorded by the CMS experiment at the LHC in 2018, corresponding to an integrated luminosity of 1.7 nb $^{-1}$ . The study focuses on the 30% most central collisions to maximize medium effects.

The methodology employs advanced jet substructure techniques to reconstruct the angular distribution of hard, collinear intrajet emissions. Key procedural steps include:

Jet Reconstruction: Jets are clustered using the anti- $k_T$ algorithm with a small radius parameter $R=0.2$ to mitigate the large underlying event (UE) background characteristic of heavy ion collisions.
D0 Meson Tagging: The analysis selects "prompt D0 jets" by reconstructing $D^0 \to K^\mp \pi^\pm$ decays. A multivariate boosted decision tree (BDT) is used to distinguish signal from combinatorial background. Prompt $D^0$ mesons are isolated from non-prompt contributions (from $b$ -hadron decays) using template fits to the distance-of-closest-approach (DCA) significance distribution.
Grooming and Declustering: The "late- $k_T$ " grooming algorithm is applied. This involves reclustering jet constituents with the Cambridge-Aachen (CA) algorithm and declustering in reverse order. The algorithm selects the last emission in the declustering sequence where the relative transverse momentum $k_T > 1$ GeV. The observable of interest is the "late- $k_T$ splitting angle" ( $\theta_l$ ), defined as the angular distance between the two prongs of this specific splitting.
Unfolding: To recover particle-level distributions, a two-dimensional unfolding procedure is performed in jet transverse momentum ( $p_T^{jet}$ ) and $\ln(1/\theta_l)$ space. This corrects for detector resolution, UE fluctuations, and migration effects using a response matrix derived from simulations (PYTHIA 8.230 embedded in HYDJET 1.9).
Comparison: The angular distributions for prompt $D^0$ jets and inclusive jets are measured in PbPb collisions and compared against each other, as well as against a reference measurement in $pp$ collisions at the same energy.

Key Contributions

First Experimental Characterization: This work presents the first experimental study of the charm-quark dead-cone effect specifically in heavy ion collisions.
Novel Observable Application: It successfully applies the late- $k_T$ grooming algorithm to heavy-flavor jets in a high-background heavy ion environment, demonstrating the feasibility of probing quark-mass effects in the QGP.
Model Discrimination: The analysis provides a direct test of theoretical hypotheses regarding whether the QGP fills the charm-quark dead cone. It compares data against the HYBRID and LIDO theoretical frameworks, which utilize different descriptions of jet-medium interactions (strongly coupled vs. perturbative transport).

Results

Angular Shift: The angular distribution of late- $k_T$ emissions in PbPb collisions shows a shift toward larger angles (smaller values of $\ln(1/\theta_l)$ ) for prompt $D^0$ jets relative to inclusive jets. This indicates a suppression of collinear splittings for $D^0$ jets, consistent with the expected dead-cone effect.
Vacuum-like Behavior: The ratio of the $D^0$ -to-inclusive angular distributions in PbPb collisions is found to be compatible with the same ratio measured in $pp$ collisions. The double ratio (PbPb/$pp$) is consistent with unity across the measured angular bins.
Theoretical Consistency: The results are consistent with theoretical scenarios where the charm-quark dead-cone angle is smaller than the medium coherence angle ( $\theta_d < \theta_c$ ). In this regime, medium-induced radiation is expected to occur predominantly outside the dead-cone region, leaving the internal structure of the dead cone approximately vacuum-like.
Model Constraints: The data disfavors hypotheses predicting a significant filling of the charm-quark dead cone by medium-induced radiation. Both the HYBRID and LIDO models predict a hierarchy between prompt- $D^0$ and inclusive jets and a double ratio close to unity, which aligns with the measurements, though the current precision offers limited discrimination power between the specific implementations of these models.

Significance
The paper claims that these results provide the first characterization of the dead-cone effect for charm quarks in the presence of a QGP. The observation that the dead-cone region remains approximately vacuum-like suggests that, within the probed kinematic phase space, the charm-quark mass effect is robust against medium modifications. This implies that for charm quarks, the coherence properties of the medium do not completely obscure the mass-dependent suppression of radiation, offering a new constraint on the flavor dependence of jet quenching mechanisms. The study confirms that jet substructure techniques can be effectively used to examine potential modifications of charged quark jets at high energies in heavy ion collisions.

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