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Medium-induced modification of azimuthal correlations of electrons from heavy-flavor hadron decays with charged particles in Pb--Pb collisions at $\sqrt{s_{\rm{NN}} = 5.02}$ TeV

This paper reports on the azimuthal correlations between electrons from heavy-flavor hadron decays and associated charged particles in Pb–Pb collisions at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV, revealing that the nuclear modification factor ( $I_{\rm{AA}}$ ) for these heavy-flavor triggers is consistent with that of light-flavor and strange-particle triggers within uncertainties.

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. This state is known as the quark-gluon plasma, a seething, super-hot fluid where the fundamental building blocks of nature—quarks and gluons—roam freely instead of being bound tightly inside protons and neutrons. To recreate this fleeting moment of cosmic history, scientists smash heavy atomic nuclei together at speeds approaching the speed of light. When these collisions occur, they generate temperatures trillions of degrees higher than the center of the sun, melting the nuclear matter into this exotic plasma. The central mystery of this field is how energy moves through such a dense medium. When a high-energy particle is created in the collision, it must punch its way out of this thick fluid, losing energy and changing its path along the way. By studying how these particles emerge, physicists can map the properties of the plasma itself, learning how it resists, absorbs, and redirects the energy of the particles passing through it.

A researcher using the ALICE detector at the Large Hadron Collider has taken a fresh look at this process by focusing on a specific type of particle: electrons that come from the decay of heavy-flavor hadrons. These heavy-flavor particles are born from the heaviest quarks, known as charm and beauty quarks, which are produced in the violent initial impact of the collision. Because these heavy quarks are so massive, they are expected to interact with the surrounding plasma differently than lighter particles, perhaps losing less energy or carving a different path through the fluid. The researcher wanted to see if this difference showed up in the way these heavy quarks and their decay products are distributed around the collision point. Specifically, they looked at the angles between the heavy-flavor electrons and other charged particles flying out of the same collision. In a vacuum, these particles would fly out in pairs, back-to-back, like two skaters pushing off from each other. But inside the hot plasma, this pattern should be distorted.

The study analyzed millions of collisions between lead nuclei, focusing on the most violent, head-on crashes as well as slightly less central ones. The researcher selected electrons with specific high energies and tracked the charged particles that appeared alongside them. They measured the distribution of these particles to see if the expected back-to-back pattern was still there or if it had been washed out or shifted by the medium. The results revealed a clear sign that the plasma is indeed modifying the behavior of these heavy particles. On the side opposite to the heavy electron, where a partner particle should have appeared, the researcher found a significant drop in the number of high-energy particles in the most central collisions. This suppression suggests that the partner particle, which is traveling through the densest part of the plasma, is losing a substantial amount of energy before it can escape. It is as if the particle is running through a thick fog that slows it down and steals its momentum.

However, the story on the same side as the heavy electron was more subtle. The researcher noticed a slight hint that the number of lower-energy particles on this side might be increased, though the evidence was not strong enough to be considered a definitive discovery. This potential increase could mean that the energy lost by the heavy quark is not simply disappearing but is being transferred to the surrounding plasma, creating a ripple effect that produces more low-energy particles nearby. When the researcher compared these findings to similar measurements made with lighter particles, they found that the overall pattern of energy loss and redistribution was surprisingly similar. This suggests that, within the limits of their current data, the heavy quarks are not behaving in a radically different way from lighter particles regarding how they lose energy to the medium. The differences expected due to the mass of the quarks were not large enough to be seen clearly with the current precision.

The study also looked at how the energy of the initial heavy quark influenced the outcome. They compared electrons from lower-energy heavy quarks with those from higher-energy ones. While the higher-energy quarks produced more associated particles overall, the way the plasma modified their paths remained consistent across the different energy levels. This indicates that the mechanism by which the plasma drains energy from these particles does not change drastically with the initial speed of the quark in the range they studied. The researcher concluded that while they have confirmed that heavy-flavor particles are indeed affected by the quark-gluon plasma, the specific details of how their mass influences this interaction remain to be pinned down with greater precision. The data provides a solid foundation for future studies, suggesting that as detectors become more sensitive and collect more data, scientists will be able to distinguish the unique fingerprints of heavy quarks moving through the early universe's most extreme environment.

Technical Summary: Medium-induced modification of azimuthal correlations of electrons from heavy-flavor hadron decays with charged particles in Pb–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV

Problem and Motivation
In ultrarelativistic heavy-ion collisions, the formation of a Quark-Gluon Plasma (QGP) leads to parton energy loss, a phenomenon known as "jet quenching." While jet quenching has been extensively studied using light-flavor hadrons, the dependence of parton energy loss on quark mass and color charge remains a critical area of investigation. Theoretical models predict that heavy quarks (charm and beauty) lose less energy than light quarks due to the "dead-cone" effect and mass-dependent interactions. However, direct measurements of heavy-flavor jet substructure and fragmentation modifications in the QGP are challenging. This paper addresses the need to characterize heavy-flavor jet properties by measuring the azimuthal correlations between electrons from heavy-flavor hadron decays and associated charged particles in Pb–Pb collisions. The study aims to quantify medium-induced modifications to the near-side (NS) and away-side (AS) correlation peaks and compare them with light-flavor triggers to investigate flavor dependence.

Methodology
The analysis utilizes data from Pb–Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.02$ TeV, recorded by the ALICE detector during LHC Run 2. The dataset corresponds to integrated luminosities of approximately 54 $\mu$ b $^{-1}$ for the 0–10% (most central) and 41 $\mu$ b $^{-1}$ for the 30–50% (semicentral) centrality classes.

Trigger and Associated Particles: The trigger particles are electrons (and positrons) from heavy-flavor hadron decays with transverse momentum ( $p_T$ ) in the range $4 < p_T^e < 16$ GeV/c. Associated charged primary particles are selected within $1 < p_T^{assoc} < 7$ GeV/c and a pseudorapidity difference $|\Delta\eta| < 1$ relative to the trigger.
Electron Identification: Electrons are identified using a combination of specific energy loss ($dE/dx$) in the Time Projection Chamber (TPC) and the energy-to-momentum ratio ( $E/p$ ) in the Electromagnetic Calorimeter (EMCal). Hadron contamination is estimated and subtracted using a sideband method on the $n\sigma_{TPC}$ distribution.
Background Subtraction: Electrons from background sources (Dalitz decays of neutral mesons and photon conversions) are subtracted using an unlike-sign (ULS) and like-sign (LS) pair technique based on invariant mass reconstruction.
Correlation Analysis: Two-dimensional azimuthal correlation distributions $C(\Delta\eta, \Delta\phi)$ are constructed and corrected for detector acceptance and inhomogeneities using a mixed-event technique. The distributions are integrated over $|\Delta\eta| < 1$ to obtain one-dimensional $\Delta\phi$ distributions.
Baseline and Peak Extraction: The correlation distributions are fitted with a baseline function incorporating anisotropic flow coefficients ( $v_2, v_3$ ) and a von Mises function to describe the NS peak. The per-trigger nuclear modification factor ( $I_{AA}$ ) is calculated as the ratio of the per-trigger associated yields in Pb–Pb collisions to those in pp collisions (at $\sqrt{s} = 5.02$ TeV) for both NS and AS peaks.

Key Contributions and Results
The paper reports the first measurement of jet-like azimuthal correlations triggered by heavy-flavor decay electrons in Pb–Pb collisions at LHC energies, providing access to heavy-flavor jet observables.

Correlation Distributions: In central (0–10%) collisions, the NS peak shape in Pb–Pb follows a trend similar to pp collisions, narrowing as $p_T^{assoc}$ increases. The AS peak shows a tendency toward suppression relative to pp collisions for $p_T^{assoc} > 3$ GeV/c.
Nuclear Modification Factor ( $I_{AA}$ ):
- Near-Side (NS): In 0–10% central collisions, the NS $I_{AA}$ shows a hint of enhancement for $p_T^{assoc} < 3$ GeV/c (significance of 1.27 $\sigma$ ), potentially indicating medium excitation or $k_T$ broadening. For $p_T^{assoc} > 3$ GeV/c, the NS $I_{AA}$ is consistent with unity.
- Away-Side (AS): The AS $I_{AA}$ is consistent with unity at low $p_T^{assoc}$ but shows a significant suppression ( $\sim$ 0.5) for $p_T^{assoc} > 4$ GeV/c in central collisions (significance of 2.5 $\sigma$ ). This suppression is attributed to path-length-dependent energy loss of the recoiling parton.
Trigger $p_T$ Dependence: The analysis compares trigger electrons in two intervals: $4 < p_T^e < 7$ GeV/c (dominated by charm) and $7 < p_T^e < 16$ GeV/c (dominated by beauty). The $I_{AA}$ values for both NS and AS peaks are consistent between the two intervals within uncertainties, suggesting that the modification of the parton shower does not strongly depend on the initiating parton's mass or energy in this kinematic regime.
Flavor Comparison: The $I_{AA}$ for heavy-flavor electron triggers is compared with results from light-flavor (charged hadrons) and strange-particle ( $K_S^0$ ) triggers. The results are consistent within uncertainties, indicating no clear mass dependence in the modification of associated yields, although kinematic differences due to decay steps and parton-to-hadron scaling are acknowledged.

Significance
The paper claims that these results provide evidence for medium-induced modifications to heavy-flavor jet fragmentation in Pb–Pb collisions. The observed enhancement of low- $p_T$ associated particles on the near-side and the suppression of high- $p_T$ particles on the away-side are consistent with the redistribution of energy lost by the parton into the medium and the modification of the parton shower. The consistency of $I_{AA}$ across different trigger flavors (heavy vs. light) within current uncertainties suggests that the modification of associated yields may not exhibit a strong quark-mass dependence in the measured kinematic range, though the author notes that kinematic differences between decay products and direct hadrons must be considered. The study establishes a baseline for future, more precise measurements with the upgraded ALICE detector during LHC Run 3, which will allow for reduced uncertainties and the use of direct heavy-flavor hadron triggers (e.g., D mesons) to further elucidate heavy-quark transport mechanisms in the QGP.

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