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Dijet hadrochemistry: tomography of jet-induced medium excitation and diffusion wake

This paper utilizes a hybrid transport-hydrodynamics model to predict that dijet hadrochemistry in heavy-ion collisions serves as a novel tomographic probe, revealing enhanced hadron ratios and a diffusion wake-induced depletion that confirms jet-induced medium excitation.

Original authors: Cong Yi, Xiang-Yu Wu, Zhong Yang, Pi Duan, Guang-You Qin, Xin-Nian Wang, Wenbin Zhao

Published 2026-10-12

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CC BY 4.0

Original authors: Cong Yi, Xiang-Yu Wu, Zhong Yang, Pi Duan, Guang-You Qin, Xin-Nian Wang, Wenbin Zhao

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 the universe's most violent collisions, where atomic nuclei smash together at nearly the speed of light, a strange and fleeting state of matter is born. This is the quark-gluon plasma, a super-hot soup of fundamental particles that existed only moments after the Big Bang. When high-energy particles, known as partons, are created in these collisions, they must punch their way out of this dense, seething fluid. As they travel, they lose energy and drag the surrounding plasma along with them, creating ripples and disturbances much like a boat moving through water. Scientists have long known that these collisions suppress the number of particles that emerge, but a deeper mystery remains: exactly how does the plasma react to this intrusion, and what does that reaction tell us about the fluid's properties? By studying the specific types of particles that emerge from these crashes, researchers can now peer into the wake left behind by these high-speed travelers, revealing a hidden layer of the plasma's behavior that was previously invisible.

A team of physicists has now taken a detailed look at this phenomenon using a sophisticated computer model that simulates the entire lifecycle of a heavy-ion collision. They focused on events where two jets of particles are produced back-to-back, shooting in opposite directions through the expanding fireball of the quark-gluon plasma. The researchers tracked how these jets interact with the medium, depositing energy and momentum as they go. Their simulations show that the plasma does not just absorb this energy; it responds by forming a complex pattern of waves. Ahead of the moving jet, the plasma compresses into a shockwave, while behind it, a trail of depleted density forms, known as a diffusion wake. This wake acts like a vacuum, pulling matter away and leaving a region where fewer particles are produced than expected. The team's work suggests that this depletion is not just a lack of particles, but a specific chemical signature that can be measured.

The researchers calculated the chemical makeup of the particles associated with these jets, specifically looking at the ratios of protons to pions and kaons to pions. In their simulations of collisions between lead nuclei, they found that the particles emerging directly from the jets were richer in protons and kaons compared to collisions in empty space. This enrichment happens because the jet's energy excites the surrounding plasma, causing thermal particles to merge with the jet's own fragments to form heavier particles. However, the most striking discovery lies in the region between the two jets. When the team subtracted the background noise and looked at the area roughly 90 degrees away from the jet paths, they found a distinct deficit. In this zone, the number of pions, kaons, and protons all dropped below zero relative to the background, confirming the presence of the diffusion wake.

Crucially, the study did not just find a hole in the particle count; it identified what was missing. The ratios of protons to pions and kaons to pions in this depleted region remained high, mirroring the enhanced ratios seen near the jets themselves. This finding is significant because it rules out the idea that the missing particles are simply a result of the jets breaking apart in a standard way. If the depletion were merely an artifact of the jets fragmenting, the missing particles would have a chemical composition similar to empty space. Instead, the fact that the missing particles are disproportionately heavy suggests they originated from the jet-excited medium itself. The plasma, disturbed by the passing jet, had been preparing to create these specific heavy particles, but the diffusion wake swept them away before they could be detected.

The researchers also examined how these effects change depending on the size of the area being measured around the jets. They found that the enhancement of protons and kaons grows stronger as the observation area expands, indicating that the influence of the medium extends far beyond the immediate path of the jet. The subleading jet, which travels a longer distance through the plasma and loses more energy, creates a more pronounced effect than the leading jet. This difference provides a way to map the energy deposition and the subsequent flow of the plasma. By measuring these specific chemical ratios in the depleted zones, scientists now have a new, sensitive tool to probe the internal structure of the quark-gluon plasma. This approach offers a way to visualize the diffusion wake, turning a subtle theoretical prediction into a concrete, observable signature of how matter behaves under the most extreme conditions.

Technical Summary: Dijet Hadrochemistry as a Tomographic Probe of Jet-Induced Medium Excitation

Problem Statement
In relativistic heavy-ion collisions, energetic partons traversing the Quark-Gluon Plasma (QGP) induce a medium response characterized by a Mach-cone-like shock front and a trailing diffusion wake. While the Mach cone has been theoretically studied, its direct experimental observation is complicated by radial flow, density gradients, and event-by-event fluctuations. Conversely, the diffusion wake, which depletes soft hadron production on the backside of the jet, has been difficult to isolate. Previous studies have established that jet-medium interactions modify the chemical composition (hadrochemistry) of jet-associated hadrons, enhancing baryon-to-meson and strangeness ratios via recombination mechanisms. However, the species composition of the depleted yield associated with the diffusion wake remains unexplored. The central problem addressed is whether the chemical signature of this depletion can serve as a distinct, experimentally accessible probe to identify the diffusion wake and constrain QGP properties.

Methodology
The authors employ the CoLBT-hydro framework, a coupled model integrating the linear Boltzmann transport (LBT) model for parton propagation with (3+1)-dimensional viscous hydrodynamics (CLVisc) for the bulk medium evolution.

Simulation Setup: The study simulates inclusive charged jets and dijet events in $p+p$ and 0–10% central $Pb+Pb$ collisions at $\sqrt{s_{NN}} = 5.02$ TeV. Initial parton showers are generated via PYTHIA and propagated through the medium, accounting for both elastic and inelastic scatterings.
Hadronization: A hybrid scheme is utilized, combining Cooper–Frye particlization, quark coalescence (including shower-thermal and shower-shower recombination), and string fragmentation (PYTHIA8). Resonance decays are handled via SMASH.
Analysis Strategy: The authors calculate background-subtracted jet-associated hadron yields ( $Y_i$ ) for pions, kaons, and protons. To isolate the diffusion wake signal in dijet events, they apply mixed-event acceptance corrections and sideband background subtraction to remove contributions from anisotropic flow and multi-parton interactions (MPI).
Key Observable: The study focuses on the $K/\pi$ and $p/\pi$ ratios as functions of the azimuthal angle difference ( $|\Delta\phi|$ ) relative to the leading jet, specifically examining regions of positive yield (jet peaks) and negative yield (diffusion wake).

Key Contributions and Results

Validation of Inclusive Jet Hadrochemistry: The model successfully reproduces preliminary ALICE measurements for inclusive jets, showing enhanced $p/\pi$ and $K/\pi$ ratios in $Pb+Pb$ compared to $p+p$ . The calculations demonstrate that quark coalescence between shower and thermal partons provides a substantial contribution to these enhancements, particularly at intermediate transverse momentum ( $1 < p_T < 4$ GeV).
Identification of the Diffusion Wake: In dijet events, the background-subtracted yields for pions, kaons, and protons become negative near $|\Delta\phi| \simeq \pi/2$ in $Pb+Pb$ collisions, while remaining consistent with zero in $p+p$ . This depletion is attributed to the diffusion wake, where the energy density and outward flow of the medium are reduced.
Hadrochemical Signature of the Wake: Crucially, the ratios of the deficits ( $|Y_p|/|Y_\pi|$ and $|Y_K|/|Y_\pi|$ ) in the negative-yield region remain finite and exceed the corresponding $p+p$ values found around jet peaks. This indicates that the "missing" hadrons originate from the jet-excited medium (via recombination) rather than from vacuum fragmentation. If the depletion were merely an artifact of over-subtracting fragmentation, the ratios would resemble vacuum values.
Cone-Radius Dependence: The study analyzes the dependence of these ratios on the jet cone radius $R$ . The $K/\pi$ enhancement dominates at small $R$ (driven by single thermal strange quarks), while the $p/\pi$ enhancement grows more rapidly and dominates at larger $R$ (driven by the quadratic scaling of baryon coalescence with thermal parton density). This crossing point provides a measure of the angular scale where medium response overtakes the modified jet shower.
Asymmetry Between Jets: The enhancements and depletion ratios are more pronounced around the subleading jet, which traverses a longer path in the medium and deposits more energy, compared to the leading jet.

Significance and Claims
The paper proposes dijet hadrochemistry as a new, experimentally accessible tomographic probe for jet-induced medium excitation. The authors claim that the finite and enhanced ratios of the depletion ( $|Y_p|/|Y_\pi|$ and $|Y_K|/|Y_\pi|$ ) constitute a robust signature of the diffusion wake. Unlike the absolute magnitude of the negative yield, which is sensitive to background subtraction normalization, these ratios characterize the species composition of the missing yield, making them less sensitive to systematic uncertainties. The authors assert that measuring these ratios, alongside statistically significant negative yields, would provide a stringent test of the diffusion wake signal and offer a new means to probe QGP properties, such as the thermalization and diffusion rates of deposited energy. The study suggests that extending these analyses to $\gamma$ - and $Z$ -tagged jets and light-ion collisions could further clarify the origins of these signatures.

Technical Summary: Dijet Hadrochemistry as a Tomographic Probe of Jet-Induced Medium Excitation

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