Observation of the jet diffusion wake using dijets in heavy ion collisions

Using dijet-hadron correlations in lead-lead and proton-proton collisions at 5.02 TeV, the CMS collaboration firmly established the existence of a jet diffusion wake—a depletion of particles opposite to the jet direction—with a statistical significance exceeding 5 standard deviations, providing new insights into quark-gluon plasma properties.

The Gist

Imagine you are at a crowded, chaotic concert. The crowd represents the Quark-Gluon Plasma (QGP), a super-hot, super-dense "soup" of subatomic particles that existed just moments after the Big Bang. Scientists at CERN recreate this soup by smashing heavy lead atoms together at nearly the speed of light.

Now, imagine a VIP guest (a high-energy particle called a quark or gluon) tries to run through this mosh pit. As they sprint through the crowd, they bump into people, knock them over, and lose energy. This is called jet quenching. Usually, scientists only see the VIP getting tired and slowing down.

But this new paper from the CMS collaboration at CERN is about something else: what happens to the crowd behind the VIP.

The "Jet Diffusion Wake"

Think of the VIP running through the crowd like a speedboat cutting through a lake.

• The Boat: The high-energy particle (the jet).
• The Water: The Quark-Gluon Plasma.
• The Wake: When a boat moves fast, it leaves a trail of disturbed water behind it. In physics, this is called a "wake."

However, there's a twist. In this specific experiment, the scientists discovered a "Diffusion Wake."
Imagine the VIP running so fast that they don't just push people aside; they actually suck the crowd away from the path they just left. It's like a vacuum cleaner moving through a room, leaving a temporary empty space behind it. The paper confirms that after the jet passes, there is a distinct depletion (a lack of particles) in the direction opposite to where the jet was going.

How Did They Find It? (The "Two-Runner" Trick)

Detecting this empty space is incredibly hard because the VIP also leaves a trail of debris in front of them (the jet itself), which hides the empty space behind. It's like trying to see the empty space behind a runner while they are still covered in a cloud of dust.

To solve this, the scientists used a clever trick involving dijets (two jets created at the same time, flying in opposite directions).

1. The Setup: They looked at pairs of jets flying back-to-back.
2. The Twist: They separated the two jets by a large distance in "pseudorapidity" (a way of measuring angle). Imagine one runner going North and the other going South, but one is far ahead of the other.
3. The Comparison:
   • Scenario A (Small Gap): When the two runners are close together, their "dust clouds" and "empty spaces" overlap and cancel each other out. You can't see the wake.
   • Scenario B (Large Gap): When the runners are far apart, the "dust" from the first runner doesn't interfere with the "empty space" behind the second runner.

By comparing the "close runners" (where the signal is hidden) with the "far runners" (where the signal is exposed), they were able to subtract the noise and clearly see the empty space (the diffusion wake) left behind the jet.

The Results

• The Evidence: They found a statistically significant "hole" in the particle count behind the jet. It was so clear that the chance of it being a fluke is less than 1 in 3.5 million (5 standard deviations).
• The Analogy: It's like looking at a photo of a crowded room, then looking at a photo of the same room after a vacuum cleaner passed through. The paper proves that the vacuum cleaner (the jet) didn't just push people aside; it actually left a temporary, empty trail behind it.

Why Does This Matter?

For years, scientists have been trying to understand how the "soup" of the early universe behaves.

• The "Mach Cone" vs. The "Wake": Scientists previously looked for a "Mach cone" (a shockwave like a sonic boom) in front of the jet. That has been hard to prove because it gets mixed up with other effects.
• The New Insight: This "diffusion wake" is a different kind of signal. It tells us how the medium (the plasma) flows and reacts to energy loss. It's like understanding the viscosity of the soup. Is it like water? Like honey? Or like a super-fluid?

The Bottom Line

This paper is the first time scientists have firmly observed this "diffusion wake" using dijets. It's a major step forward in understanding how energy moves through the hottest, densest matter in the universe.

In simple terms: They finally proved that when a high-speed particle zooms through the primordial soup of the universe, it leaves a distinct, empty trail behind it, just like a boat leaves a wake in the water. This helps us understand the "rules of the road" for the universe's most extreme conditions.

Technical Summary

1. Problem Statement

When high-energy quarks and gluons (partons) traverse the Quark-Gluon Plasma (QGP) created in ultrarelativistic heavy-ion collisions, they lose energy and momentum, a phenomenon known as jet quenching. This energy deposition is expected to induce a collective response in the medium, creating a "jet wake."

• The Challenge: Theoretical models predict two distinct components to this wake: a Mach cone (shockwave) in the direction of the jet and a diffusion wake (a depletion of particles) in the direction opposite to the jet propagation.
• The Obstacle: Historically, experimental attempts to observe these wakes have been inconclusive because the Mach cone and medium-induced modifications to the jet shower itself occur in the same direction as the jet, masking the signal. Furthermore, the diffusion wake is a subtle effect (a depletion) that is difficult to distinguish from background fluctuations.
• Previous Evidence: The CMS experiment previously reported evidence of a diffusion wake using $Z$-boson-hadron correlations (where the $Z$ boson does not interact with the QGP), but a direct observation using dijet events remained elusive due to the complexity of separating jet modifications from medium response.

2. Methodology

The analysis utilizes data from the CMS detector at the CERN Large Hadron Collider (LHC) to isolate the diffusion wake by exploiting the spatial separation of jets in dijet events.

• Data Samples:
  • PbPb Collisions: 0.66 nb$^{-1}$ recorded at $\sqrt{s_{NN}} = 5.02$ TeV (2018).
  • pp Collisions: 299 pb$^{-1}$ recorded at $\sqrt{s_{NN}} = 5.02$ TeV (2017) serving as the vacuum reference.
• Event Selection:
  • Dijets are defined as the two highest $p_T$ back-to-back jets ($\Delta\phi > 5\pi/6$).
  • Leading jet ($jet_1$): $p_T > 130$ GeV, $|\eta| < 1.0$.
  • Subleading jet ($jet_2$): $p_T > 50$ GeV, $|\eta| < 2.0$.
• Key Strategy (Pseudorapidity Separation):
  • The analysis categorizes dijets based on the pseudorapidity separation ($\Delta\eta_{jet}$) between the leading and subleading jets.
  • Small Gap ($|\Delta\eta_{jet}| < 0.5$): The subleading jet and its associated diffusion wake are spatially close to the leading jet. The wake signal is "hidden" by the large yield of particles from the jet fragmentation itself.
  • Large Gap ($|\Delta\eta_{jet}| > 0.5$): The subleading jet is displaced in $\eta$. Consequently, the diffusion wake associated with the subleading jet appears in a region of $\eta$ where there is no leading jet fragmentation, creating a distinct "dip" or depletion in the particle yield.
• Signal Extraction:
  • The correlation distributions of charged particles relative to the leading jet axis are measured for both small and large $\Delta\eta_{jet}$ samples.
  • The diffusion wake signal is extracted by subtracting the small-gap distribution ($R_{sym}$) from the large-gap distribution ($R_{asym}$). This subtraction removes the symmetric background and isolates the asymmetry caused by the wake.
  • The PbPb results are further compared to the pp reference to isolate medium-induced effects.
• Kinematic Focus: The analysis focuses on charged particles with transverse momentum $1 < p_T < 2$ GeV and $2 < p_T < 4$ GeV, where the diffusion wake effect is theoretically predicted to be strongest.

3. Key Contributions

• First Dijet Observation: This paper presents the first observation of the QGP diffusion wake using dijet events, moving beyond the previous $Z$-boson and photon-jet studies.
• Spatial Separation Technique: It validates a novel methodology using pseudorapidity-separated dijets to spatially disentangle jet-induced medium modifications (the wake) from the jet fragmentation itself.
• Quantitative Measurement: The study provides differential measurements of the wake signal as a function of collision centrality and charged-particle $p_T$.
• Model Constraints: The results provide critical constraints for theoretical models (HYBRID and CoLBT-hydro) regarding jet-medium coupling and energy loss mechanisms.

4. Results

• Observation of Depletion: In central PbPb collisions (0–30% centrality), a pronounced depletion (dip) in charged-particle yield is observed in the region opposite to the subleading jet ($\Delta\eta_{ch, jet1} < 0$) for large $\Delta\eta_{jet}$ selections.
• Statistical Significance: For charged particles in the range $1 < p_T < 2$ GeV in central collisions, the diffusion wake signal deviates from zero with a significance greater than 5 standard deviations ($>5\sigma$).
• Centrality Dependence: The strength of the depletion increases from peripheral (50–80%) to central (0–30%) collisions, consistent with the expectation that the QGP density is higher in central events.
• $p_T$ Dependence: The effect is most prominent at lower $p_T$ ($1 < p_T < 2$ GeV) and diminishes at higher $p_T$ ($2 < p_T < 4$ GeV), aligning with theoretical expectations for the diffusion wake.
• Comparison with Models:
  • PYTHIA (No Medium): Shows no depletion, as expected.
  • HYBRID and CoLBT-hydro (With Medium): Both models predict a depletion but overestimate the magnitude of the observed signal. However, they correctly predict the location (mean position) of the dip.
  • The discrepancy suggests that current models may overestimate the coupling strength or the efficiency of momentum transfer from the jet to the medium in the diffusion wake regime.

5. Significance

• Validation of QGP Dynamics: The firm establishment of the diffusion wake confirms that the QGP responds collectively to the passage of energetic partons, behaving as a fluid that can be "pushed" and "pulled" by traversing jets.
• Refining Transport Coefficients: By providing a clean observable (the depletion) that is distinct from jet quenching suppression, this measurement helps refine the extraction of QGP transport coefficients (such as the jet quenching parameter $\hat{q}$ and shear viscosity $\eta/s$).
• Methodological Advance: The success of the $\Delta\eta$-separation technique offers a new tool for future heavy-ion physics analyses to study medium response without relying on colorless probes (like $Z$ bosons), which have lower cross-sections.
• Theoretical Impact: The observation challenges existing theoretical models to better describe the non-perturbative coupling between jets and the hydrodynamic evolution of the QGP, particularly regarding the magnitude of the diffusion wake.

In conclusion, this work represents a milestone in heavy-ion physics, providing the first robust, data-driven observation of the diffusion wake in dijet events and offering new insights into the hydrodynamic nature of the quark-gluon plasma.