Evidence of medium response to hard probes using correlations of Z bosons with hadrons in heavy ion collisions

This paper presents the first evidence of medium response to hard probes in heavy ion collisions by observing significant modifications in the distributions of low transverse momentum hadrons recoiling against Z bosons in lead-lead collisions, consistent with theoretical predictions of a hydrodynamic wake generated by energy depletion in the quark-gluon plasma.

The Gist

The Big Picture: A Cosmic "Splash"

Imagine you are at a crowded, chaotic party (the Quark-Gluon Plasma, or QGP). This isn't a normal party; it's a super-hot soup of tiny particles (quarks and gluons) that usually stick together but are currently floating freely because the temperature is so high. This happens when scientists smash heavy lead atoms together at nearly the speed of light.

Usually, when a fast particle (a "hard probe") tries to run through this party, it gets bumped around, loses energy, and slows down. This is called "jet quenching." We've known this happens for a long time, but we didn't fully understand how the party crowd reacts to the runner. Does the crowd just get tired? Do they push back? Do they leave a hole behind?

The Experiment: The "Z-Boson" Flashlight

To study this, the CMS team at CERN used a very special tool: the Z boson.

Think of the Z boson as a ghostly flashlight.

• Most particles in the party (the QGP) interact with everything. If you throw a ball into the crowd, it hits people and slows down.
• The Z boson, however, is like a ghost. It doesn't interact with the crowd at all. It flies straight through the party without bumping into anyone.

When a Z boson is created, it is almost always paired with a high-speed particle (a "jet") flying in the exact opposite direction. Because the Z boson is a ghost, we know exactly how much energy the other particle (the jet) started with. It's like knowing the exact speed of a car before it hits a wall, just by looking at the ghostly car that drove away safely.

The Discovery: The "Wake" and the "Hole"

The scientists looked at what happened to the low-speed particles (the "soft" hadrons) around the Z boson. They found two surprising things:

1. The "Hole" in the Crowd (Negative Wake)
Usually, if you run through a crowd, you might expect people to bunch up behind you. But here, the scientists found a dip or a hole in the number of particles right next to the Z boson.

• The Analogy: Imagine a speedboat cutting through a calm lake. As the boat moves forward, it pushes the water aside and forward, leaving a dip or a hollow space directly behind the boat.
• The Physics: The high-speed particle (the jet) pushed the QGP medium forward, "depleting" the energy in the region right next to the Z boson. This created a "negative wake" or a "medium hole."

2. The "Splash" Behind the Runner (Positive Wake)
On the opposite side (where the jet was flying), they saw an excess of particles.

• The Analogy: This is like the wake behind the speedboat. As the boat pushes water forward, that water piles up and splashes behind it.
• The Physics: The energy lost by the jet didn't just disappear; it was transferred to the medium, causing a "splash" of new particles to form in the jet's path.

Why This Matters

Before this paper, we knew the jet got "quenched" (slowed down). But we didn't have clear proof of how the medium responded.

• Old Theory: "The jet loses energy, and that's it."
• New Evidence: "The jet loses energy, pushes the medium forward, creates a hole behind the Z boson, and causes a splash on the jet side."

This is the first time we have seen direct evidence of this "hydrodynamic wake" in heavy ion collisions. It proves that the Quark-Gluon Plasma behaves like a fluid that can be pushed, deformed, and pushed back, rather than just a static wall.

The "Recipe" Check

The scientists compared their real data against several computer simulations (theoretical recipes):

• Some recipes said, "The medium doesn't react." (These failed).
• Some recipes said, "The medium reacts, but only in a simple way." (These were okay, but not perfect).
• The recipes that included fluid dynamics (like the boat wake analogy) matched the data best.

The Bottom Line

This paper is like taking a high-speed photo of a speedboat cutting through water for the first time. We finally see the hole the boat makes in front of it and the splash it leaves behind. It confirms that the universe's most extreme state of matter (the QGP) acts like a fluid that responds dynamically to the particles moving through it.

In short: The Z boson acted as a perfect reference point, allowing scientists to see that when a high-speed particle tears through the quark-gluon plasma, it leaves a distinct "footprint" of a depleted zone and a splash of energy, proving the plasma is a responsive, fluid-like medium.

Technical Summary

1. Problem and Motivation

The primary goal of this research is to investigate the microscopic mechanisms of jet quenching and the medium response of the Quark-Gluon Plasma (QGP) created in relativistic heavy-ion collisions.

• The Challenge: While jet quenching (energy loss of high-momentum partons traversing the QGP) is well-established, the specific details of how the medium reacts to this energy loss remain elusive. Theoretical models distinguish between:
  • Energy loss mechanisms: Collisional loss and medium-induced radiation (pQCD) or drag forces (AdS/CFT).
  • Medium response: The recoil of medium constituents and the creation of hydrodynamic wakes (positive wakes behind the jet, negative wakes or "holes" on the opposite side).
• The Difficulty: Separating the signal of the recoiling parton from the medium's response is difficult because both effects occur at similar energy scales (hundreds of MeV).
• The Solution: The authors utilize Z bosons as a "calibrated" probe. Unlike jets, Z bosons do not interact strongly with the QGP. Therefore, the transverse momentum ($p_T$) of the Z boson accurately reflects the initial energy of the recoiling parton before it enters the medium. By studying hadrons produced in the direction of the Z boson (the "Z-side"), the analysis isolates the medium's response to the parton moving in the opposite direction, avoiding the complexities of the jet-side where the parton shower and medium modifications are entangled.

2. Methodology

The analysis utilizes data collected by the CMS detector at the LHC.

• Datasets:
  • Lead-Lead (PbPb) collisions: $\sqrt{s_{NN}} = 5.02$ TeV, recorded in 2018, integrated luminosity $1.67 \text{ nb}^{-1}$.
  • Proton-Proton (pp) collisions: $\sqrt{s} = 5.02$ TeV, recorded in 2017, integrated luminosity $301 \text{ pb}^{-1}$.
• Event Selection:
  • Z Boson: Reconstructed via the dimuon decay channel ($Z \to \mu^+\mu^-$) with $40 < p_T^Z < 350$ GeV and invariant mass $60-120$ GeV.
  • Centrality: PbPb events are categorized into three centrality intervals: 0–30% (most central), 30–50%, and 50–90% (peripheral).
  • Associated Hadrons: Charged hadrons with $p_T > 1$ GeV and $|\eta| < 2.4$ are correlated with the Z boson. Muons from the Z decay are excluded via a cone cut ($\Delta R > 0.0025$).
• Observables:
  • The analysis measures the normalized associated yield ($\Delta \langle N_{ch} \rangle$) as a function of:
    • Azimuthal angle difference: $\Delta \phi_{ch,Z} = \phi_Z - \phi_{ch}$.
    • Rapidity difference: $\Delta y_{ch,Z} = y_Z - \eta_{ch}$.
  • Background Subtraction: An event-mixing technique is employed to subtract the underlying event and uncorrelated background, isolating the correlation signal.
• Theoretical Comparisons: Results are compared against several Monte Carlo models:
  • PYQUEN: Radiative/collisional loss without strict local energy-momentum conservation.
  • JEWEL: pQCD-based with options for medium recoil (conserving energy/momentum) or no recoil.
  • HYBRID: AdS/CFT-based strong coupling model predicting hydrodynamic wakes (positive and negative).
  • CO-LBT: Coupled Linear Boltzmann Transport with hydrodynamics, including "reheating" and diffusion wakes.

3. Key Contributions

This paper presents the first measurement of Z boson-hadron two-particle angular correlations in bins of charged-hadron $p_T$ and event centrality. Its specific contributions include:

1. Isolation of the Z-side: By focusing on the region around the Z boson ($\Delta \phi \sim 0$), the study probes the "negative wake" or "medium hole" created by the parton recoiling opposite to the Z, a region previously difficult to access due to jet-side contamination.
2. Differential Analysis: The study breaks down correlations by hadron $p_T$ ($1-2$, $2-4$, and $4-10$ GeV) and centrality, allowing for a nuanced view of how soft and hard particles respond to the medium.
3. Model Discrimination: It provides a rigorous testbed for theoretical models, specifically distinguishing between those that include medium recoil/hydrodynamic response and those that do not.

4. Results

The analysis reveals significant deviations between PbPb and pp collisions, particularly in central events (0–30% centrality):

• Low-$p_T$ Hadrons ($1 < p_T < 2$ GeV):
  • Azimuthal Distribution ($\Delta \phi$): A significant dip (negative yield) is observed at $\Delta \phi \sim 0$ (Z-side) in PbPb compared to pp. This indicates a depletion of particles in the direction of the Z boson.
  • Rapidity Distribution ($\Delta y$): A similar dip is observed at $\Delta y \sim 0$.
  • Significance: The deviation from the pp reference has a statistical significance $> 3\sigma$.
  • Interpretation: This dip is interpreted as a negative hydrodynamic wake or "medium hole," where the high-energy parton pushes the QGP forward, leaving a depleted region behind it on the Z-side.
• High-$p_T$ Hadrons ($4 < p_T < 10$ GeV):
  • A suppression of yield is observed on the jet side ($\Delta \phi \sim \pi$), consistent with standard jet quenching.
  • On the Z-side, the yield is slightly enhanced compared to pp, potentially due to large-angle scattering or momentum broadening.
• Model Comparisons:
  • Models without medium response (e.g., PYQUEN, JEWEL without recoil) fail to describe the dip structure at small $\Delta \phi$ and $\Delta y$.
  • Models with medium response (HYBRID with wake, JEWEL with recoil, CO-LBT) provide a much better description of the data.
  • The HYBRID model (including both positive and negative wakes) and CO-LBT (including diffusion wakes) describe the central PbPb data well, particularly the negative yield on the Z-side.

5. Significance

This work provides the first experimental evidence of a probe-induced energy depletion and the resulting hydrodynamic response of the QGP.

• Confirmation of Hydrodynamic Wake: The observation of a negative yield (dip) on the Z-side confirms theoretical predictions that a hard parton traversing the QGP creates a "hole" or negative wake in the medium, distinct from the positive wake (shockwave) behind the jet.
• Validation of Medium Response: The results demonstrate that medium recoil and hydrodynamic effects are essential components of jet quenching. Models ignoring these effects cannot reproduce the data.
• New Observable: The Z-hadron correlation in the Z-side region is established as a powerful new observable for studying the transport properties of the QGP, specifically its ability to respond to energy deposition and momentum transfer.

In summary, the CMS Collaboration has successfully used Z bosons as a clean trigger to reveal the "shadow" cast by a recoiling parton in the QGP, providing direct evidence of the medium's hydrodynamic response to hard probes.