Measurement of $π^0$-hadron correlations relative to the event plane in semicentral Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

In semicentral Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, ALICE measurements of $\pi^0$-hadron correlations relative to the event plane show no significant dependence, a result that aligns with JEWEL model predictions but suggests the potential existence of energy-loss mechanisms beyond simple path-length dependence.

The Gist

The Big Picture: The Ultimate "Squish"

Imagine you have two giant, heavy balls of play-dough (lead nuclei). You smash them together at nearly the speed of light. When they collide, they don't just bounce off; they melt into a super-hot, super-dense soup for a tiny fraction of a second.

Scientists call this soup the Quark-Gluon Plasma (QGP). It's the state of matter that existed microseconds after the Big Bang. In this soup, the tiny particles that usually make up protons and neutrons (quarks and gluons) are free to swim around, but they are packed so tightly that it's like trying to run through a crowded mosh pit.

The Experiment: The "Flashlight" and the "Crowd"

In this experiment, the ALICE team at CERN's Large Hadron Collider (LHC) wanted to see how hard it is to swim through this mosh pit.

1. The Trigger (The Flashlight): They waited for a specific event: a high-energy particle called a neutral pion ($\pi^0$) to be created. Think of this pion as a bright, high-speed flashlight beam shooting out of the collision. Because it's so energetic, it's likely to punch straight through the soup.
2. The Target (The Crowd): As this "flashlight" flies through the soup, it leaves a trail. The scientists looked at the other particles (charged hadrons) that were knocked out of the soup or created by the flashlight's path.
3. The Angle (The Shape of the Room): The collision isn't perfectly round; it's more like a football or an almond shape.
   • In-plane: This is the "short way" across the almond. The path is shorter.
   • Out-of-plane: This is the "long way" across the almond. The path is longer.

The scientists wanted to know: Does the flashlight lose more energy if it shoots through the long, crowded part of the room compared to the short, less crowded part?

The Method: Filtering the Noise

The problem is that the "mosh pit" (the soup) is incredibly noisy. When the balls collide, millions of particles are created just from the soup itself, not from the flashlight. It's like trying to hear a single person whispering in a stadium full of screaming fans.

To fix this, the scientists used a clever trick called the Reaction Plane Fit (RPF):

• They measured the "whispers" (the signal) in three different directions: straight across the short way, straight across the long way, and in between.
• They mathematically modeled the "screaming crowd" (the background noise) based on how the soup naturally flows.
• They subtracted the crowd's noise from their data. What was left was the true signal of the flashlight interacting with the soup.

The Results: A Surprise in the Low-Speed Zone

After doing the math, they found two very interesting things:

1. The High-Speed Zone (Fast Particles):
When they looked at the particles moving very fast (above 3 GeV/c), the results were boring. The flashlight lost the same amount of energy whether it went through the short path or the long path. The soup didn't seem to care about the direction.

2. The Low-Speed Zone (Slower Particles):
Here is where it got exciting. When they looked at the slower particles (around 2 GeV/c), they saw a suppression.

• The Analogy: Imagine you are walking through a crowd. If you walk the "long way" (out-of-plane), you have to push through more people. If you walk the "short way" (in-plane), you have fewer people to push through.
• The Finding: The scientists found that the slower particles were significantly "thinner" (fewer of them) when the flashlight shot through the long, crowded path compared to the short path. It's as if the soup "ate" more of the slow particles when they had a longer journey.

The Theory Check: Did the Computer Get It Right?

The scientists ran their data through a supercomputer model called JEWEL. This model simulates how jets (the flashlight beams) lose energy in the soup.

• The Prediction: JEWEL said, "We don't think the direction matters much. The energy loss should be roughly the same in all directions."
• The Reality: The real data disagreed with the computer. The real soup was much more effective at stopping the slow particles on the long path than the computer predicted.

The Conclusion: There's More to the Story

The paper concludes that while our current theories (like JEWEL) explain what happens to fast particles, they are missing something important about how the soup interacts with slower particles.

The Takeaway:
The "Quark-Gluon Plasma" isn't just a simple wall that slows things down based on distance. There seems to be a more complex mechanism at play, perhaps involving how the soup "recoils" or pushes back against the particles. It suggests that the physics of this primordial soup is more intricate and "sticky" than our current maps of the universe allow for.

In short: We smashed heavy balls together, shone a light through the resulting soup, and realized that the soup is much better at stopping slow swimmers on the long path than our best computer models predicted. This means we need to rewrite the rulebook on how energy is lost in the early universe.

Technical Summary

1. Problem and Motivation

The primary goal of this research is to investigate jet quenching—the energy loss of high-energy partons (quarks and gluons) as they traverse the Quark-Gluon Plasma (QGP) created in heavy-ion collisions.

• Path-Length Dependence: Theoretical models predict that the amount of energy lost by a parton depends on the path length it travels through the medium. In non-central (semicentral) collisions, the QGP has an almond-shaped geometry. Consequently, partons emitted in-plane (parallel to the reaction plane) travel a shorter distance through the medium than those emitted out-of-plane (perpendicular to the reaction plane).
• The Challenge: While jet quenching is well-established, measuring its specific dependence on the emission angle relative to the event plane is difficult. Standard inclusive jet measurements average over all angles. Furthermore, isolating the jet signal from the massive combinatorial background of the underlying event (flow) is challenging, particularly for associated particles with lower transverse momentum ($p_T$).
• Specific Gap: Previous measurements often used charged hadrons as triggers, which introduces biases due to the fragmentation process. This study uses neutral pions ($\pi^0$) as triggers to probe the initial hard scattering more directly and examines the correlation with associated charged particles relative to the 2nd-order event plane ($\Psi_2$).

2. Methodology

The analysis utilizes data from the ALICE experiment at the LHC, collected during 2015 and 2018 runs of Pb–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV.

Experimental Setup and Selection

• Collision Centrality: Semicentral collisions (30–50% centrality) were selected to ensure significant spatial anisotropy.
• Trigger Particle ($\pi^0$): Neutral pions were reconstructed via their decay into two photons ($\pi^0 \to \gamma\gamma$) using the Electromagnetic Calorimeter (EMCal).
  • Trigger $p_T$ range: $11 < p_T(\pi^0) < 14$ GeV/c.
  • Photons were identified with an energy threshold of 2 GeV, and clusters matching charged tracks were rejected.
• Associated Particles: Charged particles were reconstructed using the Inner Tracking System (ITS) and Time Projection Chamber (TPC) within $|\eta| < 0.8$.
• Event Plane Determination: The 2nd-order event plane ($\Psi_2$) was determined using the V0 detectors (forward scintillators). The $\pi^0$ trigger orientation was categorized as in-plane, mid-plane, or out-of-plane based on its azimuthal angle relative to $\Psi_2$.

Analysis Techniques

1. Correlation Measurement: The angular correlation between the $\pi^0$ trigger and associated charged particles was measured in terms of azimuthal angle difference ($\Delta\phi$) and pseudorapidity difference ($\Delta\eta$).
2. Background Subtraction (Event Mixing): To correct for detector acceptance and efficiency, a mixed-event technique was employed, pairing triggers and associates from different events with similar centrality and vertex positions.
3. Combinatorial Background Removal: The $\pi^0$ signal was extracted from the diphoton invariant mass spectrum ($M_{\gamma\gamma}$) by fitting the peak (Gaussian) over a polynomial background. Sideband regions were used to estimate and subtract the combinatorial background contribution to the correlations.
4. Anisotropic Flow Subtraction (RPF): A critical step involved removing the background modulation caused by the collective anisotropic flow of the medium. The Reaction Plane Fit (RPF) method was used. This involves simultaneously fitting the flow coefficients ($v_n$) for both the trigger and associated particles to model the background distribution in $\Delta\phi$, allowing for the extraction of the pure jet-induced correlation yield.
5. Yield Calculation: The per-trigger yields for the near-side ($\Delta\phi \approx 0$) and away-side ($\Delta\phi \approx \pi$) were calculated by integrating the background-subtracted correlation functions.

3. Key Contributions

• Event-Plane Differential Measurement: This work provides a high-precision measurement of $\pi^0$-hadron correlations differentially in the trigger's orientation relative to the event plane, specifically isolating the path-length dependence.
• Advanced Background Handling: The application of the RPF method to $\pi^0$-triggered correlations allows for a more robust subtraction of flow background compared to traditional Zero-Yield-At-Minimum (ZYAM) methods, particularly in the low-$p_T$ region of associated particles.
• Model Comparison: The results are directly compared with the JEWEL (Jet Evolution With Energy Loss) model, which simulates jet-medium interactions including medium recoils.

4. Results

• Yield Spectra: The per-trigger yields for both near-side and away-side correlations follow a steeply falling spectrum as a function of associated particle $p_T$ ($p_T^{assoc}$).
• Event-Plane Dependence:
  • High $p_T$ ($> 3$ GeV/c): No significant difference was observed between in-plane and out-of-plane yields within uncertainties. The ratio of out-of-plane to in-plane yields is consistent with unity.
  • Low $p_T$ ($1.5 - 2.5$ GeV/c): A suppression of the associated charged-particle yields was observed for out-of-plane triggers compared to in-plane triggers. The ratio of out-of-plane/in-plane yields dropped below unity in this region for both near- and away-side peaks.
• Comparison with JEWEL:
  • The JEWEL model (both with and without medium recoils) predicts no significant modification of the yields based on the event plane orientation; it predicts a ratio close to unity across all $p_T$.
  • The experimental data deviates from the JEWEL prediction specifically in the low-$p_T$ region ($1.5 - 2.5$ GeV/c), where the data shows a stronger suppression than the model.

5. Significance and Conclusion

• Evidence of Path-Length Dependence: The observed suppression of out-of-plane yields at low $p_T$ suggests that partons traveling longer paths through the medium (out-of-plane) lose more energy, leading to a depletion of lower-momentum associated particles. This supports the hypothesis of path-length dependent energy loss.
• Limitations of Current Models: The discrepancy between the data and the JEWEL model indicates that current theoretical implementations of jet quenching may be missing significant physical mechanisms. Specifically, the model's reliance on path-length dependence alone (or its specific implementation of medium response) is insufficient to explain the strong suppression seen at low $p_T$.
• Future Directions: The authors suggest that the observed behavior may point to additional energy-loss mechanisms beyond simple path-length scaling. Future measurements using prompt photons as triggers (which are less susceptible to jet-selection biases) and more comprehensive event-by-event hydrodynamic simulations are recommended to further disentangle these effects.

In summary, this paper provides compelling evidence that jet quenching in the QGP is sensitive to the geometry of the collision, with a distinct suppression of low-momentum particles for jets traversing the longest path lengths, a phenomenon not fully captured by current state-of-the-art Monte Carlo models like JEWEL.