Modification of jet-energy flow in heavy-ion collisions

The ALICE Collaboration presents the first measurements of jet-energy flow in Pb-Pb collisions at 5.02 TeV, revealing a significant suppression of energy flow at larger radii compared to pp collisions that indicates jet narrowing and is best described by the JEWEL model without recoil, while models incorporating medium response are disfavored by the data.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle accelerator, smashing tiny bits of matter together at nearly the speed of light. When scientists smash heavy lead atoms together, they create a tiny, fleeting drop of "super-hot soup" called the Quark-Gluon Plasma (QGP). This is a state of matter so hot and dense that the usual rules of how particles stick together break down, and they float freely like a liquid.

The ALICE Collaboration (a team of scientists at CERN) wanted to understand how this "soup" affects the particles flying through it. To do this, they used jets.

The Analogy: The Firehose and the Fog

Think of a jet as a high-pressure firehose spraying water (energy) in a very tight, focused stream.

• In a normal vacuum (Proton-Proton collisions): If you turn on a firehose, the water shoots out in a tight, straight line. Most of the water stays right in the center of the stream.
• In the "Super-Hot Soup" (Lead-Lead collisions): Now, imagine that same firehose is spraying through a thick, dense fog. As the water hits the fog, some of it gets knocked sideways, some slows down, and the stream might get wider or change shape.

What They Measured: The "Energy Flow"

The scientists wanted to measure exactly how the water (energy) spreads out from the center of the stream as it travels. They invented a new way to measure this called Jet-Energy Flow.

Instead of just looking at the whole stream, they looked at it in layers, like peeling an onion:

1. They measured the energy in a very narrow tube right in the center of the jet.
2. Then, they measured the energy in a slightly wider tube around that.
3. They calculated the difference between the wide tube and the narrow tube.

This difference tells them: "How much extra energy is there in the outer ring compared to the core?"

The Big Discovery

When they compared the "firehose" in a vacuum (Proton-Proton) to the one in the "soup" (Lead-Lead), they found something surprising:

• The Vacuum Stream: As expected, most of the energy stayed tightly packed in the center. The amount of energy in the outer rings was small.
• The Soup Stream: In the heavy-ion collisions, the energy in the outer rings was significantly lower than expected.

The Metaphor: It's as if the "soup" didn't just knock the water sideways; it actually squeezed the stream tighter. The energy that usually leaks out to the edges of the jet was suppressed or pushed back into the core. The jet became "narrower" in terms of where its energy was concentrated.

The "Recoil" Mystery (The Bouncing Ball)

To understand why this happened, the scientists compared their data to computer simulations (models) that try to predict how particles behave.

• Model A (JEWEL without recoil): This model assumes that when a particle hits the "soup," the soup doesn't push back. It's like a ball hitting a wall that absorbs the impact without bouncing back. This model matched the data perfectly.
• Model B (JEWEL with recoil & JETSCAPE): These models assume that when a particle hits the soup, the soup particles bounce back (recoil) and add their own energy to the mix, potentially making the outer edges of the jet wider or more energetic. These models did not match the data. The data showed the outer edges were less energetic, not more.

Why This Matters

This is the first time scientists have measured this specific "energy flow" difference. It tells us that the "soup" (QGP) interacts with the jet in a very specific way: it seems to suppress the spreading of energy at wider angles, effectively narrowing the jet's profile.

It's like discovering that when you throw a ball through a specific type of thick fog, the fog doesn't just slow the ball down; it actually forces the ball to stay in a tighter, more focused path than it would in clear air. This gives scientists a new, sharper tool to understand the microscopic rules of how energy moves through the most extreme matter in the universe.

In short: The scientists found that in the super-hot soup created by smashing lead atoms, jets of energy get "squeezed" tighter than they do in empty space, and current computer models that assume the soup "bounces back" (recoils) don't explain this behavior.

Technical Summary

Technical Summary: Modification of Jet-Energy Flow in Heavy-Ion Collisions

Problem and Motivation
Jets, collimated sprays of particles originating from hard parton scatterings, serve as essential multi-scale probes for studying the Quark-Gluon Plasma (QGP), a deconfined state of matter formed in heavy-ion collisions. As partons traverse the QGP, they lose energy and undergo structural modifications known as jet quenching. These modifications arise from the interplay between medium-induced emissions (redistributing energy to larger angles) and incoherent energy loss of resolved subjets. While previous measurements by ALICE, ATLAS, and CMS have explored jet and jet-substructure properties, this paper introduces a new observable, the jet-energy flow (JEF), denoted as $\Delta p_T$. This observable characterizes the radial distribution of energy from the jet axis in an infrared- and collinear-safe manner, offering an event-by-event measure of medium-induced parton-shower modifications.

Methodology
The analysis utilizes data from the ALICE experiment at the Large Hadron Collider (LHC).

• Data Sets: The study employs inclusive charged jets from proton-proton (pp) collisions at $\sqrt{s} = 13$ TeV (integrated luminosity $25.81 \pm 0.43$ nb$^{-1}$) and central Pb–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV (0–10% centrality class).
• Jet Reconstruction: Jets are reconstructed using the anti-$k_T$ clustering algorithm with varying resolution parameters ($R$) ranging from 0.05 to 0.4 in steps of 0.05. The analysis focuses on charged jets with $|\eta_{jet}| \le 0.5$.
• Observable Definition: The JEF observable is defined as $\Delta p_T = p_{T, R_{i+1}} - p_{T, R_i}$, where $p_{T, R_i}$ and $p_{T, R_{i+1}}$ are the transverse momenta of matched jets reconstructed with adjacent radii ($R_i$ and $R_{i+1}$) within the same event. This difference probes the energy accumulated in the annular region between the two radii.
• Background Subtraction: In Pb–Pb collisions, a substantial background of uncorrelated soft particles is present. The analysis employs an event-by-event constituent-subtraction method to remove this thermal background, utilizing a free parameter $\Delta R_{max}$ tuned to the jet radius to avoid over- or under-subtraction.
• Unfolding: Detector effects and residual background fluctuations are corrected using an iterative Bayesian unfolding procedure. A four-dimensional response matrix is constructed using Monte Carlo (MC) simulations (PYTHIA 8 for pp; PYTHIA 6 embedded in Pb–Pb data for heavy-ion) to map detector-level distributions to particle-level truth.
• Reference Baseline: To investigate medium-induced modifications, the JEF in Pb–Pb collisions is compared to a reference from pp collisions. Due to statistics, the pp reference uses $\sqrt{s} = 13$ TeV data, while the Pb–Pb data is at 5.02 TeV. A PYTHIA-based ratio of pp collisions at 5 TeV to 13 TeV is used to correct for the beam-energy dependence of the baseline.

Key Contributions
This paper presents the first measurements of the jet-energy flow ($\Delta p_T$) observable in both pp and heavy-ion collisions. Key contributions include:

1. Measurement of $\Delta p_T$: Providing fully corrected distributions for inclusive charged jets in the transverse momentum intervals of 40–60 and 60–80 GeV/c for pp, and 60–80 GeV/c for Pb–Pb.
2. Model Comparisons: Comparing the data against several MC event generators (PYTHIA 8, HERWIG, JEWEL, and JETSCAPE) to test different implementations of parton showers, hadronization, and medium interactions.
3. Quantification of Medium Effects: Establishing a baseline in pp collisions and quantifying the modification in Pb–Pb collisions, specifically focusing on the radial narrowing of the jet energy profile.

Results

• Proton-Proton Collisions: The JEF distributions peak at $\Delta p_T = 0$ and fall steeply toward larger values. The steepness increases smoothly with increasing jet radius, confirming that most energy is concentrated in the jet core. All tested MC models (PYTHIA 8, HERWIG, JEWEL, JETSCAPE) reproduce the pp results well, with only small deviations observed in the high-$\Delta p_T$ tails.
• Pb–Pb Collisions: The JEF distributions in central Pb–Pb collisions are significantly steeper than in pp collisions. A clear suppression of energy flow is observed at larger radii ($\Delta p_T \ge 5$ GeV/c) with a statistical significance of 3.5–4.5$\sigma$. This indicates a "narrowing" of the jet energy flow in the presence of the medium.
• Model Performance in Pb–Pb:
  • JEWEL (without recoil): This model, where medium partons recoiling from jet interactions are discarded, provides a good description of the relative modification (Pb–Pb/pp ratio) observed in the data.
  • JEWEL (with recoil) and JETSCAPE: These models, which include medium response (recoiling partons hadronizing with the jet), show significant deviations. They predict increasing or more constant trends with radius that are disfavored by the data.
  • JETSCAPE: While it reproduces the pp data well, its prediction for the Pb–Pb/pp ratio exhibits a constant trend that is not supported by the measurements.

Significance
The paper claims that these measurements place new constraints on models of jet-medium interactions. The results demonstrate the sensitivity of the jet-energy flow observable to the microscopic mechanisms of parton energy loss. Specifically, the data favors models where the energy transferred to the medium (recoil) does not contribute to the jet structure in the measured kinematic range, or suggests that the redistribution of energy to large angles is not as pronounced in the high-$p_T$ core as predicted by models including full medium response. The authors note that while the current analysis is limited to smaller jet radii in Pb–Pb due to background fluctuations, extending these measurements to larger radii would be valuable to assess potential energy redistribution at large angles, a feature observed in other jet-shape measurements. The study establishes a robust baseline for future investigations into the transport properties of the QGP.