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Energy-energy correlators in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

The ALICE experiment reports the first measurement of the two-point energy-energy correlator (EEC) inside charged-particle jets in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, revealing a modification relative to pp collisions characterized by suppression at small angles and enhancement at large angles, which provides new constraints on cold nuclear matter effects relevant for understanding jet-structure modifications in heavy-ion collisions.

Original authors: ALICE Collaboration

Published 2026-06-17

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: ALICE Collaboration

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

The Big Picture: Smashing Protons and Lead Ions

Imagine the Large Hadron Collider (LHC) as a massive, high-speed racetrack. Usually, scientists crash two protons together (like two tiny marbles). But in this experiment, they crashed a single proton into a heavy lead ion (like crashing a marble into a bowling ball made of 208 smaller marbles stuck together).

When these particles smash, they don't just bounce off; they shatter into a shower of new particles. These particles fly out in tight, focused bundles called jets. Think of a jet like a powerful firehose spraying water, but instead of water, it's spraying a stream of subatomic particles.

The Tool: The "Energy-Energy Correlator" (EEC)

To understand what's happening inside these particle firehoses, the scientists used a special measuring tool called the Energy-Energy Correlator (EEC).

The Analogy: Imagine you are standing in a dark room with a friend who is throwing handfuls of confetti at you.

The Jet: The stream of confetti.
The EEC: A way to measure how the energy of the confetti is spread out. It asks: "If I pick two pieces of confetti, how far apart are they, and how heavy are they?"
- If the confetti is clumped tightly together, the angle between them is small.
- If the confetti is spread out wide, the angle is large.

By measuring this "angle vs. energy" for millions of collisions, the scientists can map out exactly how the firehose sprays its contents. This map tells them about the physics happening inside the jet, from the very first split (perturbative) to the final formation of particles (non-perturbative).

The Experiment: Comparing the "Marble" to the "Bowling Ball"

The scientists wanted to see if the environment changes how the firehose sprays.

The Control Group (pp): They looked at jets from proton-proton collisions. This is the "clean" baseline, like a firehose spraying in a vacuum.
The Test Group (p–Pb): They looked at jets from proton-lead collisions. Here, the proton has to travel through the "crowd" of the lead nucleus before it even starts its firehose. This is like spraying a firehose through a dense forest of trees.

The Discovery: A Slight "Wobble" in the Spray

When they compared the two groups, they found something interesting, but only for the lower-energy jets (those with a specific momentum range of 20–40 GeV/c).

The Result: In the proton-lead collisions, the spray pattern changed slightly compared to the proton-proton collisions.
- At small angles: The spray was suppressed (less energy close to the center).
- At large angles: The spray was enhanced (more energy pushed further out to the sides).

The Metaphor: Imagine a skilled archer shooting an arrow.

In the proton-proton collision, the arrow flies straight and tight.
In the proton-lead collision, the arrow seems to wobble slightly. It loses a tiny bit of speed right at the tip (small angles) but spreads out a bit more at the tail (large angles). It's as if the arrow hit a few invisible leaves on its way out, causing it to fan out slightly.

Crucially: For the higher-energy jets (40–80 GeV/c), there was no difference. The "firehose" was so powerful that the "forest" of the lead nucleus didn't seem to bother it at all.

What Caused This? (The Investigation)

The scientists had to figure out why the spray changed. They tested several theories:

Is it the "Crowd" inside the lead? They checked if the density of particles or the direction of the collision mattered. Result: No. The change happened regardless of how crowded the event was or which way the jet was pointing.
Is it just the "Map" of the lead? They used computer models to see if the lead nucleus simply had a different internal structure (like a different PDF map) that changed the spray. Result: No. The models that only changed the "map" couldn't explain the wobble.
Is it a "Final State" interaction? This suggests that after the jet is formed, it interacts with the "cold" nuclear matter (the lead nucleus) as it tries to escape. The models that included these interactions (like the jet bumping into other particles) came closest to matching the data, though they weren't a perfect fit yet.

The "Peak" Mystery

The paper also looked at a specific "hump" or peak in the data. This peak represents the moment when the chaotic spray of energy settles down into stable particles (hadronization).

They found that this peak happened at the exact same "energy scale" in both the proton-proton and proton-lead collisions.
The Takeaway: Even though the spray pattern changed slightly, the fundamental "moment of birth" for the particles remained the same. It's like the archer's arrow wobbling in the wind, but the moment the arrow leaves the bowstring is identical in both cases.

Summary

This paper is the first time scientists have measured this specific "energy-angle map" (EEC) inside jets created when a proton hits a lead nucleus.

They found that for lower-energy jets, the lead nucleus acts like a gentle breeze that slightly fans out the spray of particles. For higher-energy jets, the breeze is too weak to make a difference. This helps scientists understand how "cold" nuclear matter (matter that isn't a super-hot plasma) affects the behavior of particles, which is a key piece of the puzzle for understanding how the universe works at the smallest scales.

Technical Summary: Energy-Energy Correlators in p–Pb Collisions at $\sqrt{s_{NN}} = 5.02$ TeV

Problem and Motivation
This work addresses the need to disentangle initial-state effects from final-state interactions in jet evolution within a cold nuclear matter (CNM) environment. While jet substructure observables in proton-proton (pp) collisions are well-described by perturbative Quantum Chromodynamics (pQCD) and non-perturbative hadronization, the behavior of jets in proton-nucleus (p–Pb) collisions remains a critical testing ground for cold nuclear matter effects. Previous measurements of jet observables in p–Pb collisions (e.g., jet nuclear modification factor, fragmentation functions) have generally shown small or no modifications relative to pp baselines, suggesting that jets traverse a relatively dilute CNM environment. However, theoretical frameworks propose that soft multiple scattering, transverse-momentum broadening, or interactions with partonic comovers could alter the angular distribution of energy flow within jets. The Energy-Energy Correlator (EEC), an infrared- and collinear-safe observable sensitive to both perturbative splittings and non-perturbative hadronization, offers a precision probe to investigate these potential modifications.

Methodology
The ALICE Collaboration presents the first measurement of the two-point EEC inside charged-particle jets in p–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV, comparing them to a new measurement in pp collisions at the same energy.

Data Samples: The analysis utilizes Minimum Bias (MB) data from LHC Run 2, comprising 870 million pp events ( $\sqrt{s} = 5.02$ TeV, 2017) and 624 million p–Pb events ( $\sqrt{s_{NN}} = 5.02$ TeV, 2016).
Jet Reconstruction: Charged particles ( $p_{T,track} > 0.15$ GeV/c) are clustered into jets using the anti- $k_T$ algorithm with a resolution parameter $R=0.4$ . Jets are reconstructed within $|\eta_{jet}| < 0.9 - R$ .
EEC Definition: The EEC is defined as a $p_T$ -weighted two-particle correlation as a function of the angular distance $R_L$ between particle pairs within a jet. To suppress sensitivity to soft radiation and the underlying event (UE), only constituents with $p_{T,track} > 1$ GeV/c are included in the correlation sum.
Background Subtraction: A perpendicular cone method is employed to estimate and subtract contributions from the underlying event. This involves constructing "jet-perp" and "perp-perp" pairs using tracks from cones located $\pm 90^\circ$ in azimuth relative to the jet axis, assuming azimuthal symmetry of the UE in p–Pb collisions.
Corrections: Detector effects, including tracking efficiency, momentum resolution, and angular resolution, are corrected using a two-dimensional bin-by-bin method based on simulations (DPMJET for p–Pb and PYTHIA 8 for pp). Systematic uncertainties are evaluated by varying selection criteria, background subtraction parameters, and event generators.

Key Contributions and Results
The paper reports the EEC distributions for jets in three transverse momentum intervals: 20–40, 40–60, and 60–80 GeV/c.

Observation of Modification: In the 20–40 GeV/c interval, a significant modification is observed in the p–Pb EEC relative to the pp baseline. Specifically, there is a suppression at small opening angles ( $0.03 < R_L < 0.08$ ) and an enhancement at large opening angles. The p–Pb/pp ratio deviates from unity by 2.4–3.4 $\sigma$ in the suppression region and exceeds unity by 2.2 $\sigma$ at the largest reported $R_L$ .
Dependence on Kinematics: This modification is not observed in the higher $p_T$ intervals (40–60 and 60–80 GeV/c), where the p–Pb and pp results agree within uncertainties. The effect is robust against variations in the constituent $p_T$ threshold (tested from 150 MeV/c to 2 GeV/c).
Differential Studies: The modification shows no significant dependence on the jet pseudorapidity (comparing Pb-going vs. p-going jets) or on the forward event activity (comparing high- vs. low-multiplicity events). This suggests the effect is not driven by local particle density or initial collision geometry.
Transition Region Analysis: The position of the EEC peak, which marks the transition between perturbative and non-perturbative regimes, scales with $1/\langle p_{T}^{ch,jet} \rangle$ . The extracted peak position in p–Pb ( $2.43 \pm 0.07$ GeV/c) is consistent with the pp measurement ( $2.40 \pm 0.10$ GeV/c), indicating that the hadronization scale remains unchanged despite the observed angular broadening.
Model Comparisons:
- Initial-State Effects: Simulations using PYTHIA 8 with nuclear Parton Distribution Functions (nPDFs) and the Angantyr model (incorporating multi-parton interactions) fail to reproduce the observed enhancement at large angles.
- Final-State Effects: Theoretical models incorporating final-state interactions—specifically partonic comovers, transverse-momentum broadening, and twist-4 Operator Product Expansion (OPE) corrections—are compared to the data. While the comover model qualitatively describes the large-angle trend, and the $p_T$ -broadening and twist-4 OPE models capture the general shape of the 20–40 GeV/c data, none provide a complete quantitative description across all kinematic regimes. Notably, the $p_T$ -broadening model predicts modifications at higher $p_T$ that are not observed in the data.

Significance
This paper establishes the first measurement of the EEC in p–Pb collisions, providing new constraints on cold nuclear matter effects. The observation of an angular broadening of energy flow in lower- $p_T$ jets, which cannot be explained by initial-state effects alone, suggests a potential role for final-state interactions as jets propagate through the CNM. These results are crucial for disentangling initial- and final-state contributions to jet-structure modifications, a prerequisite for interpreting similar measurements in heavy-ion collisions where a quark-gluon plasma is formed. The findings indicate that while jets in p–Pb collisions generally traverse a dilute medium, specific kinematic regimes may exhibit subtle modifications driven by soft multiple scattering or interactions with the nuclear medium.