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Probing jet evolution with charged energy correlators in small systems

The ALICE Collaboration presents measurements of charged energy-energy correlators in proton-proton and proton-lead collisions at 5.02 TeV, demonstrating that these observables effectively distinguish between parton shower and hadronization models while revealing that cold nuclear matter effects on charged EECs are consistent with charge-independent behavior.

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

Imagine a high-energy particle collision at CERN's Large Hadron Collider (LHC) as a high-speed car crash. When two protons smash together, they don't just shatter into dust; they create a spray of new particles that fly outward in a cone shape. Physicists call this spray a "jet."

For decades, scientists have studied these jets to understand the fundamental rules of the universe (Quantum Chromodynamics, or QCD). However, looking at the whole jet is like looking at a blurry photo of a fireworks display. You see the explosion, but you can't easily tell how the individual sparks were created or how they traveled.

This paper introduces a new, sharper way to look at these fireworks: Charged Energy-Energy Correlators (Charged EECs).

Here is a simple breakdown of what the ALICE collaboration did and what they found, using everyday analogies.

1. The New Tool: Tracking Energy and "Charge"

Usually, scientists measure how much energy is in a jet and how far apart the particles are. This paper adds a new layer: electric charge.

Think of a jet as a crowd of people running out of a stadium.

Energy is how fast they are running.
Charge is their team jersey color (Red for positive, Blue for negative).

The "Charged EEC" is a way of measuring the relationship between how fast two people are running and how far apart they are, while also checking if they are wearing the same color jersey or different colors.

Like-sign pairs: Two people in Red jerseys running together, or two in Blue.
Unlike-sign pairs: A Red and a Blue running together.

2. Why Does This Matter? The "Recipe" of a Jet

When a jet forms, it goes through two main stages:

The Parton Shower: The initial explosion where the energy splits into smaller and smaller pieces (like a tree branching out). This is governed by strict, calculable physics.
Hadronization: The moment the energy cools down and "freezes" into actual particles (like water turning to ice). This is messy and hard to predict.

The scientists wanted to know: Can we tell which part of the "recipe" is responsible for specific patterns in the jet?

3. The Big Discovery: Separating the Ingredients

By comparing the "Red-Red" (like-sign) and "Red-Blue" (unlike-sign) pairs, the team found a clever way to separate the two stages of the jet's life:

The "Tree Branching" (Parton Shower): The scientists found that the like-sign pairs (Red-Red or Blue-Blue) are very sensitive to how the initial explosion (the shower) is modeled. If you change the rules of how the tree branches, the pattern of same-colored pairs changes.
The "Freezing" (Hadronization): The unlike-sign pairs (Red-Blue) are sensitive to how the particles "freeze" into existence. Different theories about how particles form (like "string breaking" vs. "clumping") change the pattern of mixed-color pairs.

The Analogy: Imagine trying to figure out how a cake was baked.

If you look at the Red-Red pairs, you are learning about the mixing process (how the batter was stirred).
If you look at the Red-Blue pairs, you are learning about the baking process (how the heat turned it into a cake).

The paper claims that by looking at these two groups separately, they can test computer simulations (Monte Carlo generators) much more precisely than before.

4. The "Cold Nuclear Matter" Test

The team also crashed protons into lead nuclei (p-Pb collisions). This is like running the same experiment in a slightly denser fog (cold nuclear matter) to see if the fog changes how the fireworks look.

The Result: They found that the fog changes the overall shape of the fireworks (suppressing some angles, enhancing others), but it does not care about the jersey colors. The rules for Red-Red, Blue-Blue, and Red-Blue pairs changed exactly the same way.
The Takeaway: The "fog" affects the energy flow, but it doesn't mess with the electric charge rules.

5. What the Computer Models Got Right (and Wrong)

The scientists compared their real-world data against five different computer simulations of how jets work.

The Good News: Some models (like Herwig and SHERPA) did a great job predicting the overall shape of the data.
The Bad News: No single model got everything right.
- Some models predicted that the "Red-Blue" pairs would peak at a different angle than the "Red-Red" pairs. The data might show this, but the measurements aren't precise enough yet to be sure.
- The Mystery: At very small angles, the data showed a "rebalancing" effect (the ratio of colors shifted back toward equality) that none of the computer models predicted. It's as if the fireworks display did something unexpected at the very end that the physics textbooks didn't account for.

Summary

In simple terms, this paper is about sorting the noise. By separating particles based on their electric charge, the ALICE collaboration created a new lens to look at particle jets. They discovered that:

Same-charge pairs tell us about the initial explosion.
Opposite-charge pairs tell us about how particles form.
Cold nuclear matter (lead collisions) changes the energy flow but respects the charge rules.
Current computer models are good but missing some details, specifically regarding how the jet behaves at the very smallest angles.

This work doesn't just confirm what we know; it highlights exactly where our current understanding of the universe's building blocks is still a bit fuzzy.

Technical Summary: Probing Jet Evolution with Charged Energy Correlators in Small Systems

Problem and Motivation
Jets produced in high-energy collisions evolve through a hierarchy of energy scales in Quantum Chromodynamics (QCD), transitioning from perturbative parton showers to non-perturbative hadronization. While the two-point energy–energy correlator (EEC) has emerged as a powerful observable for studying jet substructure and energy flow, it traditionally treats all charged particles equally. The paper addresses the need to disentangle the specific contributions of energy scales and charge conservation mechanisms within jets. By introducing "charged EECs," which segregate particle pairs based on electric charge (like-sign vs. unlike-sign) or weight them by charge products, the authors aim to probe the interplay between energy distribution and charge conservation. This approach offers a novel method to constrain hadronization mechanisms and parton shower models, which are currently described by phenomenological models (e.g., Lund string breaking vs. cluster models) rather than first-principles QCD. Furthermore, the study investigates whether these charge-dependent dynamics are modified by cold nuclear matter effects in proton–lead (p–Pb) collisions compared to proton–proton (pp) collisions.

Methodology
The analysis utilizes data from the ALICE experiment at the LHC, collected during Run 2 at a center-of-mass energy of $\sqrt{s_{NN}} = 5.02$ TeV for both pp and p–Pb collisions.

Data Selection: Charged-particle jets with transverse momentum $20 < p_{T,ch}^{jet} < 80$ GeV/c are reconstructed using the anti- $k_T$ algorithm with a radius parameter $R=0.4$ . Only jets fully within the TPC acceptance ( $|\eta_{jet}| < 0.5$ ) are considered.
Observable Definition: The study measures four distinct correlators as a function of the pair opening angle $R_L = \sqrt{\Delta\phi^2 + \Delta\eta^2}$ $R_{L} = Δ ϕ^{2} + Δ η^{2}$ :
1. Inclusive EEC ( $\Sigma_{EEC}$ ): Sum of energy-weighted pairs regardless of charge.
2. Charge-Selected EECs: $\Sigma^{++}_{EEC}$ and $\Sigma^{--}_{EEC}$ (like-sign) and $\Sigma^{+-}_{EEC}$ (unlike-sign).
3. Charge-Weighted EEC ( $\Sigma^Q_{EEC}$ ): Incorporates the product of particle charges ( $q_i q_j$ ) into the energy weighting.
4. Derived Ratios: Charged EEC asymmetry ( $\Sigma^Q_{EEC}/\Sigma_{EEC}$ ) and the like-sign ratio ( $\Sigma^{++}_{EEC}/\Sigma^{--}_{EEC}$ ).
Corrections and Background Subtraction:
- For pp collisions, no underlying event (UE) subtraction is applied beyond a $p_T > 1$ GeV/c cutoff for constituents.
- For p–Pb collisions, a perpendicular cone subtraction method is employed to remove UE contributions, alongside an area-based $p_T$ correction.
- Detector effects (tracking efficiency, $p_T$ resolution, and pair reconstruction efficiency) are corrected using a two-dimensional bin-by-bin method based on Monte Carlo (MC) simulations (PYTHIA 8, DPMJET, GEANT3).
Model Comparison: Measurements are compared against five MC event generators: PYTHIA 8 (Monash tune and Vincia shower), Herwig 7 (cluster hadronization), and SHERPA 3 (with both AHADIC++ cluster and Lund string hadronization). Comparisons also include state-of-the-art perturbative QCD (pQCD) calculations based on light-ray operator product expansion.

Key Results

Differential Sensitivity to QCD Modeling:
- Parton Shower Sensitivity: The like-sign EECs ( $\Sigma^{++}_{EEC}$ and $\Sigma^{--}_{EEC}$ ) exhibit sensitivity to the parton shower modeling (e.g., differences between PYTHIA's $p_T$ -ordered shower and Vincia's antenna shower).
- Hadronization Sensitivity: Conversely, the unlike-sign EEC ( $\Sigma^{+-}_{EEC}$ ) is primarily sensitive to the hadronization scheme (e.g., differences between SHERPA's AHADIC cluster model and Lund string model).
- Inclusive EEC Limitations: Due to the dominance of unlike-sign pairs in inclusive jets (driven by the prevalence of gluon-initiated jets), the inclusive EEC is less sensitive to parton shower variations than the like-sign EEC.
Transition Region Characterization:
- The peak of the EEC distribution, representing the hadronization transition, scales with $1/\langle p_{T,ch}^{jet} \rangle$ .
- MC models predict a hierarchy where the transition scale for unlike-sign pairs is smaller than for like-sign pairs. However, current experimental uncertainties prevent a definitive observation of this hierarchy in the data.
- The peak height remains constant across $p_T$ intervals when scaled appropriately.
Charged EEC Asymmetry:
- The asymmetry $\Sigma^Q_{EEC}/\Sigma_{EEC}$ reaches a minimum of approximately $-0.3$ at the transition angle, indicating a dominance of unlike-sign pairs.
- At angles smaller than the transition scale (low $p_T$ jets), the data shows a "rebalancing" where the asymmetry becomes less negative (closer to zero). This feature is qualitatively predicted by Herwig but is largely missing or less pronounced in other models (PYTHIA, SHERPA).
Cold Nuclear Matter Effects (p–Pb vs. pp):
- Measurements in p–Pb collisions show modifications to the EEC distribution (suppression at small angles, enhancement at large angles) consistent with previous inclusive EEC results.
- Crucially, these modifications are charge-independent; the ratios of charged EECs in p–Pb to pp are consistent with unity within uncertainties.
Theoretical Agreement:
- A pQCD calculation based on light-ray operator product expansion (incorporating twist-2 and twist-4 terms) agrees well with the measured p–Pb/pp ratios, demonstrating that theoretical frameworks can now account for cold nuclear matter effects in these observables.

Significance and Claims
The paper claims to present the first LHC measurements of energy–energy correlators specifically for charged particles within charged-particle jets, utilizing charge information to separate energy scales. The primary significance lies in the demonstration that charged EECs can isolate specific stages of jet evolution:

They provide a tool to disentangle perturbative shower modeling from non-perturbative hadronization mechanisms, which are often convoluted in inclusive observables.
They offer a new constraint on hadronization models, showing that unlike-sign pairs are a sensitive probe for hadronization schemes, while like-sign pairs probe the parton shower.
The observation of charge-independent cold nuclear matter effects suggests that the modification of jet substructure in small systems is driven by mechanisms that do not distinguish between particle charges.
The discrepancy between data and models regarding the "rebalancing" of the asymmetry at small angles highlights missing or mismodeled dynamics in current event generators, particularly for quark-initiated jets or low- $p_T$ regimes.

The authors conclude that while no single MC model provides a consistent description across all charged EEC observables, the differential nature of these measurements offers a refined path for validating and improving QCD phenomenological models.