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Measurement of $J/\psi$ -jet correlations in $pp$ and Pb+Pb collisions at $\sqrt{s_{\text{NN}}}=5.02$ TeV with the ATLAS detector

The ATLAS experiment measured $J/\psi$ -jet correlations in $pp$ and Pb+Pb collisions at $\sqrt{s_{\text{NN}}}=5.02$ TeV by categorizing $J/\psi$ mesons based on their isolation from jets and promptness, revealing that strong suppression persists up to high transverse momenta and providing new constraints on charmonium production mechanisms and their interaction with the quark-gluon plasma.

Original authors: ATLAS Collaboration

Published 2026-07-01

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: ATLAS 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 the Large Hadron Collider (LHC) as the world's most powerful particle accelerator, smashing tiny particles together at nearly the speed of light. When scientists smash heavy lead atoms together, they create a fleeting, super-hot "soup" called the Quark-Gluon Plasma (QGP). Think of this soup as a dense, chaotic fog where the fundamental building blocks of matter (quarks) are free to roam, rather than being stuck together in pairs or groups.

This paper is a report from the ATLAS experiment, one of the giant detectors watching these collisions. The scientists are trying to solve a mystery: Why do certain heavy particles, called "charmonia" (specifically the J/ψ particle), disappear or get "suppressed" when they travel through this hot soup?

To understand why they disappear, the scientists needed to know exactly how these particles are born and how they interact with the "traffic" around them. Here is a breakdown of their investigation using simple analogies:

1. The Mystery of the Disappearing Act

In normal collisions (like smashing two protons together), J/ψ particles are born and fly away. But in the heavy lead collisions, many of them vanish. Scientists believe the hot QGP fog is "melting" them or blocking their path. To figure out how this happens, they need to know: Is the J/ψ particle born alone, or is it born inside a busy "jet" of other particles?

2. The Two Types of "Births"

The scientists realized J/ψ particles come from two different "families":

The "Prompt" Family: These are born instantly, right at the moment of the crash. They are like a spark that appears the second two cars collide.
The "Non-Prompt" Family: These are born a split-second later from the decay of heavier particles (B-hadrons). They are like a spark that appears a moment after the crash, after some debris has already fallen.

3. The "Jet" Connection

In particle physics, when a particle is created, it often drags a shower of other particles with it, forming a "jet."

Isolated J/ψ: Imagine a lone wolf walking through a forest. It has no jet attached to it.
Non-Isolated J/ψ: Imagine a wolf walking right in the middle of a pack. It is surrounded by a jet of other particles.

The Innovation: Previous studies often looked only at events where a jet was already there. This paper is special because it looked at all J/ψ particles, regardless of whether they were alone or in a pack. They separated the "lone wolves" from the "pack wolves" to see if the QGP fog treats them differently.

4. The Investigation

The team analyzed data from two types of races:

Proton-Proton (pp) Collisions: The "control group." This is like a clear day with no fog.
Lead-Lead (Pb+Pb) Collisions: The "test group." This is the heavy fog (QGP).

They counted how many J/ψ particles survived in the fog compared to the clear day. They also checked if the "lone wolves" (isolated) survived better or worse than the "pack wolves" (non-isolated).

5. The Findings

The Fog is Tough: In the heavy lead collisions, the J/ψ particles were indeed suppressed (they disappeared more often than in the clear proton collisions). This suppression happened even for the fastest, highest-energy particles.
The "Pack" vs. The "Lone Wolf":
- The scientists found a subtle but significant difference between the "lone wolves" and the "pack wolves."
- The non-isolated J/ψ particles (those born inside a jet) were suppressed more than the isolated ones.
- The Analogy: Imagine walking through a dense crowd. If you are walking alone, you might get pushed around, but you can still find a path. If you are walking inside a large, chaotic group (a jet), the crowd pushes on the whole group, making it much harder for anyone inside to get through. The data suggests that J/ψ particles born inside these jets suffer more from the "crowd" (the QGP) than those born alone.
Timing Matters: The "Non-Prompt" (delayed birth) particles were found to be mostly "isolated" (lone wolves) at high speeds, while the "Prompt" (instant birth) particles were more likely to be found in jets.

6. Why This Matters

This study doesn't just count particles; it changes the rules of the game for theoretical models.

Before, models struggled to explain exactly how these particles are made and how they lose energy in the plasma.
By separating the "lone wolves" from the "pack wolves," the scientists have given theorists a new, sharper tool. They now know that the "pack" (the jet) plays a crucial role in how much energy the J/ψ loses.

In Summary:
The ATLAS team acted like detectives at a crime scene. They found that the "hot soup" of the early universe suppresses heavy particles. But more importantly, they discovered that how those particles are born (alone or in a jet) changes how much they get suppressed. The "pack" gets hit harder by the soup than the "lone wolf." This helps scientists understand the invisible forces at play when matter is heated to trillions of degrees.

Technical Summary: Measurement of $J/\psi$ –jet correlations in $pp$ and Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV with the ATLAS detector

Problem and Motivation
Charmonium production, specifically the $J/\psi$ meson (a $c\bar{c}$ bound state), is strongly suppressed in heavy-ion collisions relative to proton-proton ($pp$) collisions due to interactions with the quark-gluon plasma (QGP). While mechanisms such as color screening, in-medium dissociation, and regeneration are understood, the precise production mechanisms of charmonia remain unresolved. Specifically, it is unclear how charmonia correlate with jet activity and whether high- $p_T$ charmonia experience medium-induced energy loss analogous to jet quenching. Previous measurements of $J/\psi$ –jet correlations by CMS and LHCb relied on event samples triggered by the presence of jets or defined the $J/\psi$ as being "in a jet" based on the jet's four-momentum (which includes the $J/\psi$ itself). These approaches introduce potential biases and do not fully separate the kinematics of the $J/\psi$ from the jet. This paper aims to quantify $J/\psi$ –jet correlations by analyzing all $J/\psi$ events, separating the $J/\psi$ kinematics from jet kinematics, and distinguishing between prompt (directly produced) and non-prompt (from $b$ -hadron decays) $J/\psi$ mesons.

Methodology
The analysis utilizes data collected by the ATLAS detector at the LHC, corresponding to $1.82 \text{ nb}^{-1}$ of Pb+Pb collisions (2015 and 2018) and $0.26 \text{ fb}^{-1}$ of $pp$ collisions (2017), all at $\sqrt{s_{NN}} = 5.02$ TeV.

Event Selection and Reconstruction: $J/\psi$ candidates are reconstructed in the dimuon decay channel ( $\mu^+\mu^-$ ) with $p_T > 9$ GeV and $|y| < 2$ . Muons must satisfy the "Medium" working point and be matched to trigger-level muons.
Jet Association: Jets are reconstructed using the anti- $k_t$ algorithm with a radius parameter $R=0.2$ and a transverse momentum threshold of $p_T > 20$ GeV. Crucially, the jet reconstruction excludes the signal from the $J/\psi$ meson.
Classification: $J/\psi$ $J / ψ$ candidates are categorized into three classes:
1. Non-isolated: Matched to a jet within $\Delta R < 0.4$ .
2. Inclusive: All selected $J/\psi$ candidates.
3. Isolated: Defined as the difference between inclusive and non-isolated yields ( $N_{iso} = N_{incl} - N_{non-iso}$ ).
Signal Extraction: Yields for prompt and non-prompt $J/\psi$ are extracted using unbinned maximum-likelihood fits to the dimuon invariant mass ( $m_{\mu\mu}$ ) and pseudo-proper lifetime ( $\tau_{\mu\mu}$ ) distributions. The fit model includes signal components (Crystal Ball + Gaussian) and background components (exponentials) for both prompt and non-prompt sources.
Corrections: Raw distributions are corrected for acceptance, trigger efficiency, reconstruction efficiency, and jet-related effects (including jet energy scale, resolution, and background jets from the underlying event or multi-parton interactions estimated via the orthogonal-cone method).
Observables: The study reports differential cross-sections in $pp$, yields in Pb+Pb (normalized by event count and nuclear overlap function $\langle T_{AA} \rangle$ ), nuclear modification factors ( $R_{AA}$ ), isolated fractions, and non-prompt fractions.

Key Contributions

Decoupling Kinematics: The analysis separates the $J/\psi$ kinematics from the jet kinematics by reconstructing jets without the $J/\psi$ signal and analyzing the full $J/\psi$ sample rather than jet-triggered events.
Isolation Definition: It introduces a rigorous definition of "isolated" $J/\psi$ mesons based on the absence of a nearby high- $p_T$ jet, allowing for a direct comparison between isolated and non-isolated production mechanisms.
Prompt vs. Non-prompt Separation: The measurement provides separate results for prompt and non-prompt $J/\psi$ mesons across all categories (isolated, non-isolated, inclusive).
Systematic Treatment: The paper details systematic uncertainties arising from jet corrections, background subtraction, and fit models, specifically addressing the impact of the underlying event in Pb+Pb collisions on jet tagging.

Results

Isolated Fractions: The fraction of isolated $J/\psi$ mesons starts at unity for $p_T \approx 9$ GeV and decreases to between 0.6 and 0.8 at $p_T \approx 50$ GeV. The isolated fraction is systematically 10–20% higher for prompt $J/\psi$ compared to non-prompt $J/\psi$ in both $pp$ and Pb+Pb collisions. No significant difference in the isolated fraction is observed between $pp$ and Pb+Pb collisions or across different centrality intervals.
Non-prompt Fractions: The non-prompt fraction for inclusive $J/\psi$ increases from $\approx 0.3$ at $p_T = 10$ GeV to $\approx 0.6$ at $p_T = 50$ GeV. For non-isolated $J/\psi$ with $p_T > 15$ GeV, the non-prompt fraction is approximately 0.7 and shows no $p_T$ dependence.
Nuclear Modification Factors ( $R_{AA}$ ):
- Strong suppression is observed for all classes up to the highest measured $p_T$ .
- For prompt $J/\psi$ , the $R_{AA}$ values for isolated and inclusive classes are consistent across the entire $p_T$ range.
- However, for $p_T > 30$ GeV, the $R_{AA}$ for non-isolated prompt $J/\psi$ is systematically lower than that for isolated prompt $J/\psi$ . This difference has a significance of 3.5 $\sigma$ (via Stouffer's test) and 3.3 standard deviations (via slope comparison).
- No significant difference in $R_{AA}$ is observed among the three isolation classes for non-prompt $J/\psi$ .
Fragmentation Function: The raw fragmentation function for prompt $J/\psi$ in $pp$ collisions is qualitatively consistent with Pythia 8 predictions for non-prompt production but differs significantly from predictions for prompt production, suggesting that prompt $J/\psi$ production late in the parton shower is underestimated in the model.

Significance
The paper claims that these results provide new constraints on charmonium production mechanisms and their in-medium suppression. By demonstrating that non-isolated prompt $J/\psi$ mesons (those associated with jets) experience stronger suppression ( $R_{AA}$ ) than isolated ones at high $p_T$ , the authors suggest that $J/\psi$ produced in association with a more developed parton shower or fragmentation system may be subject to enhanced energy loss or dissociation in the medium. The results offer critical input for theoretical models aiming to describe $J/\psi$ production in both $pp$ and Pb+Pb collisions, particularly regarding the interplay between jet quenching and charmonium suppression.