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Observation of structures in the $J/\psi +\psi(2S)$ mass spectrum with the ATLAS detector

Using 140 fb $^{-1}$ of 13 TeV proton-proton collision data, the ATLAS experiment observes a significant excess near 6.9 GeV in the $J/\psi +\psi(2S)$ mass spectrum with a combined significance of 8.9 $\sigma$ , while finding no significant signal near 7.2 GeV and determining the ratio of partial decay widths between the $J/\psi +\psi(2S)$ and di- $J/\psi$ channels for the 6.9 GeV resonance.

Original authors: ATLAS Collaboration

Published 2026-06-26

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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

The Big Picture: Hunting for "Ghost" Cars in a Traffic Jam

Imagine the Large Hadron Collider (LHC) at CERN as a massive, high-speed racetrack where tiny particles called protons are smashed together at nearly the speed of light. When they crash, they create a chaotic explosion of debris, much like a massive pile-up of cars at a busy intersection.

Most of the time, this debris is just standard "traffic"—common particles that physicists see all the time. But sometimes, for a split second, a rare and exotic "ghost car" appears. This ghost car is a tetraquark, a particle made of four "charm" quarks stuck together. These are exotic because quarks usually come in pairs or triplets, not foursomes.

In 2020, scientists first spotted one of these ghost cars, named X(6900), because it appeared to weigh about 6.9 billion electron-volts (GeV). However, they only saw it when the ghost car fell apart into two specific pieces (two J/psi particles).

This new paper asks a simple question: If this ghost car exists, does it also fall apart into a different set of pieces? Specifically, does it break into one J/psi and one psi(2S)?

The Investigation: Two Different Lenses

The ATLAS team at CERN acted like detectives looking at the crash site through two different camera lenses:

Lens A (4 Muons): They looked for the ghost car breaking into four muons (a type of heavy electron).
Lens B (4 Muons + 2 Pions): They looked for the ghost car breaking into four muons plus two pions (another type of particle). This second lens is like a wider-angle view; it captures more of the crash debris, making it easier to spot the rare event.

They analyzed data from 140 "inverse femtobarns" of collisions (a fancy way of saying they looked at a huge amount of data, equivalent to 140 trillion trillion collisions).

The Discovery: A Loud Siren in the Noise

When the scientists sorted through the data, they found something exciting. In both camera lenses, there was a distinct "bump" or excess of events right around the 6.9 GeV mark.

The Analogy: Imagine you are listening to a crowded room full of people talking (the background noise). Suddenly, a specific person shouts a name. In the first lens, you heard the shout clearly. In the second lens, you heard it even louder.
The Result: The signal was so strong that the chance of it being a random fluke is less than one in a billion. In scientific terms, they achieved a significance of 8.9 sigma. (For context, 5 sigma is the standard required to claim a discovery; 8.9 is a massive, undeniable confirmation).

This confirms that the X(6900) ghost car exists and that it can indeed decay into the J/psi + psi(2S) combination.

The "What If" Test: Is There a Second Ghost Car?

The scientists also wondered if there might be a second ghost car, heavier than the first one, sitting around 7.2 GeV. They called this hypothetical particle X(7200).

The Analogy: It's like checking if there is a second, heavier car hidden in the same pile-up.
The Result: They looked very carefully but found no evidence of this second car. They set an upper limit, essentially saying, "If this second car exists, it is so rare that we didn't see it in this dataset."

The Connection: How the Ghost Car Breaks Apart

One of the most interesting findings is how the X(6900) breaks apart. The team compared how often it breaks into two J/psi particles versus how often it breaks into a J/psi and a psi(2S).

The Analogy: Imagine a magic box that can open in two ways: either it drops two red balls or one red ball and one blue ball. The scientists found that the box drops these two combinations at almost exactly the same rate.
The Result: The ratio of these two decay paths is roughly 1:1. This tells us that the internal structure of this tetraquark is very specific and symmetric, favoring one type of breakup just as much as the other.

Summary of Findings

Confirmation: The ATLAS experiment has confirmed the existence of the X(6900) tetraquark by seeing it decay into a new combination of particles (J/psi + psi(2S)).
Strength: The evidence is extremely strong (8.9 sigma), making this a definitive observation.
No Second Car: There is no evidence for a heavier partner particle (X(7200)) in this dataset.
Decay Pattern: The particle decays into the new channel at nearly the same rate as it does into the old channel, suggesting a balanced internal structure.

In short, the scientists have successfully found the "ghost car" in a new disguise, proving it is a real, stable (though short-lived) object in the quantum world, and they have learned a bit more about how it falls apart.

Technical Summary: Observation of structures in the $J/\psi + \psi(2S)$ mass spectrum with the ATLAS detector

Problem and Motivation
The nature of exotic hadrons, particularly all-charm tetraquarks ( $T_{cccc}$ ), remains a critical area for understanding Quantum Chromodynamics (QCD) in the non-perturbative regime. Following the 2020 observation by the LHCb Collaboration of a narrow resonance near 6.9 GeV ( $X(6900)$ ) in the di- $J/\psi$ mass spectrum, subsequent confirmations by ATLAS and CMS, and recent measurements of quantum numbers by CMS, there is a need to investigate whether similar resonant structures exist in other decay channels. Specifically, this study addresses the search for resonant structures in the $J/\psi + \psi(2S)$ mass spectrum. While the $J/\psi + \psi(2S)$ channel was previously studied by ATLAS, this analysis extends the investigation by reconstructing the $\psi(2S) \to J/\psi \pi^+ \pi^-$ decay mode, which offers a total branching fraction more than twice that of the $\psi(2S) \to \mu^+\mu^-$ mode, thereby increasing the potential signal yield.

Methodology
The analysis utilizes proton–proton collision data at $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 140 fb $^{-1}$ , recorded by the ATLAS detector during 2015–2018. The study focuses on two decay channels:

$J/\psi + \psi(2S) \to 4\mu$
$J/\psi + \psi(2S) \to 4\mu + 2\pi$ (via $\psi(2S) \to J/\psi \pi^+ \pi^-$ )

Event Selection and Background Estimation:
Events are selected using di-muon and tri-muon triggers. Signal candidates require four muons forming two opposite-charge pairs consistent with $J/\psi$ and $\psi(2S)$ masses. For the $4\mu+2\pi$ channel, two additional opposite-sign charged tracks are required. A simultaneous fit is performed using Signal Regions (SRs) and Control Regions (CRs). The SRs and CRs are defined based on the angular distance $\Delta R$ between the two charmonia candidates ( $\Delta R < 0.25$ for SRs, $\ge 0.25$ for CRs). This strategy allows for the constraint of background normalizations and validation of the Single Parton Scattering (SPS) background shape.

Backgrounds are categorized as:

Others: Events with at most one real charmonium (estimated via data-driven sideband methods).
Nonprompt: Charmonium from $b$ -hadron decays (estimated via MC, corrected in a dedicated CR).
Double Parton Scattering (DPS): Estimated via MC, constrained in a CR with high di-charmonium mass.
SPS: Prompt di-charmonium production (estimated via MC, with shape constrained by the Pythia parameter $pT0timesMPI$).

For the $4\mu+2\pi$ channel, a Boosted Decision Tree (BDT) is employed to suppress feed-up background from the di- $J/\psi$ channel where random tracks are misidentified as pions.

Signal Modeling and Fit Strategy
Unbinned maximum likelihood fits are performed on the $m_{4\mu}$ and $m_{4\mu+2\pi}$ mass spectra. Three representative signal models are considered to account for potential interference effects:

Model A: Two interfering S-wave Breit–Wigner (BW) resonances (assuming the $X(6900)$ observed in di- $J/\psi$ also decays to $J/\psi+\psi(2S)$ ).
Model B: One resonance interfering with the SPS background and a second standalone resonance.
Model C: A single standalone BW resonance (considering only $J/\psi+\psi(2S)$ channels).

The fit simultaneously extracts signal yields, masses, and widths. The ratio of partial decay widths, $R = \Gamma(X(6900) \to J/\psi+\psi(2S)) / \Gamma(X(6900) \to \text{di-}J/\psi)$ , is treated as a free parameter in Models A and B. A search for a second resonance near 7.2 GeV ( $X(7200)$ ) is also conducted by adding a standalone component to the models.

Key Results

Observation of $X(6900)$ : A significant excess is observed near 6.9 GeV in both the $4\mu$ and $4\mu+2\pi$ channels. The combined significance reaches 8.9 $\sigma$ (calculated with signal shape parameters fixed to best-fit values).
Resonance Properties: The fitted mass is consistently around 6.9 GeV across all models. The width varies by model: approximately 0.08 GeV for Model A and 0.18 GeV for Models B and C.
Partial Width Ratio: Under the assumption of Model A (nominal) and Model B (systematic assessment), the ratio of partial decay widths is determined to be $R = 1.08 \pm 0.20 \pm 0.40$ (statistical and systematic uncertainties). This suggests the branching fractions to $J/\psi+\psi(2S)$ and di- $J/\psi$ are comparable.
Search for $X(7200)$ : No significant signal is observed near 7.2 GeV. An upper limit on the yield of a potential $X(7200)$ relative to $X(6900)$ is set at 0.41 at the 95% confidence level.
Background Validation: The background estimation methods show good agreement with data in the control regions, validating the SPS background shape and normalization.

Significance and Claims
The paper claims that this analysis confirms the presence of structures in the $J/\psi+\psi(2S)$ channels, favoring a single-resonance hypothesis near 6.9 GeV. The result supersedes previous measurements from the same collaboration (Ref. [13]) by providing more precise mass and width determinations and achieving a higher statistical significance (8.9 $\sigma$ compared to 4.7 $\sigma$ in the previous di- $J/\psi$ -only analysis). The finding that the partial decay width ratio is near unity supports scenarios where the $X(6900)$ couples similarly to both di-charmonium final states. The paper concludes that while the existence of a resonance near 7.2 GeV is not supported by current data, future datasets will be required to precisely determine the nature, quantum numbers, and potential additional resonances of these exotic states.