The strong force, unrivaled in strength yet confined to minuscule distances, produces unique bound states of quarks known as quarkonia. These are the strong-force equivalent of the hydrogen atom, consisting of a quark and its antiquark in place of a proton and an electron, with new states corresponding to a new energy level. A particularly well-known example is charmonium, formed from a charm quark and its antiquark. The J/ψ charmonium is a ground state, and the ψ(2S) is a radial excitation of J/ψ.

The discovery of the J/ψ in 1974 sparked the “November Revolution” in particle physics, revealing the existence of more than three types of quarks and laying the groundwork for the Standard Model of particle physics as we know it today. Decades later, LHC experiments continue to study the J/ψ in detail.

But what happens when not two, but four charm quarks bind together? This results in an exotic state known as an all-charm tetraquark. The four charm quarks can be in either a "molecular" or a compact state. One such tetraquark, called X(6900), has been observed by the LHCb, ATLAS and CMS Collaborations. When produced, it appears as two charmonia in ground (J/ψ) or excited (ψ(2S)) states. Physicists have now set out to precisely measure the mass, decay width and decay modes of the all-charm tetraquark — all essential steps toward understanding the nature and formation of these exotic states.

In 2022, the ATLAS Collaboration reported a hint of excesses in the J/ψ+ψ(2S)→4𝜇 spectrum around 6.9 and 7.2 GeV. This led to several open questions. Were these excesses due to data fluctuations? Were there other excesses? Could the di-J/ψ and J/ψ+ψ(2S) channels be jointly analysed to extract the partial decay width of the all-charm tetraquark?

To answer these questions, the ATLAS Collaboration used the full LHC Run-2 dataset (collected 2015–2018) to analyse all-charm tetraquark decays into two channels: J/ψ+ψ(2S) producing four muons (4𝜇) and, for the first time, producing four muons with two pions (4𝜇+2𝜋). This second channel is especially promising due to its higher decay probability (branching fraction).

One of the challenges in this analysis was in estimating the background originating from the di-J/ψ channel, where four muons are combined with two random pions, thus mimicking the 4𝜇+2𝜋 channel. ATLAS physicists trained a boosted decision tree (BDT) to distinguish signal from background and used a control region with two same-sign pions to estimate the remaining background contribution after a requirement on the BDT. They performed a simultaneous fit on the 4𝜇 and 4𝜇+2𝜋 mass spectra to extract the signal parameters from the data.

As shown in Figure 1, a resonance appears near 6.9 GeV in both channels. The combined significance of the signal is 8.9𝜎, confirming the presence of an all-charm tetraquark candidate X(6900) decaying into J/ψ+ψ(2S). Physicists also considered models where the X(6900) observed in the di-J/ψ channel is assumed to also decay into J/ψ+ψ(2S). In this case, the X(6900) in both the di-J/ψ and J/ψ+ψ(2S) channels are considered the same resonance, although current data does not exclude the possibility that the resonance in the J/ψ + ψ(2S) channel is a separate state. They performed a combined fit of the di-J/ψ and J/ψ+ψ(2S) channels, with statistical input for the di-J/ψ channel from the previous ATLAS analysis.

ATLAS researchers then measured the ratio of branching fractions for X(6900) → J/ψ+ψ(2S) and X(6900) → di-J/ψ. Surprisingly, the ratio was found to be close to 1. This was unexpected, as the di-J/ψ channel has a larger available phase space, and thus the J/ψ+ψ(2S) decay mode was expected to be relatively suppressed.

They also looked for the presence of a second resonance, X(7200), also previously hinted at in LHCb and CMS data. The ratio of signal yields for X(7200) to X(6900) was measured to be 0.12 ± 0.11, with an upper limit of 0.41 at a 95% confidence level. The existence of a potential resonance near 7.2 GeV is not supported by the current data. The event display above shows a candidate for an all-charm tetraquark, produced in a proton–proton collision within the ATLAS detector.

With LHC Run 3 data taking underway, the accumulation of additional data is expected to clarify the nature of this excess, constraining its underlying model and quantum numbers, and probing the existence of further resonances.

Learn more

• Observation of structures in the J/ψ+ψ(2S) mass spectrum with the ATLAS detector (Submitted to PRL, arXiv:2509.13101, see figures)
• Observation of an Excess of Dicharmonium Events in the Four-Muon Final State with the ATLAS Detector (Phys. Rev. Lett. 131 (2023) 151902, arXiv:2304.08962, see figures)
• LHCb Collaboration: Observation of structure in the J/𝜓-pair mass spectrum (Science Bulletin 65 (2020) 1983, arXiv:2006.16957)
• CMS Collaboration: New Structures in the J/𝜓 J/𝜓 Mass Spectrum in Proton–Proton Collisions at 13 TeV (Phys. Rev. Lett. 132 (2024) 111901, arXiv:2306.07164)
• CMS Collaboration: Determination of the spin and parity of all-charm tetraquark (arXiv:2506.07944)