CMS Teams at Nanjing Normal, Tsinghua, and Fudan Universities Reveal an All-Charm Tetraquark Family and Its Quantum Characteristics in the journal Nature
In 2024, the CMS experiment at CERN’s Large Hadron Collider (LHC)—the world’s most powerful particle accelerator in Geneva, Switzerland—reported resonant structures: X(6600), X(6900), and X(7100), decaying into pairs of J/ψ (spin-1 charm-anticharm) mesons. The result, with “Run 2” data, was published in Physical Review Letters, and highlighted as an “Editor’s Suggestion” [1]. Nanjing Normal University (NNU) and Tsinghua University (THU) were united in a CMS team, led by Professor Kai Yi of NNU, which took the leading role for this result [2].
The three structures, favored as possible candidates for exotic tetraquarks composed of four charm (c) quarks[3], attracted broad attention as a potential new form of hadronic matter (the class of strongly interacting particles, to which the proton and neutron belong). Yet these results also raised several crucial questions: The significance of X(7100) was below statistical “certainty'' (i.e., <5 standard deviations [sigma])—so, was it actually real or a statistical fluke? The three structures appeared to participate in the phenomenon of quantum interference---in which, under the right conditions, the peaks can partially cancel each other---but this effect was also not statistically compelling. But, if true, this has the critical implication that these structures would share common quantum numbers—in turn, suggesting that they are a family of states. So, were they truly interfering? Are there additional family members? Does the triplet decay into other channels? And, ultimately: what is their true nature?
Plagued by these unresolved questions, Professor Kai Yi initiated a program to probe deeper into CMS data. As early as 2020, he led an undergraduate innovation project to search for potential structures in the ψ(2S) J/ψ channel—where ψ(2S) represents the first radial excitation of J/ψ—a plausible but distinct decay mode. After the student Jinjing Gu moved to Tsinghua University (THU) for her PhD studies, Prof. Yi coordinated with Prof. Zhen Hu at THU to continue the investigation. Following the release of the preliminary triplet result in 2022 [4], Prof. Yi reached out to CMS teams experienced in spin-parity analyses to establish a collaboration focused on measuring the triplet’s quantum properties using 2016–18 data. Prof. Andrei Gritsan from Johns Hopkins University (JHU) enthusiastically joined the initiative, and later, Prof. Yi brought in Prof. Chengping Shen of Fudan University (FDU) to strengthen long-term efforts analyzing CMS data collected after 2022. Preliminary results from these three collaborative efforts were released by CMS last April [5,6,7], and the spin-parity findings have now just been published in Nature [8] with a news release and news and views article [9], more details can be found in CMS news briefing [10].
In the spin-parity analysis [7,8], the quantum numbers of the three states were measured for the first time using CMS data collected from 2016 to 2018 (Run 2). Angular analysis techniques originally developed for Higgs boson studies were applied, yielding parity (P) and charge-conjugation (C) values of P = C = +1. The spin J was found to be consistent with 2, while 0 and 1 were excluded at the 95% and 99% confidence levels, respectively. Spin-2 particles are rare in nature, and these are the first well-established exotic particles with spin 2. The results are consistent with an internal diquark–antidiquark (i.e., [cc]-[c̄c̄]) structure, in which both the diquark and antidiquark carry spin 1, with spins aligned so as to sum to 2, and zero orbital angular momentum between them—marking the first glimpse of a probable internal configuration.
These first round studies are being extended with new data collected from 2022 to 2024 (Run 3), and all toll add up to 315 fb⁻¹ of proton–proton collisions—yielding 3.6 times more J/ψ J/ψ pairs than before. This will give the updated analyses a much sharper view of the three states. A preliminary report from last April of this larger sample showed that all structures, as well as the interference among them, were now observed with statistical significances above five standard deviations (i.e.,>5sigma) [5]. Although the JPC quantum numbers for these structures was then unknown, the presence of interference already indicated that they share common JPC values: pointing to a familial relationship of the X's, and providimg a solid foundation for a collective spin-parity measurement. At the same time, in a ψ(2S) J/ψ analysis, CMS explicitly claimed the first observation (i.e.,>5sigma) of a new decay mode, X(6900) → J/ψ ψ(2S) → μ⁺μ⁻μ⁺μ⁻, and also found evidence for X(7100) in this same mode. A hint of interference between X(6900) and X(7100) was also seen, corroborating the J/ψ J/ψ results [5]. These updated results, and an improved understanding of the X triplet that they provide, will soon be submitted for formal publication.
Tao Han, a theorist and founding Director of Pittsburgh Particle Physics, Astrophysics and Cosmology Center, former chair of the American Physical Society Division of Particles and Fields (APSDPF), a Fellow of the American Physical Society (APS) and a fellow of the American Association for the Advancement of Science (AAAS), commented: “Those are important measurements and exciting discoveries! Their results revealed the unexpected and unexplained rich structure of exotic heavy hadrons. The multiple states and their decay patterns discovered by the CMS team offer a new avenue for theoretical exploration. The systems with four heavy quarks provide a laboratory to study their underlying dynamics, conceivably in the framework of non-relativistic QCD. I fully anticipate much more work to follow both in theory and experiments. Congratulations!”
Joel Butler, review committee chair for one of the analyses, former CMS spokesperson, former chair of the APSDPF, and a fellow of APS and AAAS, commented: “The new results bring us closer to a coherent picture of all charmed tetraquarks, further enriching this vibrant field. They once again demonstrate CMS’s outstanding capabilities, made possible by the excellent detector, the enlarged dataset, and the tireless efforts of the analysis teams. In particular, the CMS groups from China have made a big difference by taking leading roles in these analyses. I look forward to many more significant results from them in this exciting new era.”
Hesheng Chen, academician of the Chinese Academy of Sciences and former representative of China at the LHC, remarked: “Congratulations to the CMS team on their latest achievement in revealing the triplet structure, following their earlier discovery of unexpected new particles led by the Chinese CMS groups. CMS has now outlined a preliminary picture of these triplet states, and I look forward to their continued leadership and further groundbreaking progress in the study of all-heavy exotic hadrons.”
Reflecting on the achievement, Prof. Yi emphasized the importance of collaboration among CMS teams, in particular the efforts of Nanjing Normal University, Tsinghua University, Johns Hopkins University, and Fudan University. Sun Yat-sen University, University of Iowa, University of Shanghai for Science and Technology and Henan Normal University also contributed to the results.
The new round of analyses relied heavily on contributions from early-career researchers in the Chinese CMS groups. Former NNU undergraduate Yilin Zhou, now a PhD student at FDU, graduate student Shiyi Huang and undergraduate Yufei Chen from NNU, as well as PhD student Zhengcheng Liang from THU, made significant contributions to the J/ψ J/ψ analysis using new data. Former NNU undergraduate Jinjing Gu, now a PhD student at Tsinghua University, together with PhD student Liangliang Chen and undergraduate Yufei Chen from NNU, made important contributions to the ψ(2S)J/ψ analysis. PhD student Xining Wang from THU, along with Andrei Gritsan’s team from JHU, played a key role in the spin-parity analysis. For the J/ψ J/ψ analysis using new data: Yilin and Shiyi delivered the pre-approval and approval talks, Yufei contributed a special note summarizing theoretical studies of width patterns, and Hui Wang, a faculty member at NNU, presented the unblinding talk for the J/ψ J/ψ analysis. For the ψ(2S) J/ψ analysis: Jinjing and Liangliang gave the pre-approval and approval talks for the ψ(2S) J/ψ analysis, while Jingqing Zhang, a faculty member at NNU, served as the contact person. For the spin-parity analysis: Jingqing delivered the inaugural talk, and Xining presented the pre-approval talk. Gerry Bauer, Guest Scientist at NNU, provided senoir guidance and insights on these analyses.
With the much larger upcoming dataset up to 2026—about 10 times J/ψ pair of previous runs—the program will now be entering a precision era. Cross-section measurements of the di-J/ψ spectrum will be achievable with greatly improved accuracy, while full amplitude analyses may allow definitive determinations of JPC quantum numbers. The expanded dataset will also make it possible to revisit open questions, such as whether there is a resonance at the J/ψ J/ψ threshold, and whether X(7100) decays into the ψ(2S) J/ψ channel. At the same time, the extended mass reach will enable a broader search for additional high-mass resonances in the di-J/ψ system. This expanded dataset also opens up exciting opportunities to explore even heavier sectors, particularly those involving bottom quarks. In this regime, searches can target new structures composed of both charm and bottom quarks, as well as fully bottom-quark tetraquark states. For example, the studies in the J/ψ + Υ and Υ + Υ channels have already been initiated, aiming to uncover potential resonant states that could shed further light on the dynamics of heavy-quark interactions.
These achievements were made possible through sustained support from National Natural Science Foundation of China and Ministry of Science and Technology of the People’s Republic of China, Institute of Physics Frontiers and Interdisciplinary Sciences and the School of Physics and Technology as well as Ministry of Education Key Laboratory of NSLSCS of Nanjing Normal University, whose contributions have enabled young scientists to actively participate in cutting-edge research at the LHC.
REFERENCES:
[1] CMS Collaboration, “New structures in the J/ψ J/ψ mass spectrum in proton-proton collisions at √s = 13 TeV”, Phys. Rev. Lett. 132 (2024) 111901, doi:10.1103/PhysRevLett.132.111901, arXiv:2306.07164
[2] https://en.njnu.edu.cn/news/nanjing-normal-university-and-tsinghua-unive... , https://news.njnu.edu.cn/info/1044/113117.htm, 2024
[3] CMS Collaboration observes new all-heavy quark structures, Ingrid Fadelli, Phys.org, 2024, https://phys.org/news/2024-04-cms-collaboration-heavy-quark.html
[4] CMS Collaboration, “Observation of new structures in the J/ψ J/ψ mass spectrum in pp collisions at √s=13 TeV”, 2022, https://cds.cern.ch/record/2815336
[5] CMS Collaboration, “Observation of a family of all-charm tetraquark candidates at the LHC”, 2025, http://cds.cern.ch/record/2929472
[6] CMS Collaboration, “Observation of X(6900) and evidence for X(7100) in the J/ψ ψ(2S) → μ+μ−μ+μ− mass spectrumin 314 fb−1 of pp collisions at CMS”, 2025, http://cds.cern.ch/record/2929529
[7] CMS Collaboration, “Spin and symmetry properties of all-charm tetraquarks”, 2025,
http://cds.cern.ch/record/2929695
[8] CMS Collaboration, “Determination of the spin and parity of all-charm tetraquarks”, 2025, https://www.nature.com/articles/s41586-025-09711-7, https://arxiv.org/abs/2506.07944
[9] Nature Press Release: "Quantifying a quirky quark quartet", https://www.natureasia.com/en/info/press-releases/detail/9171; Elena Santopinto, "Quarks in ‘exotic’ quartets prefer to stick together", https://www.nature.com/articles/d41586-025-03591-7
[10] CMS news briefing (2025): https://cms.cern/news/cms-gets-acquainted-family-all-charm-tetraquarks
Figure 1.The J/ψ J/ψ invariant-mass spectrum up to 9 GeV for Run2 and Run 2+3 data, illustrating the contrast with the enlarged data set.
Figure 2.The J/ψ ψ(2S) invariant-mass spectrum up to 9 GeV for Run 2+3 data.
Figure 3.Results of statistical tests comparing different JiP hypotheses with the 2+m model. Black dots show the observed values, while shaded regions indicate the expected ranges for each model. The data mostly fall within the blue band, consistent with the 2+m model.
Figure 4.Group photo of the faculty and student teams who contributed to the three analyses discussed in this work.
Figure 5.Illustration of the exotic hadron landscape: the J/ψ J/ψ mass spectrum is depicted as a mountain with three prominent peaks, with interference phenomena shown between them. Each peak represents a possible diquark–antidiquark configuration within the spectrum.