Nanjing Normal University and Tsinghua University CMS team sights a charming family of tetraquarks at CMS

The CMS experiment at the LHC, the world’s largest particle collider located in Geneva (Switzerland), has reported a triplet of structures, dubbed X(6600), X(6900), and X(7100), which are prime candidates for a family of particles made of four charm quarks—a novel form of matter. The result is published in the leading physics journal Physical Review Letters (http://link.aps.org/doi/10.1103/PhysRevLett.132.111901), and is recommended by the journal’s “Editor’s Suggestion”, which states: “Three structures, X(6900) and two new ones around 6.64 and 7.13 GeV, are seen in the J/ψJ/ψ mass spectrum that are consistent with being part of a family of radial excitations” (Fig. 1 (left)). Physicists from Nanjing Normal University (NNU) and Tsinghua University (THU), led by Professor Kai Yi, played a leading role in obtaining these results, along with physicists from the University of Iowa, Fudan University, the Moscow Institute of Physics and Technology, and Henan Normal University. 

For almost four decades all strongly interacting particles (hadrons), like the proton, were thought to be made only of pairs, or triplets, of fundamental particles called quarks; but since the discovery of the X(3872) in 2003, there has been intense interest in particles composed of quartets or quintets of quarks---known as `exotic hadrons.’ Although many such states are now known, they all involve light (u,d,s) quarks, which are difficult to theoretically model, leaving the substructure of such states a topic of debate. On the other hand, systems of heavy ( c,b ) quarks are much easier to understand. But it was only in 2020 that the first candidate of an all-heavy tetraquark, the X(6900), was found by the LHCb experiment.

CMS has now reported three such structures. But, as remarked by Prof. Kai Yi, “It has been a long journey to go from a hint of the two lower mass structures that was actually seen back in 2013, in data that CMS collected during 2011 and 2012 (Run I), to observe the strong signals now seen in data from 2016 to 2018 (Run II).” Having those hints in mind from LHC’s Run I, the analysis team designed a search strategy in Run II without actually looking at the data. This approach is called a `blind’ analysis which is very effective in avoiding potential biases—such as fooling oneself into seemingly find what one thinks one is supposed to, or wants to, find! The three structures jumped out after the new data was finally unblinded.

 
Fig. 1: The online notice for the CMS paper in Physical Review Letters (left). The measured double-J/ψ mass distribution and the results of fit with three signal peaks superimposed on a continuum background, including the effects of quantum interference among the three states (right).

CMS’s three structures, decaying into a pair of J/ψ  particles, are shown in Fig. 1 (right), with X(6600) and X(7100), being newly sighted. The J/ψ is a charm-anticharm bound state, and thus these structures are naturally interpreted as all-charm tetraquarks. However, finding two new structures is not just adding two more of the same: having multiple candidates allows qualitatively new comparisons among them because one can look for patterns among them, and theoretical predictions are more reliable for comparative measurements between particles.

In this vein, CMS found the three structures displaying a quirky phenomenon known as quantum interference among all three, which demands that they all have the same quantum charateristics. This suggests that these states form a family of excited tetraquarks. As such, the triplet provides important new clues to unravel their nature, and more generally, unlock the internal structure of exotics, which in turn represents a new regime in which to study the theory of strong interactions, quantum chromodynamics. 

Echoing this, theorist Stan Brodsky from Stanford University noted: “This observation opens up an important new field of doubly-heavy tetraquarks… Clearly this is just the beginning — we predict a huge array of tetraquarks: diquark/antidiquark bound states with all possible flavors.”As well, Wolfgang Adam, a senior physicist at the Institute of High Energy Physics of the Austrian Academy of Sciences, who has closely followed the progress of this study as chair of its CMS-internal review body, said, “Recently, the study of exotic hadrons has attracted substantial interest and constitutes a very active field of research. The new result adds an exciting new element to this domain and opens the path to further exploration. The publication demonstrates CMS’s potential in this field, thanks to the excellent capacities of the detector, in particular in the reconstruction of muons, the large dataset, and the relentless efforts of the groups performing the analysis of these data. In particular, CMS groups from China made significant contributions to the CMS detector and took a leading role in this analysis, and I am looking forward to many important contributions from them.” 

Hesheng Chen, an Academician of the Chinese Academy of Sciences, and the former leader of the Chinese LHC Project, exclaimed, "Congratulations to the Chinese CMS team! This is the first time that a Chinese CMS team led the  discovery of unexpected new particles in the CMS experiment for 30 years, since the introduction of the LHC project to IHEP (Beijing) by Carlo Rubbio (the general director of CERN at that time) in 1993."

The LHC is currently collecting more proton-proton collision data at an energy of 13.6 TeV, and CMS is preparing to improve their measurements of the properties of these states, such as their spin-parity, cross section, etc. This new data presents a new exciting possibility—CMS is seeking potential exotic states that are composed purely of the even heavier bottom quarks.

The China CMS teams receive support from the Chinese Academy of Sciences (CAS), the Ministry of Science and Technology of China (MoST), and the National Science Foundation of China (NSFC). The NNU/THU CMS team is funded by the NSFC under Grants No. 11975010, No. 12075123, and No. 12061141002, as well as by MoST, the Nanjing Normal University research start-up funding project, and Tsinghua University's Initiative Scientific Research and Dushi Programs. NNU was previously part of the joint THU-NNU team and became a full-fledged member of CMS in April 2023.