It was learned from Tsinghua University that at the 41st International High Energy Physics Conference held in Italy recently, the cooperative group of the compact muon solenoid detector (CMS) of the Large Hadron Collider (LHC) of the European Center for nuclear research reported that they found a strange particle family that may be composed of four charm quarks. Professor Yi Kai of China CMS group reported this result on behalf of the CMS cooperation group. The "Tsinghua Nanshi" CMS group jointly formed by researchers from Tsinghua University and Nanjing Normal University played an original and leading role in this research. "Because these particles are internally composed of the same heavy flavor quarks, the physical image is relatively simple and easy to understand, which provides a new window for us to study strong interactions." Yi Kai said that this is the first time that the Chinese experimental team has observed all charm four quark particles on the LHC, and it is also the first time that China has led the discovery of new particles in the CMS experiment. In the framework of the standard model of particle physics, the basic particles of the material world are quarks, leptons and so on. Two quarks or three quarks form the traditional mesons and baryons respectively, which are collectively referred to as "traditional" hadrons. Other hadrons, such as those composed of four or even five quarks, or those containing gluons, are called "strange" hadrons. Theorists predicted the existence of traditional hadrons and strange hadrons decades ago. However, until the last 20 years, scientists have observed relatively clear four quark or five quark strange hadrons in experiments. The possible strange hadrons discovered in the early stage only include some lighter mesons, while the possible strange meson x (3872) discovered by Belle cooperation group in 2003 has set off an upsurge of research on "heavy flavor" strange hadrons in the world in recent 20 years, and has become one of the long-term hot spots of high-energy physics. Later, the charged strange hadron ZC (3900) + discovered by the Chinese BESIII cooperation group confirmed the odd characteristic of this kind of strange hadron, which is different from the traditional charmolin - electrification, and further pushed the research of heavy flavor strange hadrons to a climax. But the experimental study of strange hadrons is often full of twists and turns. For example, the X (4140) particle was found in the CDF experiment in 2009, and was denied by other experiments. It took lhcb 8 years to reconfirm its existence with a larger statistic in 2017. Another example is the five quark state. Although it first appeared in many experiments at the beginning of this century (which was later verified to be statistical fluctuations), only the five quark state discovered by lhcb in 2015 was first widely recognized and was selected as one of the top ten breakthroughs in world physics that year. "Previously, we have not found all strange hadron families that may be composed of pure heavy flavor quarks, that is, strange particles composed of charm quarks or bottom quarks." Yi Kai said. In 2013, Yi Kai used the collision data collected from 2011 to 2012 to analyze J/ ψ J/ ψ The mass spectrum is preliminarily studied. In 2019, Yi Kai, a professor of the school of physical science and technology of Nanjing Normal University and a visiting professor of Tsinghua University, and Hu Zhen, an associate professor of the Department of physics of Tsinghua University, formed a research team at Tsinghua University and Nanjing Normal University. Relying on Tsinghua University, researchers from the two universities joined the CMS experiment and jointly formed the "Tsinghua Nanshi" CMS group, with Yi Kai as the head and Hu Zhen as the deputy head. The "Tsinghua Nanshi" CMS team conducted a comprehensive and systematic study on the data collected from 2016 to 2018 in the low-quality multi lepton terminal state. Based on all the "proton proton" collision data collected from 2016 to 2018, the CMS cooperation group observed the full charm four quark particle family (a group of strange hadrons composed of heavy quarks with the same flavor), and the "Tsinghua Nanshi" CMS group played a leading role in this discovery. The three formants in this family are named x (6600), X (6900) and X (7300) according to mass, of which X (6600) and X (7300) particles are all observed for the first time in the world; The evidence of the existence of X (6900) particle was observed by lhcb experiment two years ago, and the results of CMS confirmed the existence of X (6900). Yi Kai said that the discovery is an important breakthrough in the field of strange hadrons: "this unique and easy to understand system can help theorists build the final model for dozens of strange hadrons discovered in the past 20 years." For the research results of CMS experiment, academicians of the Chinese Academy of Sciences Zhao Guangda, a theoretical particle physicist at Peking University, commented: "The discovery of the full charm four quark state family is of great significance to our understanding of strong interactions. In this system, because there is no light quark component, it is unlikely to form a 'common' molecular state structure formed by exchanging light mesons, which provides us with a simpler and easier to understand physical image. I am very happy that Chinese experimental particle physicists have played a leading role in this discovery, and I hope Make persistent efforts in future work. " Dmitry Denisov, deputy director of Brookhaven National Laboratory and former spokesman of d0 experiment, participated in the report and discussion of the results at the meeting, He said: "understanding strong interactions is one of the most important issues in particle physics research. These new resonances discovered by CMS Cooperation Group on LHC pave the way for a deeper understanding of the interaction between fundamental particles. We look forward to more exciting discoveries in the future!" The CMS cooperation group is composed of more than 4000 members from more than 50 countries and about 240 units. Among them, the members of the China group come from the Institute of high energy, Chinese Academy of Sciences, Peking University, University of science and technology of China, Beijing University of Aeronautics and Astronautics, Tsinghua University, Sun Yat sen University, Zhejiang University, Fudan University, Nanjing Normal University and other units. In recent years, China CMS group has made outstanding achievements in Higgs particle property measurement and multi boson research. The discovery of the full charm four quark state family has further improved the status of China group in international cooperation. (Xinhua News Agency)