According to the quark model, all hadrons consist of 2 or 3 quarks. However, quantum chromodynamics (QCD) allows the existence of multi-quarks, hybrid states and glueballs. Based on that, the observations of those new hadronic states are important to test and develop the QCD and hadron phenomenology models, which is also one of the most important goals in high energy physics experiments. In the past few years, Lattice QCD(LQCD) has gone through many developments and reached important achievements. It provides important theoretical predictions for the spectroscopy and new hadron searches, and predicts that the ground state of the pseudo-scalar glueball has a mass around 2.3~2.6 GeV/c2.
In BESIII experiment, a lot of charmonium particles are produce in electron-positron collisions. Radiative decays of the J/ψ meson are ideally suited for light hadron spectroscopy studies, including in particular searches for exotic hadrons, e.g., of glueballs and hybirds. At present, the available knowledge in the mass region larger than 2 GeV/c2 is limited. In 2012, the X(2370) was firstly observed in the J/ψ radiative decays which showed to be consistent with the LQCD prediction on the ground state mass of the pseudo-scalar glueball. Intensive and extensive investigations are necessary to explore the structure of particles, search for more particles and shrink the blinded region with the mass larger than 2 GeV/c2. Such progress would help to bring better understanding for new hardon spectroscopy.
By far, a series of new particles were observed in the π+π-η' mass spectrum via J/ψ radiative decays, including the X(1835), X(2120), X(2370) and X(2600). These experimental results provide important clues and research portal for understanding of the new hadron spectroscopy.
The PRL publication: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.042001