The Beijing Spectrometer Experiment (BESIII) announced the observation of more new charmonium-like states. The results were published in the latest issue of Physical Review Letters.

The experiment measured the e⁺e^- _{→π⁺π^-J/ψ} and e⁺e^- → _π⁺π^-h_c cross sections with high precision, using 9fb^-1 data collected with the BESIII detector at the BEPCII accelerator, and two resonant structures were observed in each decay mode.

The first structure, Y(4220), with a mass of about 4.22 GeV, was seen in both modes. The Y(4320) was seen in the e⁺e^- _{→ π⁺π^-J/ψ}  process at about 4.32 GeV, and the Y(4390) was seen in the e⁺e^- →  _π⁺π^-h_c mode at about 4.39 GeV. They are all vector particles, and they are different from any of the known vector charmonium and charmonium-like states.

More and more vector particles have been observed between 4 and 4.6 GeV, which is the mass region for many kinds of exotic hadrons containing a pair of charm-anticharm quarks. To investigate these states is at the top of the priority list for theorists.

The possibility of these new states being quark-gluon hybrids, tetraquark states, hadron molecules, hadro-charmonia, or something more exotic, needs to be further studied with the new measurements from the BESIII experiment.

The data analyzed in these papers were taken during 2013-2015. With the same data sample, BESIII has previously discovered the charged charmonium-like states Z_c(3900) and Z_c(4020), as well as the X(3823).

BESIII is collecting more data this year, looking for further discoveries in this area. With these data, the collaboration hopes to strengthen the study of exotic hadrons and better understand the quark confinement of the strong interaction.

Figure: Comparison of the new states observed in e⁺e^- →π⁺π^-J/ψ (open circles) and e⁺e^- → π⁺π^-h_c (red dots) , and the known charmonium and charmonium-like states (Image by IHEP)