Charmonium and Charmoniumlike States at the BESIII Experiment

Charmonium is a bound state of a charmed quark and a charmed antiquark, and a charmoniumlike state is a resonant structure that contains a charmed quark and antiquark pair but has properties that are incompatible with a conventional charmonium state. While operating at center-of-mass energies from 2 to 4.9 GeV, the BESIII experiment can access a wide mass range of charmonium and charmoniumlike states, and has contributed significantly in this field. We review BESIII results involving conventional charmonium states, including the first observation of M1 transition $\psi(3686) \to \gamma \eta_c(2S)$ and the discovery of the $\psi_2(3823)$; and report on studies of charmoniumlike states, including the discoveries of the $Z_c(3900)$ and $Z_c(4020)$ tetraquark candidates, the resolution of the fine structure of the $Y(4260)$, the discovery of the new production process $e^+e^-\to \gamma X(3872)$, and the uncovering of strong evidence for the commonality among the $X(3872)$, $Y(4260)$, and $Z_c(3900)$ states. The prospects for further research at BESIII and proposed future facilities are also presented.