XYZ physics at BESIII experiment

With the ability to run above 4~GeV, the BESIII experiment located in the Beijing Electron Positron Collider (BEPCII), has becoming a pioneer in searching and studying charmoniumlike states ($XYZ$ particles). In 2013, BESIII Collaboration discovered a charged charmoniumlike state $Z_c(3900)$, which is confirmed immediately experimentally, and provides the best candidate for a four quark state by now. Continuous studies by BESIII Collaboration show new decay behavior of $Z_c(3900)$, and there are possible partner particle $Z_c(4020)/Z_c(4025)$ existing. By scanning above 4~GeV, BESIII also reveals the potential connection between $Y(4260)$ and $X(3872)$ for the first time, which may help us understand $XYZ$ particles in a new sight.

Observation of $Z_c(3900)$ both by BESIII and Belle

Using $525$ pb$^{-1}$ data collected with the BESIII detector at $e^+e^-$ central-of-mass energy 4.26 GeV, the BESIII Collaboration observed a charged charmoniumlike state $Z_c(3900)topi^{pm}J/psi$ with mass $M[Z_c(3900)]=3899.0pm 3.6 pm 4.9$ MeV/c$^2$, and width $Gamma[Z_c(3900)]=46pm 10 pm 20$ MeV. The significance of this state was estimated to be $>8sigma$ in all kinds of systematic tests. Using $967$ fb$^{-1}$ data recorded by the Belle detector on or near $Upsilon(nS)$, n=1, 2, $cdots$, 5 resonances, the $e^+e^- to pi^+pi^-J/psi$ process has been studied through initial-state-radiation (ISR) method. Except for the $Y(4260)$ resonance, an intermediate state $Z(3900)^{pm}$ has been observed in the $pi^{pm}J/psi$ mass spectrum, with significance $>5.2sigma$. The measured mass $M[Z(3900)^{pm}]=3894.5pm6.6pm4.5$ MeV/c$^2$ and width $Gamma[Z(3900)^{pm}]=63pm24pm26$ MeV, shows good agreement with the BESIII measurement within errors, which means $Z_c(3900)$ and $Z(3900)^{pm}$ is the same state.