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 the $psi(1^3D_2)$ state in $e^+e^-topi^+pi^-gammachi_{c1}$ at BESIII

We report the observation of the $X(3823)$ in the process $e^+e^-to pi^+pi^-X(3823) to pi^+pi^-gammachi_{c1}$ with a statistical significance of $6.2sigma$, in data samples at center-of-mass energies $sqrt{s}=$4.230, 4.260, 4.360, 4.420 and 4.600~GeV collected with the BESIII detector at the BEPCII electron positron collider. The measured mass of the $X(3823)$ is $(3821.7pm 1.3pm 0.7)$~MeV/$c^2$, where the first error is statistical and the second systematic, and the width is less than $16$~MeV at the 90% confidence level. The products of the Born cross sections for $e^+e^-to pi^+pi^-X(3823)$ and the branching ratio $mathcal{B}[X(3823)to gammachi_{c1,c2}]$ are also measured. These measurements are in good agreement with the assignment of the $X(3823)$ as the $psi(1^3D_2)$ charmonium state.

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.

Observation of $e^{+}e^{-} to eta J/psi$ at center-of-mass energy $sqrt{s}=4.009$ GeV

Using a 478 pb$^{-1}$ data sample collected with the BESIII detector operating at the BEPCII storage ring at a center-of-mass energy of $sqrt{s}=4.009$ GeV, the production of $e^{+}e^{-}to eta Jpsi$ is observed for the first time with a statistical significance of greater than $10sigma$. The Born cross section is measured to be $(32.1pm 2.8 pm 1.3)$ pb, where the first error is statistical and the second systematic. Assuming the $eta Jpsi$ signal is from a hadronic transition of the $psi(4040)$, the fractional transition rate is determined to be ${cal B}(psi(4040)to eta Jpsi)=(5.2pm 0.5pm 0.2pm 0.5)times 10^{-3}$, where the first, second, and third errors are statistical, systematic, and the uncertainty from the $psi(4040)$ resonant parameters, respectively. The production of $e^{+}e^{-} to pi^0 Jpsi$ is searched for, but no significant signal is observed, and ${cal B}(psi(4040)to pi^0 Jpsi) < 2.8 times 10^{-4}$ is obtained at the 90% confidence level.

Higher-order multipole amplitude measurement in $psi(2S)togammachi_{c2}$

Using $106times10^6$ $psi(2S)$ events collected with the BESIII detector at the BEPCII storage ring, the higher-order multipole amplitudes in the radiative transition $psi(2S)togammachi_{c2}togammapipi/gamma KK$ are measured. A fit to the $chi_{c2}$ production and decay angular distributions yields $M2=0.046pm0.010pm0.013$ and $E3=0.015pm0.008pm0.018$, where the first errors are statistical and the second systematic. Here $M2$ denotes the normalized magnetic quadrupole amplitude and $E3$ the normalized electric octupole amplitude. This measurement shows evidence for the existence of the $M2$ signal with $4.4sigma$ statistical significance and is consistent with the charm quark having no anomalous magnetic moment.