M. N. Achasov

We study the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}$ at a center-of-mass energy of 4.260 GeV using a $525\text{ }\text{ }{\mathrm{pb}}^{\ensuremath{-}1}$ data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be $(62.9\ifmmode\pm\else\textpm\fi{}1.9\ifmmode\pm\else\textpm\fi{}3.7)\text{ }\text{ }\mathrm{pb}$, consistent with the production of the $Y(4260)$. We observe a structure at around $3.9\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ in the ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}J/\ensuremath{\psi}$ mass spectrum, which we refer to as the ${Z}_{c}(3900)$. If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}J/\ensuremath{\psi}$ invariant mass spectrum, neglecting interference, results in a mass of $(3899.0\ifmmode\pm\else\textpm\fi{}3.6\ifmmode\pm\else\textpm\fi{}4.9)\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and a width of $(46\ifmmode\pm\else\textpm\fi{}10\ifmmode\pm\else\textpm\fi{}20)\text{ }\text{ }\mathrm{MeV}$. Its production ratio is measured to be $R=(\ensuremath{\sigma}\mathbf{(}{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{Z}_{c}(3900{)}^{\ensuremath{\mp}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}\mathbf{)}/\ensuremath{\sigma}({e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}))=(21.5\ifmmode\pm\else\textpm\fi{}3.3\ifmmode\pm\else\textpm\fi{}7.5)%$. In all measurements the first errors are statistical and the second are systematic.

We study ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{h}_{c}$ at center-of-mass energies from 3.90 to 4.42 GeV by using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross sections are measured at 13 energies and are found to be of the same order of magnitude as those of ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}$ but with a different line shape. In the ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{h}_{c}$ mass spectrum, a distinct structure, referred to as ${Z}_{c}(4020)$, is observed at $4.02\text{ }\text{ }\mathrm{GeV}/{c}^{2}$. The ${Z}_{c}(4020)$ carries an electric charge and couples to charmonium. A fit to the ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{h}_{c}$ invariant mass spectrum, neglecting possible interferences, results in a mass of $(4022.9\ifmmode\pm\else\textpm\fi{}0.8\ifmmode\pm\else\textpm\fi{}2.7)\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and a width of $(7.9\ifmmode\pm\else\textpm\fi{}2.7\ifmmode\pm\else\textpm\fi{}2.6)\text{ }\text{ }\mathrm{MeV}$ for the ${Z}_{c}(4020)$, where the first errors are statistical and the second systematic. The difference between the parameters of this structure and the ${Z}_{c}(4025)$ observed in the ${D}^{*}{\overline{D}}^{*}$ final state is within $1.5\ensuremath{\sigma}$, but whether they are the same state needs further investigation. No significant ${Z}_{c}(3900)$ signal is observed, and upper limits on the ${Z}_{c}(3900)$ production cross sections in ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{h}_{c}$ at center-of-mass energies of 4.23 and 4.26 GeV are set.

We study the process e(+)e(-) -> (D* (D) over bar*)(+/-)pi(-/+) at a center-of-mass energy of 4.26 GeV using a 827 pb(-1) data sample obtained with the BESIII detector at the Beijing Electron Positron Collider. Based on a partial reconstruction technique, the Born cross section is measured to be (137 +/- 9 +/- 15) pb. We observe a structure near the (D* (D) over bar*)(+/-) threshold in the pi(-/+) recoil mass spectrum, which we denote as the Z(c)(+/-) (4025). The measured mass and width of the structure are (4026.3 +/- 2.6 +/- 3.7) MeV/c(2) and (24.8 +/- 5.6 +/- 7.7) MeV, respectively. Its production ratio sigma(e(+)e(-) -> Z(c)(+/-)(4025)pi(-/+)-> (D* (D) over bar*)(+/-)pi(-/+)/sigma(e(+)e(-) -> (D* (D) over bar*)(+/-)pi(-/+) is determined to be 0.65 +/- 0.09 +/- 0.06. The first uncertainties are statistical and the second are systematic.

With data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies from 4.009 to 4.420 GeV, the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}X(3872)$ is observed for the first time with a statistical significance of $6.3\ensuremath{\sigma}$. The measured mass of the $X(3872)$ is $(3871.9\ifmmode\pm\else\textpm\fi{}0.{7}_{\text{stat}}\ifmmode\pm\else\textpm\fi{}0.{2}_{\text{syst}})\text{ }\text{ }\mathrm{MeV}/{c}^{2}$, in agreement with previous measurements. Measurements of the product of the cross section $\ensuremath{\sigma}[{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}X(3872)]$ and the branching fraction $\mathcal{B}[X(3872)\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}]$ at center-of-mass energies 4.009, 4.229, 4.260, and 4.360 GeV are reported. Our measurements are consistent with expectations for the radiative transition process $Y(4260)\ensuremath{\rightarrow}\ensuremath{\gamma}X(3872)$.

Using a sample of $1.06\ifmmode\times\else\texttimes\fi{}{10}^{8}$ ${\ensuremath{\psi}}^{\ensuremath{'}}$ decays collected by the BESIII detector, ${\ensuremath{\chi}}_{c0}$ and ${\ensuremath{\chi}}_{c2}$ decays into ${\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ and $\ensuremath{\eta}\ensuremath{\eta}$ are studied. The branching fraction results are $\mathrm{Br}({\ensuremath{\chi}}_{c0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0})=(3.23\ifmmode\pm\else\textpm\fi{}0.03\ifmmode\pm\else\textpm\fi{}0.23\ifmmode\pm\else\textpm\fi{}0.14)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, $\mathrm{Br}({\ensuremath{\chi}}_{c2}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0})=(8.8\ifmmode\pm\else\textpm\fi{}0.2\ifmmode\pm\else\textpm\fi{}0.6\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, $\mathrm{Br}({\ensuremath{\chi}}_{c0}\ensuremath{\rightarrow}\ensuremath{\eta}\ensuremath{\eta})=(3.44\ifmmode\pm\else\textpm\fi{}0.10\ifmmode\pm\else\textpm\fi{}0.24\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, and $\mathrm{Br}({\ensuremath{\chi}}_{c2}\ensuremath{\rightarrow}\ensuremath{\eta}\ensuremath{\eta})=(6.5\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\pm\else\textpm\fi{}0.5\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, where the uncertainties are statistical, systematic due to this measurement, and systematic due to the branching fractions of ${\ensuremath{\psi}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{cJ}$. The results provide information on the decay mechanism of ${\ensuremath{\chi}}_{c}$ states into pseudoscalars.