Observation of theχc2(2P)meson in the reactionγγ→DD¯atBABAR

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Abstract

A search for the $Z(3930)$ resonance in $\ensuremath{\gamma}\ensuremath{\gamma}$ production of the $D\overline{D}$ system has been performed using a data sample corresponding to an integrated luminosity of $384\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ recorded by the BABAR experiment at the PEP-II asymmetric-energy electron-positron collider. The $D\overline{D}$ invariant mass distribution shows clear evidence of the $Z(3930)$ state with a significance of $5.8\ensuremath{\sigma}$. We determine mass and width values of $(3926.7\ifmmode\pm\else\textpm\fi{}2.7\ifmmode\pm\else\textpm\fi{}1.1)\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and $(21.3\ifmmode\pm\else\textpm\fi{}6.8\ifmmode\pm\else\textpm\fi{}3.6)\text{ }\text{ }\mathrm{MeV}$, respectively. A decay angular analysis provides evidence that the $Z(3930)$ is a tensor state with positive parity and $C$ parity (${J}^{PC}={2}^{++}$); therefore we identify the $Z(3930)$ state as the ${\ensuremath{\chi}}_{c2}(2P)$ meson. The value of the partial width ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}\ifmmode\times\else\texttimes\fi{}\mathcal{B}(Z(3930)\ensuremath{\rightarrow}D\overline{D})$ is found to be $(0.24\ifmmode\pm\else\textpm\fi{}0.05\ifmmode\pm\else\textpm\fi{}0.04)\text{ }\text{ }\mathrm{keV}$.

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