First Observation of the M1 Transition ψ(3686)→γηc(2S)

Le résumé fourni par la source

Using a sample of $106\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $\ensuremath{\psi}(3686)$ events collected with the BESIII detector at the BEPCII storage ring, we have made the first measurement of the $M1$ transition between the radially excited charmonium $S$-wave spin-triplet and the radially excited $S$-wave spin-singlet states: $\ensuremath{\psi}(3686)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{c}(2S)$. Analyses of the processes $\ensuremath{\psi}(3686)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{c}(2S)$ with ${\ensuremath{\eta}}_{c}(2S)\ensuremath{\rightarrow}{K}_{S}^{0}{K}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$ and ${K}^{+}{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}$ give an ${\ensuremath{\eta}}_{c}(2S)$ signal with a statistical significance of greater than 10 standard deviations under a wide range of assumptions about the signal and background properties. The data are used to obtain measurements of the ${\ensuremath{\eta}}_{c}(2S)$ mass ($M\mathbf{(}{\ensuremath{\eta}}_{c}(2S)\mathbf{)}=3637.6\ifmmode\pm\else\textpm\fi{}{2.9}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{1.6}_{\mathrm{syst}}\text{ }\text{ }\mathrm{MeV}/{c}^{2}$), width ($\ensuremath{\Gamma}\mathbf{(}{\ensuremath{\eta}}_{c}(2S)\mathbf{)}=16.9\ifmmode\pm\else\textpm\fi{}{6.4}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{4.8}_{\mathrm{syst}}\text{ }\text{ }\mathrm{MeV}$), and the product branching-fraction ($\mathcal{B}\mathbf{(}\ensuremath{\psi}(3686)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{c}(2S)\mathbf{)}\ifmmode\times\else\texttimes\fi{}\mathcal{B}\mathbf{(}{\ensuremath{\eta}}_{c}(2S)\ensuremath{\rightarrow}K\overline{K}\ensuremath{\pi}\mathbf{)}=(1.30\ifmmode\pm\else\textpm\fi{}{0.20}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.30}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$). Combining our result with a BABAR measurement of $\mathcal{B}\mathbf{(}{\ensuremath{\eta}}_{c}(2S)\ensuremath{\rightarrow}K\overline{K}\ensuremath{\pi}\mathbf{)}$, we find the branching fraction of the $M1$ transition to be $\mathcal{B}\mathbf{(}\ensuremath{\psi}(3686)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{c}(2S)\mathbf{)}=(6.8\ifmmode\pm\else\textpm\fi{}{1.1}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{4.5}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.