Observation of Two Narrow States Decaying into ${\mathit{\Xi }}_{\mathit{c}}^{+}\mathit{\gamma }$ and ${\mathit{\Xi }}_{\mathit{c}}^0\mathit{\gamma }$

We report the first observation of two narrow charmed strange baryons decaying to ${\Xi }_c^{+}\gamma$ and ${\Xi }_c^0\gamma$, respectively, using data from the CLEO II detector at the Cornell Electron Storage Ring. We interpret the observed signals as the ${\Xi }_c^{+\prime }\left(c\{\mathrm{su}\}\right)$ and ${\Xi }_c^{0\prime }\left(c\{\mathrm{sd}\}\right)$, the symmetric partners of the well-established antisymmetric ${\Xi }_c^{+}\left(c\right[\mathrm{su}\left]\right)$ and ${\Xi }_c^0\left(c\right[\mathrm{sd}\left]\right)$. The mass differences $M\left({\Xi }_c^{+\prime }\right)-M\left({\Xi }_c^{+}\right)$ and $M\left({\Xi }_c^{0\prime }\right)-M\left({\Xi }_c^0\right)$ have been measured to be $107.8\pm 1.7\pm 2.5$ and $107.0\pm 1.4\pm 2.5\mathrm{MeV}{/c}^2$, respectively.