Observation of an Excited Charm Baryon ${\Omega }_c^{*}$ Decaying to ${\Omega }_c^0\gamma$

We report the first observation of an excited singly charmed baryon ${\Omega }_c^{*}$ ($css$) in the radiative decay ${\Omega }_c^0\gamma$, where the ${\Omega }_c^0$ baryon is reconstructed in the decays to the final states ${\Omega }^{-}{\pi }^{+}$, ${\Omega }^{-}{\pi }^{+}{\pi }^0$, ${\Omega }^{-}{\pi }^{+}{\pi }^{-}{\pi }^{+}$, and ${\Xi }^{-}{K}^{-}{\pi }^{+}{\pi }^{+}$. This analysis is performed using a data set of $230.7{\mathrm{fb}}^{-1}$ collected by the BABAR detector at the PEP-II asymmetric-energy $B$ factory at the Stanford Linear Accelerator Center. The mass difference between the ${\Omega }_c^{*}$ and the ${\Omega }_c^0$ baryons is measured to be $70.8\pm 1.0(\mathrm{stat})\pm 1.1(\mathrm{syst})\mathrm{MeV}/c^2$. We also measure the ratio of inclusive production cross sections of ${\Omega }_c^{*}$ and ${\Omega }_c^0$ in $e^{+}{e}^{-}$ annihilation.

Figure 1

The invariant mass distributions of ${\Omega }_c^0$ candidates reconstructed in the ${\Omega }_c^0$ decay modes into (a) ${\Omega }^{-}{\pi }^{+}$, (b) ${\Omega }^{-}{\pi }^{+}{\pi }^0$, (c) ${\Omega }^{-}{\pi }^{+}{\pi }^{-}{\pi }^{+}$, and (d) ${\Xi }^{-}{K}^{-}{\pi }^{+}{\pi }^{+}$. For all of these, we require $x_p({\Omega }_c^0)>0.5$. Here $M_{{\Omega }_c^0}$ is the reconstructed mass of the ${\Omega }_c^0$ candidates, and $X_h$ denotes the daughter hyperon. The points with error bars represent the data, the dashed line represents the combinatorial background, and the solid line the sum of signal and background.

Figure 2

The invariant mass distributions of ${\Omega }_c^{*}\to {\Omega }_c^0\gamma$ candidates, with ${\Omega }_c^0$ reconstructed in the decay modes (a) ${\Omega }^{-}{\pi }^{+}$, (b) ${\Omega }^{-}{\pi }^{+}{\pi }^0$, (c) ${\Omega }^{-}{\pi }^{+}{\pi }^{-}{\pi }^{+}$, (d) ${\Xi }^{-}{K}^{-}{\pi }^{+}{\pi }^{+}$, and (e) for the combined decay modes [(1, 2, 3, 4)]. For all of these, we require $x_p({\Omega }_c^{*})>0.5$. Here $M_{{\Omega }_c^0}\gamma$ is the reconstructed mass of the ${\Omega }_c^{*}$ candidates, and $M_{{\Omega }_c^0}$ is the reconstructed mass of the ${\Omega }_c^0$. The points with error bars represent the data, the dashed line represents the combinatorial background, and the solid line the sum of signal and background. The shaded histograms represent the mass distribution expected from the mass sideband of ${\Omega }_c^0$.