Observation of the charmed baryonΣc+and measurement of the isospin mass splittings of theΣc

We observe the [Sigma][sub [ital c]][sup +] baryon decaying to [Lambda][sub [ital c]][sup +][pi][sup 0] and measure the mass difference [ital M]([Sigma][sub [ital c]][sup +])[minus][ital M]([Lambda][sub [ital c]][sup +]) to be 168.5[plus minus]0.2 MeV/[ital c][sup 2]. We also measure the mass differences [ital M]([Sigma][sub [ital c]][sup ++])[minus][ital M]([Lambda][sub [ital c]][sup +]) and [ital M]([Sigma][sub [ital c]][sup 0])[minus][ital M]([Lambda][sub [ital c]][sup +]) with improved precision and determine the isospin mass splittings [ital M]([Sigma][sub [ital c]][sup ++])[minus][ital M]([Sigma][sub [ital c]][sup 0]) and [ital M]([Sigma][sub [ital c]][sup +])[minus][ital M]([Sigma][sub [ital c]][sup 0]) to be 1.1[plus minus]0.4[plus minus]0.1 MeV/[ital c][sup 2] and 1.4[plus minus]0.5[plus minus]0.3 MeV/[ital c][sup 2], respectively. Our results indicate that the light quark mass difference does not dominate the isospin mass splitting in [Sigma][sub [ital c]].