Observation of Charmed Baryons in Electron-Positron Annihilations at 10GEV Center of Mass Energy.

The charmed baryons Lambda_ {c} and Sigma_{c } have been observed using the ARGUS detector in the DORIS II e^{+}e ^{-} storage at DESY. The ARGUS experiment is a 4pi magnetic solenoidal detector used to observe e^{+ }e^{-} annihilations in the center-of-mass energy region near 10GeV. The Lambda_{c} and its charge conjugate, were observed in three decay modes. In its decay to pK^{ -}pi^{+} the measured sigma cdot Br was 9.0 +/- 1.2 +/- 1.0 pb, for Lambdapi^ {+}pi^{-}pi^ {+} sigma cdot Br. was observed to be 5.5 +/- 1.3 +/- 0.8, and for K^0 p sigma cdot Br was 5.6 +/- 1.1 +/- 0.7pb. These measurements were all made at a mean centre-of-mass energy of 10.2GeV. The mean of the Lambda_{c} mass measured in these decay channels was 2283.1 +/- 1.7 +/- 2.0MeV/c^2 . Two members of the lowest isospin-triplet charmed baryons, Sigma_{c} were observed. The Sigma_sp{c }{++} and the Sigma _sp{c}{0} were observed to decay to Lambda_{c}^ {+}pi^{+} and Lambda_{c}^{+ }pi^{-} respectively, where the Lambda_{c} was then observed in the decay modes already mentioned. The mass difference between the Sigma_sp {c}{++} and the Lambda _{c} was 168.5 +/- 0.5MeV/c^2, and the Sigma_sp{c}{0} Lambda _{c} mass difference was 167.0 +/- 0.5MeV/c^2. The mean Sigma_{c} Lambda _{c} mass difference was 167.8 +/- 1.8 +/- 2.0. The isospin mass splitting between the Sigma _sp{c}{++} and the Sigma_sp{c}{0} was 1.5 +/- 0.7 +/- 0.3MeV/c^2. Studies were made into the nature of charmed baryon production in e^{+}e ^{-} annihilations. Limits for the direct hadronic production of Lambda _{c}'s from the Upsilon (1S) and Upsilon(2S) resonances, relative to the continuum, of 15% and 20% respectively, were established. The momentum spectrum of the Lambda_{c} was quite hard. Fitting the x_{p}^ectrum of the Lambda_{c} with the well known Peterson splitting function gave a value of epsilon = 0.236 +/- .035. The x_{p}^ectrum for the Sigma_{c} was softer, and had a Peterson epsilon of 0.59 +/- 0.16. The rate of Lambda _{c} production from Sigma _{c} decays was measured to be 39 +/- 12%.