Charmed Baryon Decays Observed in Ee− Annihilation at SPEAR.

Book excerpt: Various weak decays of the charmed baryon ..lambda../sub c/ are observed in the Mark II detector at the SLAC ee− storage ring SPEAR. Hadronic decays ..lambda../sub c/ ..-->.. pK−..pi..+ and ..lambda../sub c/+ ..-->.. pK/sub s/° and their conjugates are observed as peaks in invariant mass spectra at m(..lambda../sub c/) = (2286 +- 6) MeV/c2. An estimate of the charmed baryon production cross section, sigma(..lambda../sub c/) + sigma (anti ..lambda../sub c/) = (1.7 +- 0.4) nb, derived from Mark II measurements of the inclusive baryon cross sections R/sub p/ and R/sub ..lambda../ as functions of center-of-mass energy, is used to calculate branching ratios for these hadronic decays: BR (..lambda../sub c/+ ..-->.. p K− ..pi..+) = (2.0 +- 0.8)%; BR (..lambda../sub c//sup +/ ..-->.. p k/sub s/°)/BR (..lambda../sub c//sup +/ ..-->.. p K− ..pi../sup +/) = (36 +- 16)%. An attempt is also made to observe higher mass charmed baryons by reconstructing cascade decays ..sigma../sub c/ ..-->.. ..lambda../sub c/..pi ... Evidence for the observation of semileptonic decays of the charmed baryon is presented. Direct electrons are observed in events containing antiprotons, lambdas and antilambdas. The number of electrons per baryon event, after background subtraction and efficiency correction, is used, together with an estimate of the charmed baryon content of proton and lambda events, to calculate inclusive and semi-inclusive semileptonic branching ratios of the ..lambda../sub c.