First observation of the Cabibbo-suppressed decays Ξc+→Σ+π−π+ and Ξc+→Σ−π+π+ and measurement of their branching ratios

We report the first observation of two Cabibbo-suppressed decay modes, Ξc+→Σ+π−π+ and Ξc+→Σ−π+π+. We observe 59±14 over a background of 87, and 22±8 over a background of 13 events, respectively, for the signals. The data were accumulated using the SELEX spectrometer during the 1996-1997 fixed target run at Fermilab, chiefly from a 600GeV/c Σ− beam. The branching ratios of the decays relative to the Cabibbo-favored Ξc+→Ξ−π+π+ are measured to be B(Ξc+→Σ+π−π+)/B(Ξc+→Ξ−π+π+)=0.48±0.20, and B(Ξc+→Σ−π+π+)/B(Ξc+→Ξ−π+π+)=0.18±0.09, respectively. We also report branching ratios for the same decay modes of the Λc+ relative to Λc+→pK−π+.