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

Abstract We report the first observation of two Cabibbo-suppressed decay modes, Ξ c + → Σ + π − π + and Ξ c + → 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 600 GeV / c Σ − beam. The branching ratios of the decays relative to the Cabibbo-favored Ξ c + → Ξ − π + π + are measured to be B ( Ξ c + → Σ + π − π + ) / B ( Ξ c + → Ξ − π + π + ) = 0.48 ± 0.20 , and B ( Ξ c + → Σ − π + π + ) / B ( Ξ c + → Ξ − π + π + ) = 0.18 ± 0.09 , respectively. We also report branching ratios for the same decay modes of the Λ c + relative to Λ c + → p K − π + .