First Observation of the Cabibbo-suppressed Decays Xi+(c) ---> Sigma+ pi- pi+ and Xi+(c) ---> Sigma- pi+ pi+ and Measurement of their Branching Ratios

The authors report the first observation of two Cabibbo-suppressed decay modes, {Xi}{sub c}{sup +} {yields} {Sigma}{sup +}{pi}{sup -}{pi}{sup +} and {Xi}{sub c}{sup +} {yields} {Sigma}{sup -} {pi}{sup +}{pi}{sup +}. They observe 56 {+-} 13 over a background of 21, and 23 {+-} 7 over a background of 12 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 {Sigma}{sup -} beam. The branching ratios of the decays relative to the Cabibbo-favored {Xi}{sub c}{sup +} {yields} {Xi}{sup -}{pi}{sup +}{pi}{sup +} are measured to be B({Xi}{sub c}{sup +} {yields} {Sigma}{sup +}{pi}{sup -}{pi}{sup +})/B({xi}{sub c}{sup +} {yields} {Xi}{sup -} {pi}{sup +}{pi}{sup +}) = 0.50 {+-} 0.20, and B({Xi}{sub c}{sup +} {yields} {Sigma}{sup -}{pi}{sup +}{pi}{sup +})/B({Xi}{sub c}{sup +} {yields} {Xi}{sup -}{pi}{sup +}{pi}{sup +}) = 0.23 {+-} 0.11, respectively. They also report branching ratios for the same decay modes of the {Lambda}{sub c}{sup +} relative to {Lambda}{sub c}{sup +} {yields} pK{sup -}{pi}{sup +}.