Observation of the Cabibbo-Suppressed Decay ${\Xi }_{\mathit{c}}^{+}\to {\mathit{pK}}^{-}{\pi }^{+}$

We report the first observation of the Cabibbo-suppressed charm baryon decay ${\Xi }_c^{+}\to {\mathrm{pK}}^{-}{\pi }^{+}$. We observe $150\pm 22\pm 5$ events for the signal. The data were accumulated using the SELEX spectrometer during the 1996–1997 fixed target run at Fermilab, chiefly from a $600\mathrm{GeV}/c$ ${\Sigma }^{-}$ beam. The branching fractions of the decay relative to the Cabibbo-favored ${\Xi }_c^{+}\to {\Sigma }^{+}{K}^{-}{\pi }^{+}$ and ${\Xi }_c^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+}$ are measured to be $B({\Xi }_c^{+}\to {\mathrm{pK}}^{-}{\pi }^{+})/B({\Xi }_c^{+}\to {\Sigma }^{+}{K}^{-}{\pi }^{+})=0.22\mathrm{}\pm 0.06\pm 0.03$ and $B({\Xi }_c^{+}\to {\mathrm{pK}}^{-}{\pi }^{+})/B({\Xi }_c^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+})=0.20\mathrm{}\pm 0.04\pm 0.02$, respectively.