First Observation of the Doubly Cabibbo-Suppressed Decay of a Charmed Baryon: ${\mathrm{\Lambda }}_c^{+}\to p{K}^{+}{\pi }^{-}$

We report the first observation of the decay ${\mathrm{\Lambda }}_c^{+}\to p{K}^{+}{\pi }^{-}$ using a $980{\mathrm{fb}}^{-1}$ data sample collected by the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. This is the first observation of a doubly Cabibbo-suppressed decay of a charmed baryon. We measure the branching ratio of this decay with respect to its Cabibbo-favored counterpart to be $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}^{+}{\pi }^{-})/\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+})=(2.35\pm 0.27\pm 0.21)\times {10}^{-3}$, where the uncertainties are statistical and systematic, respectively.

Figure 1

Typical external (internal) $W$-emission diagrams for (a) [(c)] ${\mathrm{\Lambda }}_c^{+}\to p{K}^{+}{\pi }^{-}$ and (b) [(d)] ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$, and (e) a typical $W$-exchange diagram of ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$.

Figure 2

Distribution of $M(p{K}^{-}{\pi }^{+})$. The curves indicate the fit result: the full fit model (solid) and the combinatoric background only (dashed).

Figure 3

Distribution of $M(p{K}^{+}{\pi }^{-})$ (top) and residuals of data with respect to the fitted combinatorial background (bottom). Curves are drawn as described in Fig. 2.

Figure 4

Invariant mass squared of $K^{-}{\pi }^{+}$ versus $p{K}^{-}$ within $2.2746\mathrm{GeV}/c^2<M(p{K}^{-}{\pi }^{+})<2.2986\mathrm{GeV}/c^2$ in real data (top) and estimated efficiency using the MC (bottom). The bin widths of $x$ and $y$ axes are $0.016{\mathrm{GeV}}^2/c^4$ and $0.027{\mathrm{GeV}}^2/c^4$, respectively.