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Observation of the Doubly Cabibbo-Suppressed Decay $D^+_s \to K^+ K^+ π^-$

We report the first observation of the doubly Cabibbo-suppressed decay $D^+_s\to K^+K^+π^-$ using 605 fb$^{-1}$ of data collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The branching ratio with respect to its Cabibbo-favored counterpart $\mathcal{B}(D^+_s\to K^+K^+π^-)$ / $\mathcal{B}(D^+_s\to K^+K^-π^+)$ is (0.229$\pm0.028\pm$0.012)%, where the first uncertainty is statistical and the second is systematic. We also report a significantly improved measurement of the doubly Cabibbo-suppressed decay $D^+\to K^+π^+π^-$, with a branching ratio $\mathcal{B}(D^+\to K^+π^+π^-)$ / $\mathcal{B}(D^+\to K^-π^+π^+)$=(0.569$\pm0.018\pm$0.014)%.