First observation of $\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^0$ and evidence of $\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^{+}\ddot{I}{€}^{-}$

Abstract

We present the first observation of the singly Cabibbo-suppressed decay $\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^0$ with a significance of $5.7\ddot{I}ƒ$ and the first evidence of $\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ decay with a significance of $3.1\ddot{I}ƒ$, based on $e^{+}{e}^{-}$ annihilation data recorded by the BESIII detector at the BEPCII collider. The data correspond to an integrated luminosity of $6.4\mathrm{f}{\mathrm{b}}^{-1}$, in the center-of-mass energy range from $4.600\mathrm{GeV}$ to $4.950\mathrm{GeV}$. We determine the branching fractions of $\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^0$ and $\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ relative to their Cabibbo-favored counterparts to be $\frac{\mathcal{B}(\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^0)}{\mathcal{B}(\hat{I}{›}_c^{+}\to \hat{I}›\ddot{I}{€}^{+}\ddot{I}{€}^0)}=(2.09\pm 0.39_{stat.}\pm 0.07_{syst.})\times 10^{-2}$ and $\frac{\mathcal{B}(\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^{+}\ddot{I}{€}^{-})}{\mathcal{B}(\hat{I}{›}_c^{+}\to \hat{I}›\ddot{I}{€}^{+}\ddot{I}{€}^{+}\ddot{I}{€}^{-})}=(1.13\pm 0.41_{stat.}\pm 0.06_{syst.})\times 10^{-2}$, respectively. Moreover, by combining our measured result with the world average of $\mathcal{B}(\hat{I}{›}_c^{+}\to \hat{I}›\ddot{I}{€}^{+}\ddot{I}{€}^0)$, we obtain the branching fraction $\mathcal{B}(\hat{I}{›}_c^{+}\to \hat{I}›K^{+}\ddot{I}{€}^0)=(1.49\pm 0.27_{stat.}\pm 0.05_{syst.}\pm 0.08_{ref.})\times 10^{-3}$. This result significantly departs from theoretical predictions based on quark $SU(3)$ flavor symmetry, which is underpinned by the presumption of meson pair $S$-wave amplitude dominance.