First observation of $B^-\to D^0 K^-\pi^+\pi^-$ Decays to CP even final states

We report a study of the suppressed $B^- \to D^0 K^-\pi^+\pi^-$ decays, where $D^0 \to K^- \pi^+$, $D^0 \to K^- K^+$ and $D^0 \to \pi^- \pi^+$. These decays are interesting because of the potential sensitivity to the weak phase $\gamma$, when the $D^0$ decays to a final state that is also accessible to $\bar{D^0}$. Here, we report first observations of this decay, where the neutral $D$ meson is observed to decay into the CP even eigenstates, $K^+ K^-$ and $\pi^+\pi^-$. The corresponding $B^- \to D^0\pi^-\pi^+\pi^-$ decays provide a normalization in which many systematic uncertainties cancel. From these rates, we measure the following CP-sensitive relative decay rate \begin{align*} R_{CP+}&=0.95\pm0.11(stat)\pm0.02(syst). \\ \end{align*} \noindent. We also measure several $B^{\pm}$ decay rate asymmetries and find, \begin{align*} A^{CP+}_{s} &= -0.14\phantom{0} \pm 0.10\phantom{0} (stat)\pm0.01\phantom{0}(syst) \\ A^{K^-\pi^+}_{s} &= -0.009 \pm 0.028(stat)\pm0.013(syst) \\ A^{CP+}_{d} &= -0.018 \pm 0.018 (stat)\pm0.007(syst) \\ A^{K^-\pi^+}_{d} &= -0.006 \pm 0.006(stat)\pm0.010(syst), \\ \end{align*} \noindent where the subscripts $d$ and $s$ refer to the favored $B^-\ to D^0\pi^-\pi^+\pi^-$ and suppressed $B^- \to D^0 K^-\pi^+\pi^-$ final states, respectively. The largest sensitivity to $\gamma$ amongst these modes is expected in $A^{CP+}_{s}$.