Measurement of the branching fraction and decay rate asymmetry of B- --> D_{pi+pi-pi0}K-

We report the observation of the decay $B^- \to D_{\pi^+\pi^-\pi^0} K^-$, where $D_{\pi^+\pi^-\pi^0}$ indicates a neutral $D$ meson detected in the final state $\pi^+\pi^-\pi^0$, excluding $K_S \pi^0$. This doubly Cabibbo-suppressed decay chain can be used to measure the CKM phase $\gamma$. Using about 229 million $e^+e^- \to B \bar B$ events recorded by the BaBar experiment at the PEP-II $e^+e^-$ storage ring, we measure the branching fraction ${\cal B}(B^- \to D_{\pi^+\pi^-\pi^0} K^-) = (5.5 \pm 1.0 {\rm(stat.)} \pm 0.7 {\rm(syst.)})\times 10^{-6}$ and the decay rate asymmetry $A = 0.02 \pm 0.16 {\rm(stat.)} \pm 0.03 {\rm(syst.)}$ for the full decay chain.