First observations of the Cabibbo-suppressed decays $\bar{B}^0\to D^{+}K^{-}\pi^{+}\pi^{-}$ and $B^{-}\to D^{0} K^{-}\pi^{+}\pi^{-}$

Using 35 pb$^{-1}$ of data collected in $pp$ collisions at $\sqrt{s}$ = 7 TeV by the LHCb experiment, we report first observations of the Cabibbo-suppressed decays $\bar{B}^0\to D^{+}K^{-}\pi^{+}\pi^{-}$ and $B^{-}\to D^{0} K^{-}\pi^{+}\pi^{-}$, and measure their branching fractions relative to the corresponding Cabibbo-favored modes, $\bar{B}^{0}\to D^{+}\pi^{-}\pi^{+}\pi^{-}$ and $B^{-}\to D^{0}\pi^{-}\pi^{+}\pi^{-}$, respectively. The ratios of branching fractions are: \begin{align} {{\cal{B}}(\bar{B}^0\to D^+K^-\pi^+\pi^-)\over {\cal{B}}(\bar{B}^0\to D^+\pi^-\pi^+\pi^-)} = (5.2\pm 0.9({\rm stat})\pm 0.5({\rm syst}))\times 10^{-2} \nonumber \\ {{\cal{B}}(B^-\to D^0 K^-\pi^+\pi^-)\over {\cal{B}}(B^-\to D^0\pi^-\pi^+\pi^-)} = (9.6\pm 1.5({\rm stat})\pm0.8({\rm syst}))\times10^{-2}. \nonumber \\ \end{align} Decays such as these will be of use to determine the CKM Unitarity Triangle angle $\gamma$.