Search for the decay $B^+ \to D^{*-}K^+\pi^+$

The $B^+ \to D^{*-}K^+\pi^+$ decay is searched for in a sample, collected with the LHCb detector, of proton-proton collision data at centre-of-mass energies $\sqrt{s} = 7$ and $8 ~\text {TeV}$, corresponding to an integrated luminosity of $3 \text {fb}$. A clear signal is observed, and the ratio of its branching fraction to that of the $B^+ \to D^{*-}\pi^+\pi^+$ normalisation channel is measured to be \begin{equation*} \frac{ \mathcal{B} (B^+ \to D^{*-}K^+\pi^+) }{\mathcal{B} (B^+ \to D^{*-}\pi^+\pi^+)} = 0.0649 \pm 0.0026 \mathrm{\,(stat)} \pm 0.0049 \mathrm{\,(syst)} \, . \end{equation*}