We report results of searches for charmless hadronic $B$ meson decays to pseudoscalar( ${\pi }^{\pm }$, $K^{\pm }$, ${\pi }^0$, or $K_S^0$)-vector( $\rho$, $K^{*}$, or $\omega$) final states. By using $9.7\times {10}^6$ $B\overline{B}$ pairs collected with the CLEO detector, we report the first observation of $B^{-}\to {\pi }^{-}{\rho }^0$, ${\overline{B}}^0\to {\pi }^{\pm }{\rho }^{\mp }$, and $B^{-}\to {\pi }^{-}\omega$, which are expected to be dominated by hadronic $b\to u$ transitions. The measured branching fractions are $({10.4}_{-3.4\mathrm{}}^{+3.3\mathrm{}}\pm 2.1)\times {10}^{-6\mathrm{}}$, $({27.6}_{-7.4\mathrm{}}^{+8.4\mathrm{}}\pm 4.2)\times {10}^{-6\mathrm{}}$, and $({11.3}_{-2.9\mathrm{}}^{+3.3\mathrm{}}\pm 1.4)\times {10}^{-6\mathrm{}}$, respectively. Branching fraction upper limits are set for all of the other decay modes investigated.

• Received 30 May 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.2881