Search for Radiative Penguin Decays $B^{+}\to {\rho }^{+}\gamma$, $B^0\to {\rho }^0\gamma$, and $B^0\to \omega \gamma$

A search for the decays $B\to \rho (770)\gamma$ and $B^0\to \omega (782)\gamma$ is performed on a sample of $211\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ events collected by the BABAR detector at the SLAC PEP-II asymmetric-energy $e^{+}{e}^{-}$ storage ring. No evidence for the decays is seen. We set the following limits on the individual branching fractions: $\mathcal{B}(B^{+}\to {\rho }^{+}\gamma )<1.8\times {10}^{-6}$, $\mathcal{B}(B^0\to {\rho }^0\gamma )<0.4\times {10}^{-6}$, and $\mathcal{B}(B^0\to \omega \gamma )<1.0\times {10}^{-6}$ at the 90% confidence level. We use the quark model to limit the combined branching fraction $\overline{\mathcal{B}}[B\to (\rho /\omega )\gamma ]<1.2\times {10}^{-6}$, from which we determine a constraint on the ratio of Cabibbo-Kobayashi-Maskawa matrix elements $|V_{td}|/|V_{ts}|$.