Observation ofb→dγand Determination of|Vtd/Vts|

We report the observation of the flavor-changing neutral current process $b\to d\gamma$ using a sample of $386\times {10}^6$ $B$ meson pairs accumulated by the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We measure branching fractions for the exclusive modes $B^{-}\to {\rho }^{-}\gamma$, ${\overline{B}}^0\to {\rho }^0\gamma$, and ${\overline{B}}^0\to \omega \gamma$. Assuming that these three modes are related by isospin, we find $\mathcal{B}\mathbf{(}\overline{B}\to (\rho ,\omega )\gamma \mathbf{)}=\mathbf{(}{1.32}_{-0.31}^{+0.34}(\mathrm{stat}{)}_{-0.09}^{+0.10}(\mathrm{syst})\mathbf{)}\times {10}^{-6}$ with a significance of $5.1\sigma$. This result is used to determine the ratio of Cabibbo-Kobayashi-Maskawa matrix elements $|V_{td}/V_{ts}|$ to be ${0.199}_{-0.025}^{+0.026}(\exp {)}_{-0.015}^{+0.018}(\mathrm{theor})$.