Search for theb→dγprocess

We report the results of a search for the flavor-changing neutral current process

b \to d γ

using a sample of

275 \times 10^{6}

B

meson pairs accumulated by the Belle detector at KEKB. We find no significant signal for the exclusive decays

B^{-} \to ρ^{-} γ

{\bar{B}}^{0} \to ρ^{0} γ

, or

{\bar{B}}^{0} \to ω γ

. Assuming an isospin relation between the three modes, we set an upper limit for the combined branching fraction

B (B \to (ρ, ω) γ)

, of

1.4 \times 10^{- 6}

at the 90% confidence level. This limit can be used to constrain the ratio of CKM matrix elements

| V_{t d} / V_{t s} |

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