Search for the b --> d gamma process

We report search results for the flavor-changing neutral current process b --> d gamma with a data sample containing 274 million B meson pairs accumulated at the Upsilon(4S) resonance with the Belle detector at KEKB. We studied the exclusive decays B- --> rho- gamma, B0bar --> rho0 gamma, and B0bar --> omega gamma, and find no significant signal. We set an upper limit for a combined branching fraction Br(B --> (rho,omega) gamma) < 1.4 x 10-6 at the 90% confidence level, which is normalized to that of B- --> rho- gamma assuming an isospin relation between the three modes.