Mohapatra, D (reprint author), Virginia Polytech Inst & State Univ, Blacksburg, VA 24061 USA.
We report the observation of the flavor-changing neutral current process b -> d gamma using a sample of 386 x 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- -> rho(-)gamma, (B) over bar (0) -> rho(0)gamma, and (B) over bar (0) -> omega gamma. Assuming that these three modes are related by isospin, we find B((B) over bar -> (rho, omega)gamma) = (1.32(-0.31)(+0.34)(stat)(-0.09)(+0.10)(syst)) x 10(-6) with a significance of 5.1 sigma. This result is used to determine the ratio of Cabibbo-Kobayashi-Maskawa matrix elements vertical bar V-td/V-ts vertical bar to be 0.199(-0.025)(+0.026)(exp)(-0.015)(+0.018)(theor).