We report the observation of the radiative decay $B^{+}\to K_1(1270{)}^{+}\gamma$ using a data sample of $140{\mathrm{fb}}^{-1}$ taken at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We find the branching fraction to be $\mathcal{B}\mathbf{(}{B}^{+}\to K_1(1270{)}^{+}\gamma \mathbf{)}=\mathbf{(}4.3\pm 0.9(\mathrm{stat}.)\pm 0.9(\mathrm{syst}.)\mathbf{)}\times {10}^{-5}$ with a significance of $7.3\sigma$. We find no significant signal for $B^{+}\to K_1(1400{)}^{+}\gamma$ and set an upper limit $\mathcal{B}\mathbf{(}{B}^{+}\to K_1(1400{)}^{+}\gamma \mathbf{)}<1.5\times {10}^{-5}$ at the 90% confidence level. We also measure inclusive branching fractions for $B^{+}\to K^{+}{\pi }^{+}{\pi }^{-}\gamma$ and $B^0\to K^0{\pi }^{+}{\pi }^{-}\gamma$ in the mass range $1\mathrm{GeV}/c^2<M_{K^{+(0)}{\pi }^{+}{\pi }^{-}}<2\mathrm{GeV}/c^2$.

• Received 12 December 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.111802