Observation ofB+→K1(1270)+γ

We report the observation of the radiative decay ${B}^{+}\ensuremath{\rightarrow}{K}_{1}(1270{)}^{+}\ensuremath{\gamma}$ using a data sample of $140\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ taken at the $\ensuremath{\Upsilon}(4S)$ resonance with the Belle detector at the KEKB ${e}^{+}{e}^{\ensuremath{-}}$ collider. We find the branching fraction to be $\mathcal{B}\mathbf{(}{B}^{+}\ensuremath{\rightarrow}{K}_{1}(1270{)}^{+}\ensuremath{\gamma}\mathbf{)}=\mathbf{(}4.3\ifmmode\pm\else\textpm\fi{}0.9(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.9(\mathrm{syst}.)\mathbf{)}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ with a significance of $7.3\ensuremath{\sigma}$. We find no significant signal for ${B}^{+}\ensuremath{\rightarrow}{K}_{1}(1400{)}^{+}\ensuremath{\gamma}$ and set an upper limit $\mathcal{B}\mathbf{(}{B}^{+}\ensuremath{\rightarrow}{K}_{1}(1400{)}^{+}\ensuremath{\gamma}\mathbf{)}l1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ at the 90% confidence level. We also measure inclusive branching fractions for ${B}^{+}\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\gamma}$ and ${B}^{0}\ensuremath{\rightarrow}{K}^{0}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\gamma}$ in the mass range $1\text{ }\text{ }\mathrm{GeV}/{c}^{2}l{M}_{{K}^{+(0)}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}}l2\text{ }\text{ }\mathrm{GeV}/{c}^{2}$.