We report the first observation of the radiative hyperonic $B$ decay $B^{+}\to p\overline{\Lambda }\gamma$, using a $140{\mathrm{fb}}^{-1}$ data sample recorded on the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$ collider. The measured branching fraction is $\mathcal{B}(B^{+}\to p\overline{\Lambda }\gamma )=({2.16}_{-0.53}^{+0.58}\pm 0.20)\times {10}^{-6}$. We examine its $M_{p\overline{\Lambda }}$ distribution and observe a peak near threshold. This feature is expected by the short-distance $b\to s\gamma$ transition. A search for $B^{+}\to p{\overline{\Sigma }}^0\gamma$ yields no significant signal, and we set a 90% confidence-level upper limit on the branching fraction of $\mathcal{B}(B^{+}\to p{\overline{\Sigma }}^0\gamma )<4.6\times {10}^{-6}$.

• Received 21 March 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.061802